How to Use This Section
This chapter is examined in a very particular way. Almost every question is "name the stage", "what happens at this stage", "give N and C for this cell", or a comparison between two stages that sound alike. There is nothing to derive and nothing to calculate - the marks sit in the exact stage names, the exact events and the one line that separates two look-alike stages.
That makes this chapter one of the easiest to score full marks in, and one of the easiest to lose marks in through a single swapped word. The centromere splits at anaphase of mitosis and at anaphase II, but not at anaphase I. Synapsis is zygotene, crossing over is pachytene, chiasmata are diplotene and terminalisation is diakinesis. Telophase I gives a dyad and telophase II gives a tetrad. S phase doubles the DNA but not the chromosome number. Each of those pairs has cost more marks than any reasoning question in the chapter.
Work through the three tiers in order.
- Tier 1 - Concept Checks. One fact per question, straight from the text. If you cannot answer these without stopping, go back to the section they came from before moving on.
- Tier 2 - Stage Identification and Application. A cell is described - what is happening inside it, what is visible, what its N and C are - and you name the stage and justify the name. This is exactly how this chapter is set, so this tier is the one to spend time on.
- Tier 3 - Comparisons and Long Answers. The two-column differences and the joined-up descriptions - mitosis against meiosis, anaphase against anaphase I, metaphase against metaphase I, prophase against prophase I, and the full walk of N and C through the whole cycle.
A working habit that pays in every question of this chapter: before you name a stage, ask what the centromere is doing and what the homologues are doing. Those two answers alone separate almost every pair of stages in the chapter.
The sixteen chapter-end exercises are all answered somewhere in this chapter. The last block lists where each one is answered and works out the one that is not covered anywhere else.
The Facts These Questions Draw On
What the cell cycle is. Cell division is a very important process in all living organisms. During the division of a cell, DNA replication and cell growth also take place, and all these processes have to take place in a coordinated way to ensure correct division and formation of progeny cells containing intact genomes. The cell cycle is the sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells. Cell growth, in terms of cytoplasmic increase, is a continuous process, but DNA synthesis occurs only during one specific stage in the cell cycle. The replicated chromosomes are then distributed to daughter nuclei by a complex series of events during cell division, and these events are themselves under genetic control.
Durations. A typical eukaryotic cell cycle is illustrated by human cells in culture, which divide once in approximately every 24 hours. This duration can vary from organism to organism and also from cell type to cell type - yeast can progress through the cell cycle in only about 90 minutes. In the 24 hour average duration of the cell cycle of a human cell, cell division proper lasts for only about an hour, and the interphase lasts more than 95 per cent of the duration of the cell cycle.
The two basic phases. The cell cycle is divided into two basic phases: interphase and M phase (mitosis phase). The M phase represents the phase when the actual cell division or mitosis occurs. The interphase represents the phase between two successive M phases. The M phase starts with the nuclear division, corresponding to the separation of daughter chromosomes - karyokinesis - and usually ends with division of the cytoplasm - cytokinesis. The interphase, though called the resting phase, is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner. It is not a rest at all.
Inside interphase. The interphase is divided into three further phases: phase (Gap 1), S phase (Synthesis) and phase (Gap 2). corresponds to the interval between mitosis and initiation of DNA replication, during which the cell is metabolically active and continuously grows but does not replicate its DNA. S phase marks the period during which DNA synthesis or replication takes place; the amount of DNA per cell doubles, so if the initial amount of DNA is denoted as then it increases to , but there is no increase in the chromosome number - a cell with a diploid or number of chromosomes at still has after S phase. In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm. In , proteins are synthesised in preparation for mitosis while cell growth continues.
The phase. Some cells in adult animals do not appear to exhibit division - heart cells are the example - and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. These cells that do not divide further exit the phase to enter an inactive stage called the quiescent stage, or , of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism. Note the two separate ideas: inactive with respect to division, but still metabolically active, and the exit is from .
Who divides by mitosis. In animals, mitotic cell division is only seen in the diploid somatic cells. There are a few exceptions where haploid cells divide by mitosis - for example, male honey bees. Plants can show mitotic divisions in both haploid and diploid cells. In some lower plants and in some social insects, haploid cells also divide by mitosis.
M phase and the four stages. This is the most dramatic period of the cell cycle, involving a major reorganisation of virtually all components of the cell. Since the number of chromosomes in the parent and progeny cells is the same, mitosis is also called equational division. Though for convenience mitosis has been divided into four stages of nuclear division (karyokinesis), cell division is a progressive process and very clear-cut lines cannot be drawn between the various stages. Karyokinesis involves four stages: prophase, metaphase, anaphase and telophase.
Prophase. Prophase, the first stage of karyokinesis of mitosis, follows the S and phases of interphase, in which the new DNA molecules formed are not distinct but intertwined. Prophase is marked by the initiation of condensation of chromosomal material, and the chromosomal material becomes untangled during the process of chromatin condensation. Chromosomal material condenses to form compact mitotic chromosomes, and chromosomes are seen to be composed of two chromatids attached together at the centromere. The centrosome, which had undergone duplication during interphase, begins to move towards opposite poles of the cell. Each centrosome radiates out microtubules called asters, and the two asters together with spindle fibres form the mitotic apparatus. Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope.
Metaphase. The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, and hence the chromosomes are spread through the cytoplasm of the cell. By this stage, condensation of chromosomes is completed and they can be observed clearly under the microscope; this is the stage at which the morphology of chromosomes is most easily studied. The metaphase chromosome is made up of two sister chromatids, which are held together by the centromere. Small disc-shaped structures at the surface of the centromeres are called kinetochores, and these serve as the sites of attachment of spindle fibres to the chromosomes. Metaphase is characterised by all the chromosomes coming to lie at the equator, with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole. The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate. The two key events are: spindle fibres attach to kinetochores of chromosomes, and chromosomes are moved to the spindle equator and get aligned along the metaphase plate.
Anaphase. At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously, and the two daughter chromatids, now referred to as daughter chromosomes of the future daughter nuclei, begin their migration towards the two opposite poles. As each chromosome moves away from the equatorial plate, the centromere of each chromosome remains directed towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind. The two key events are: centromeres split and chromatids separate, and chromatids move to opposite poles.
Telophase. At the beginning of telophase the chromosomes that have reached their respective poles decondense and lose their individuality. The individual chromosomes can no longer be seen, and each set of chromatin material tends to collect at each of the two poles. The key events are: chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements; nuclear envelope develops around the chromosome clusters at each pole, forming two daughter nuclei; nucleolus, golgi complex and ER reform. Telophase is the end of karyokinesis, not of cell division - at its close there are two nuclei but still only one cell.
Cytokinesis. Mitosis accomplishes not only the segregation of duplicated chromosomes into daughter nuclei, which is karyokinesis, but the cell itself is divided into two daughter cells by the separation of cytoplasm, called cytokinesis, at the end of which cell division gets completed. In an animal cell, cytokinesis is achieved by the appearance of a furrow in the plasma membrane, and the furrow gradually deepens and ultimately joins in the centre, dividing the cell cytoplasm into two - the cut runs from the outside inward. Plant cells, however, are enclosed by a relatively inextensible cell wall, and therefore they undergo cytokinesis by a different mechanism. In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls. The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate, that represents the middle lamella between the walls of two adjacent cells - built from the inside outward. At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells. In some organisms karyokinesis is not followed by cytokinesis, as a result of which a multinucleate condition arises, leading to the formation of a syncytium - for example, the liquid endosperm in coconut.
Significance of mitosis. Mitosis, the equational division, is usually restricted to the diploid cells only. However, in some lower plants and in some social insects, haploid cells also divide by mitosis. Mitosis usually results in the production of diploid daughter cells with identical genetic complement - the chromosome number is unchanged and the genetic complement is identical. The growth of multicellular organisms is due to mitosis. Cell growth results in disturbing the ratio between the nucleus and the cytoplasm, and it therefore becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio. A very significant contribution of mitosis is cell repair - the cells of the upper layer of the epidermis, the cells of the lining of the gut, and blood cells are being constantly replaced. Mitotic divisions in the meristematic tissues - the apical and the lateral cambium - result in a continuous growth of plants throughout their life.
Why meiosis exists. The production of offspring by sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes. Gametes are formed from specialised diploid cells. This specialised kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells, and this kind of division is called meiosis. Meiosis ensures the production of the haploid phase in the life cycle of sexually reproducing organisms, whereas fertilisation restores the diploid phase. We come across meiosis during gametogenesis in plants and animals, and this leads to the formation of haploid gametes.
The four key features of meiosis. Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II, but only a single cycle of DNA replication. Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase. Meiosis involves pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes. Four haploid cells are formed at the end of meiosis II. Meiotic events are grouped as meiosis I - prophase I, metaphase I, anaphase I, telophase I - and meiosis II - prophase II, metaphase II, anaphase II, telophase II.
Prophase I. Prophase of the first meiotic division is typically longer and more complex when compared to prophase of mitosis, and it has been further subdivided into five phases based on chromosomal behaviour: leptotene, zygotene, pachytene, diplotene and diakinesis.
- Leptotene - during this stage the chromosomes become gradually visible under the light microscope, and the compaction of chromosomes continues throughout leptotene.
- Zygotene - chromosomes start pairing together, and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Chromosome synapsis is accompanied by the formation of a complex structure called the synaptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad.
- Pachytene - the four chromatids of each bivalent chromosome become distinct and clearly appear as tetrads. This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. It is an enzyme-mediated process, and the enzyme involved is called recombinase. Crossing over leads to recombination of genetic material on the two chromosomes. Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over. The first two stages of prophase I are relatively short-lived compared to pachytene.
- Diplotene - the beginning of diplotene is recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers. These X-shaped structures are called chiasmata. In oocytes of some vertebrates, diplotene can last for months or years.
- Diakinesis - the final stage of meiotic prophase I, marked by terminalisation of chiasmata. During this phase the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation. By the end of diakinesis the nucleolus disappears and the nuclear envelope also breaks down. Diakinesis represents transition to metaphase.
Metaphase I to telophase I. Metaphase I - the bivalent chromosomes align on the equatorial plate, and the microtubules from the opposite poles of the spindle attach to the kinetochore of homologous chromosomes. Anaphase I - the homologous chromosomes separate, while sister chromatids remain associated at their centromeres. The centromere does not split at anaphase I, so each chromosome reaches the pole still made of two chromatids, and each pole receives half the chromosome number of the parent cell - which is why meiosis I is the reductional division. Telophase I - the nuclear membrane and nucleolus reappear, cytokinesis follows, and this is called a dyad of cells. Although in many cases the chromosomes do undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus.
Interkinesis. The stage between the two meiotic divisions is called interkinesis, and it is generally short lived. There is no replication of DNA during interkinesis. Interkinesis is followed by prophase II, a much simpler prophase than prophase I. Do not confuse it with interphase, which has , S and and DNA replication in S phase.
Meiosis II. Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis I, meiosis II resembles a normal mitosis. Prophase II - the nuclear membrane disappears by the end of prophase II, and the chromosomes again become compact; there are no sub-stages, no synapsis, no crossing over and no chiasmata. Metaphase II - the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids. Anaphase II - it begins with the simultaneous splitting of the centromere of each chromosome, which was holding the sister chromatids together, allowing them to move toward opposite poles of the cell by shortening of microtubules attached to kinetochores. Telophase II - the two groups of chromosomes once again get enclosed by a nuclear envelope; cytokinesis follows, resulting in the formation of a tetrad of cells, that is four haploid daughter cells. Meiosis II is itself equational - the chromosome number stays .
N and C. N is the number of chromosomes per cell and a chromosome counts as one whether it is made of one chromatid or two - in practice, count centromeres. C is the amount of DNA per cell. C doubles only in S phase and is halved at each cytokinesis. N rises only when centromeres split, and falls only when a cell divides.
| Stage | N (chromosomes per cell) | C (DNA per cell) | Why |
|---|---|---|---|
| The reference state; no DNA replication in | |||
| S phase | from to | DNA replicates; each chromosome gains a second chromatid | |
| Only proteins are synthesised | |||
| Prophase (mitosis) | Condensation changes shape, not number | ||
| Metaphase (mitosis) | Chromosomes only line up on the metaphase plate | ||
| Anaphase (mitosis) | in the cell | Centromeres split, so each chromatid is now counted as a chromosome; no DNA is made | |
| Telophase (mitosis) | in the cell, per nucleus | in the cell, per nucleus | Two nuclei form around the two clusters |
| After cytokinesis (mitosis) | per cell | per cell | Back to the parent value, so mitosis is equational |
| Prophase I | Pairing and crossing over change neither count | ||
| Metaphase I | Bivalents align on the equatorial plate | ||
| Anaphase I | in the cell, to each pole | in the cell, to each pole | Homologues separate but the centromere does not split |
| Telophase I and cytokinesis - each cell of the dyad | The chromosome number is halved here; each chromosome still has two chromatids | ||
| Interkinesis | There is no replication of DNA during interkinesis | ||
| Prophase II | The chromosomes only become compact again | ||
| Metaphase II | Chromosomes align at the equator | ||
| Anaphase II | in the cell, to each pole | in the cell, to each pole | Centromeres split simultaneously and sister chromatids separate |
| Telophase II and cytokinesis - each cell of the tetrad | Four haploid daughter cells; meiosis II is equational |
Significance of meiosis. Meiosis is the mechanism by which conservation of the specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half. Meiosis also increases the genetic variability in the population of organisms from one generation to the next, and variations are very important for the process of evolution.
Mitosis against meiosis. Mitosis - one nuclear and cell division; meiosis - two sequential cycles of nuclear and cell division. Both have only a single cycle of DNA replication. Mitosis gives two daughter cells, meiosis gives four. Mitotic daughter cells are , the same as the parent; meiotic daughter cells are . Mitosis is equational; meiosis is reductional overall, though meiosis II is itself equational. Pairing of homologous chromosomes, crossing over and chiasmata are absent in mitosis and present in meiosis. Mitotic prophase is short and simple; prophase I is long and complex with five sub-stages. In mitotic anaphase the centromeres split and the sister chromatids separate; in anaphase I the homologous chromosomes separate while sister chromatids remain associated at their centromeres. Mitosis occurs in somatic cells; meiosis occurs in diploid cells destined to form gametes, during gametogenesis. Mitosis gives daughter cells with identical genetic complement; meiosis gives genetically variable daughter cells. Mitosis serves growth, restoration of the nucleo-cytoplasmic ratio, repair and replacement; meiosis serves gamete formation and variation.
Tier 1 - Concept Checks
Question 1
Q. Define the cell cycle.
Answer. The cell cycle is the sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells. Keep all three parts of that sentence - duplicates the genome, synthesises the other constituents, divides into two daughter cells - because questions are built by dropping one of them.
Question 2
Q. How long does a typical eukaryotic cell cycle take, how long does division proper take, and how long does yeast take?
Answer. A typical eukaryotic cell cycle is illustrated by human cells in culture, which divide once in approximately every 24 hours. In that 24 hour average duration, cell division proper lasts for only about an hour, so the interphase lasts more than 95 per cent of the duration of the cell cycle. This duration can vary from organism to organism and also from cell type to cell type - yeast can progress through the cell cycle in only about 90 minutes.
Question 3
Q. Name the two basic phases of the cell cycle and say what each one is.
Answer. The cell cycle is divided into two basic phases: interphase and M phase (mitosis phase). The M phase represents the phase when the actual cell division or mitosis occurs. The interphase represents the phase between two successive M phases. The M phase starts with the nuclear division, corresponding to the separation of daughter chromosomes - karyokinesis - and usually ends with division of the cytoplasm - cytokinesis.
Question 4
Q. Interphase is called the resting phase. Is the cell actually resting?
Answer. No. The interphase, though called the resting phase, is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner. It is the busiest part of the cycle, and it occupies more than 95 per cent of the duration of the cell cycle. The word "resting" only means resting from division, not resting from work.
Question 5
Q. Name the three phases of interphase, and say what happens in the first of them.
Answer. The interphase is divided into three further phases: phase (Gap 1), S phase (Synthesis) and phase (Gap 2). phase corresponds to the interval between mitosis and initiation of DNA replication. During it the cell is metabolically active and continuously grows but does not replicate its DNA.
Question 6
Q. What happens to the DNA content and to the chromosome number during S phase, and what else happens in an animal cell?
Answer. S phase, or synthesis phase, marks the period during which DNA synthesis or replication takes place. The amount of DNA per cell doubles - if the initial amount of DNA is denoted as then it increases to . But there is no increase in the chromosome number: if the cell had a diploid or number of chromosomes at , even after S phase the number of chromosomes remains the same, that is . In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.
Question 7
Q. What happens during the phase?
Answer. Proteins are synthesised in preparation for mitosis while cell growth continues. No DNA is made here - DNA synthesis occurs only during S phase. The cell stays at , throughout .
Question 8
Q. What is the or quiescent phase, which phase do cells leave to enter it, and give an example.
Answer. Some cells in adult animals do not appear to exhibit division - heart cells, for example - and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. These cells that do not divide further exit the phase to enter an inactive stage called the quiescent stage, or , of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism. Two points decide the marks: the cell leaves from , and it is inactive only with respect to division, not metabolically inactive.
Question 9
Q. Name the four stages of karyokinesis, and say why mitosis is called equational division.
Answer. Karyokinesis involves four stages: prophase, metaphase, anaphase and telophase. Since the number of chromosomes in the parent and progeny cells is the same, mitosis is also called equational division. Note the caution the chapter adds: though for convenience mitosis has been divided into four stages, cell division is a progressive process and very clear-cut lines cannot be drawn between the various stages.
Question 10
Q. State the events that complete prophase of mitosis.
Answer. Prophase is marked by the initiation of condensation of chromosomal material, and the chromosomal material becomes untangled during the process of chromatin condensation. Its completion is marked by:
- Chromosomal material condenses to form compact mitotic chromosomes, and chromosomes are seen to be composed of two chromatids attached together at the centromere.
- The centrosome, which had undergone duplication during interphase, begins to move towards opposite poles of the cell. Each centrosome radiates out microtubules called asters, and the two asters together with spindle fibres form the mitotic apparatus.
Question 11
Q. Which four structures are not seen in a cell at the end of prophase?
Answer. Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope. Learn all four as one list, because the question is normally set as "which of these is still present".
Question 12
Q. What is a kinetochore, what is the metaphase plate, and at which stage is chromosome morphology most easily studied?
Answer. Small disc-shaped structures at the surface of the centromeres are called kinetochores, and these structures serve as the sites of attachment of spindle fibres to the chromosomes. The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate. Metaphase is the stage at which the morphology of chromosomes is most easily studied, because by this stage condensation of chromosomes is completed and they can be observed clearly under the microscope.
Question 13
Q. State the key events of anaphase, and describe how a chromosome is oriented as it moves.
Answer. The two key events are centromeres split and chromatids separate, and chromatids move to opposite poles. At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously, and the two daughter chromatids, now referred to as daughter chromosomes of the future daughter nuclei, begin their migration towards the two opposite poles. As each chromosome moves away from the equatorial plate, the centromere of each chromosome remains directed towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind.
Question 14
Q. State the key events of telophase.
Answer. Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements. Nuclear envelope develops around the chromosome clusters at each pole, forming two daughter nuclei. Nucleolus, golgi complex and ER reform. The way to hold it is as prophase running backwards - the chromosomes that have reached their respective poles decondense and lose their individuality, and everything that vanished at prophase comes back.
Question 15
Q. What is cytokinesis, and what is a syncytium? Give the example.
Answer. Cytokinesis is the division of the cell itself into two daughter cells by the separation of cytoplasm, at the end of which cell division gets completed. In some organisms karyokinesis is not followed by cytokinesis, as a result of which a multinucleate condition arises, leading to the formation of a syncytium - for example, the liquid endosperm in coconut. Also remember that at the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells.
Question 16
Q. Give the significance of mitosis.
Answer. Mitosis, the equational division, is usually restricted to the diploid cells only, though in some lower plants and in some social insects, haploid cells also divide by mitosis. Its significance:
- Mitosis usually results in the production of diploid daughter cells with identical genetic complement.
- The growth of multicellular organisms is due to mitosis.
- Cell growth results in disturbing the ratio between the nucleus and the cytoplasm, so it becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio.
- A very significant contribution of mitosis is cell repair - the cells of the upper layer of the epidermis, the cells of the lining of the gut, and blood cells are being constantly replaced.
- Mitotic divisions in the meristematic tissues - the apical and the lateral cambium - result in a continuous growth of plants throughout their life.
Question 17
Q. State the four key features of meiosis.
Answer.
- Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II, but only a single cycle of DNA replication.
- Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.
- Meiosis involves pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes.
- Four haploid cells are formed at the end of meiosis II.
Question 18
Q. Name the five sub-stages of prophase I in order, with the one event that defines each.
Answer. Leptotene, zygotene, pachytene, diplotene and diakinesis, subdivided on the basis of chromosomal behaviour.
| Sub-stage | The defining event |
|---|---|
| Leptotene | The chromosomes become gradually visible under the light microscope; compaction continues throughout |
| Zygotene | Chromosomes start pairing - synapsis - and the synaptonemal complex forms; a pair of synapsed homologues is a bivalent or a tetrad |
| Pachytene | The four chromatids of each bivalent appear as tetrads; recombination nodules appear and crossing over occurs |
| Diplotene | The synaptonemal complex dissolves and the homologues separate except at the sites of crossovers - the X-shaped chiasmata |
| Diakinesis | Terminalisation of chiasmata; chromosomes fully condensed; spindle assembled; nucleolus disappears and nuclear envelope breaks down |
Question 19
Q. What is crossing over, where does it occur, and which enzyme carries it out?
Answer. Crossing over is the exchange of genetic material between two homologous chromosomes. It occurs at pachytene of prophase I, at the recombination nodules, and it takes place between non-sister chromatids of the homologous chromosomes. Crossing over is an enzyme-mediated process, and the enzyme involved is called recombinase. Crossing over leads to recombination of genetic material on the two chromosomes, and recombination is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.
Question 20
Q. What is interkinesis, and what is the difference between a dyad and a tetrad of cells?
Answer. The stage between the two meiotic divisions is called interkinesis, and it is generally short lived. There is no replication of DNA during interkinesis, and it is followed by prophase II, a much simpler prophase than prophase I. As for the two counting words - telophase I gives a dyad, that is two cells, and telophase II gives a tetrad, that is four haploid daughter cells.
Tier 2 - Stage Identification and Application
Question 21
Q. A cell has just come out of mitosis. It is metabolically active and is growing steadily, but no new DNA is being made in it. Name the phase.
Answer. It is phase of interphase. phase corresponds to the interval between mitosis and initiation of DNA replication, and during it the cell is metabolically active and continuously grows but does not replicate its DNA. The clue "just come out of mitosis and no DNA being made" fixes it - also makes no DNA, but comes after S phase, not straight after mitosis. The cell is at , .
Question 22
Q. An animal cell is examined and found to contain twice the DNA it had an hour ago, but exactly the same number of chromosomes. Its centriole has also just doubled. Name the phase and explain the apparent contradiction.
Answer. It is S phase. The amount of DNA per cell doubles - if the initial amount of DNA is denoted as then it increases to - but there is no increase in the chromosome number: a cell that had a diploid or number of chromosomes at still has after S phase. There is no contradiction, because each chromosome now simply has two chromatids instead of one, and a chromosome counts as one whether it has one chromatid or two. The centriole confirms it - in animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.
Question 23
Q. Under the microscope a cell shows chromosomes condensing into compact threads, each made of two chromatids joined at the centromere, with the two centrosomes moving apart and radiating microtubules. The nucleolus and nuclear envelope are gone. Name the stage.
Answer. It is prophase of mitosis. Prophase is marked by the initiation of condensation of chromosomal material, chromosomes are seen to be composed of two chromatids attached together at the centromere, and the centrosome, which had undergone duplication during interphase, begins to move towards opposite poles of the cell while each centrosome radiates out microtubules called asters. The two asters together with spindle fibres form the mitotic apparatus. The missing structures fit as well - cells at the end of prophase do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope.
Question 24
Q. In a dividing cell, all the chromosomes have come to lie in one plane at the centre, with one chromatid of each chromosome connected by its kinetochore to one pole and its sister connected to the opposite pole. Name the stage, and name the plane.
Answer. The stage is metaphase of mitosis. Metaphase is characterised by all the chromosomes coming to lie at the equator, and one of its two key events is that chromosomes are moved to the spindle equator and get aligned along the metaphase plate through spindle fibres to both poles. The plane is the metaphase plate - the plane of alignment of the chromosomes at metaphase. Note that the chromosomes are only lying at the plate here; nothing has split yet.
Question 25
Q. A cell is seen in which the centromere of every chromosome has just split, the freed threads are moving apart, and each one travels with its centromere pointing at the pole and its arms trailing. Name the stage, and say what the moving threads are now called.
Answer. The stage is anaphase of mitosis. Its two key events are centromeres split and chromatids separate and chromatids move to opposite poles. At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously, and the two daughter chromatids are now referred to as daughter chromosomes of the future daughter nuclei. The orientation confirms it - the centromere of each chromosome remains directed towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind. In a cell the count momentarily rises to while the DNA is still .
Question 26
Q. In a cell the chromosomes at each pole have decondensed and can no longer be told apart, two nuclear envelopes have formed, and the nucleolus, golgi complex and ER are back. The cytoplasm is still undivided. Name the stage, and say how many cells there are.
Answer. The stage is telophase of mitosis, and there is still only one cell. The chromosomes that have reached their respective poles decondense and lose their individuality, a nuclear envelope develops around the chromosome clusters at each pole, forming two daughter nuclei, and the nucleolus, golgi complex and ER reform. Telophase ends karyokinesis, not cell division - the cell itself is divided into two daughter cells by the separation of cytoplasm, called cytokinesis, which comes next.
Question 27
Q. A plant cell is seen with a partition forming in the middle of the cell and growing outwards towards the side walls. Name the process, name the structure, and say what would result if this step were skipped.
Answer. The process is cytokinesis in a plant cell, and the structure is the cell-plate. Plant cells are enclosed by a relatively inextensible cell wall, and therefore they undergo cytokinesis by a different mechanism - in plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls, and the formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate, that represents the middle lamella between the walls of two adjacent cells. If the step were skipped, karyokinesis would not be followed by cytokinesis, a multinucleate condition would arise, leading to the formation of a syncytium - as in the liquid endosperm in coconut.
Question 28
Q. A cell in prophase I is seen with its chromosomes coming together side by side in pairs, and an electron micrograph shows a ladder-like structure holding each pair together. Name the sub-stage, the process and the structure.
Answer. The sub-stage is zygotene. During this stage, chromosomes start pairing together, and this process of association is called synapsis, and such paired chromosomes are called homologous chromosomes. The structure is the synaptonemal complex - electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of a complex structure called the synaptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad.
Question 29
Q. A cell shows bivalents whose four chromatids are now clearly distinct, with small nodules sitting where segments are being exchanged between chromatids of the two different homologues. Name the sub-stage, the nodules, the process and the enzyme.
Answer. The sub-stage is pachytene. During this stage, the four chromatids of each bivalent chromosome become distinct and clearly appear as tetrads. The nodules are recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. The process is crossing over - the exchange of genetic material between two homologous chromosomes, and the enzyme is recombinase, since crossing over is an enzyme-mediated process. Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.
Question 30
Q. In a meiotic cell the ladder-like complex has dissolved and the two homologues of each pair are pulling apart everywhere except at a few X-shaped points where they stay joined. Name the sub-stage, name those points, and name the sub-stage at which they are terminalised.
Answer. The sub-stage is diplotene. The beginning of diplotene is recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers, and these X-shaped structures are called chiasmata. Each chiasma is the visible evidence of a crossover that happened at pachytene. Terminalisation of chiasmata happens one stage later, at diakinesis, which is also where the nucleolus disappears and the nuclear envelope breaks down. Remember too that in oocytes of some vertebrates, diplotene can last for months or years.
Question 31
Q. A dividing cell has pairs of chromosomes lying on the equatorial plate, and the spindle microtubules from opposite poles are attached to the kinetochore of the two homologues of each pair. Name the stage, and say how it differs from mitotic metaphase.
Answer. The stage is metaphase I of meiosis. Metaphase I - the bivalent chromosomes align on the equatorial plate, and the microtubules from the opposite poles of the spindle attach to the kinetochore of homologous chromosomes. The difference from mitotic metaphase is what lies on the plate and what faces the poles: in mitosis the unit on the plate is an individual chromosome, with one chromatid connected by its kinetochore to one pole and its sister chromatid to the opposite pole; in metaphase I the unit is a bivalent, with one whole homologue facing one pole and its partner facing the other. The cell is at , .
Question 32
Q. In a cell whose chromosome number is , the centromeres have all split simultaneously and the sister chromatids are moving to opposite poles as the microtubules shorten. Name the stage, and say why it cannot be anaphase I.
Answer. The stage is anaphase II. Anaphase II begins with the simultaneous splitting of the centromere of each chromosome, which was holding the sister chromatids together, allowing them to move toward opposite poles of the cell by shortening of microtubules attached to kinetochores. It cannot be anaphase I, for two reasons. First, in anaphase I the homologous chromosomes separate, while sister chromatids remain associated at their centromeres - the centromere does not split at anaphase I. Second, anaphase I happens in a cell that is still , whereas this cell is already , so it is in meiosis II. Sister chromatids separate at anaphase II, not at anaphase I.
Tier 3 - Comparisons and Long Answers
Question 33
Q. List the main differences between mitosis and meiosis.
Answer. Give the anchor difference first, then hang the rows on it: meiosis has two sequential cycles of nuclear and cell division on only a single cycle of DNA replication, and it pairs homologous chromosomes; mitosis does neither.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One nuclear and cell division | Two sequential cycles of nuclear and cell division - meiosis I and meiosis II |
| Number of DNA replications | One, in S phase | One only, in S phase before meiosis I; there is no replication of DNA during interkinesis |
| Number of daughter cells | Two | Four |
| Chromosome number of daughter cells | The same as the parent, | Half that of the parent, |
| Nature of the division | Equational | Reductional overall, though meiosis II is itself equational - the reduction happens at meiosis I |
| Pairing of homologous chromosomes | Absent | Present - synapsis at zygotene, forming a bivalent |
| Crossing over | Absent | Present - at pachytene, between non-sister chromatids of homologous chromosomes |
| Chiasmata | Absent | Present - X-shaped structures seen at diplotene, later terminalised at diakinesis |
| Prophase | Short and simple | Prophase I is typically longer and more complex, with five sub-stages; prophase II is short and simple |
| Anaphase | The centromeres split and the sister chromatids separate | In anaphase I the homologous chromosomes separate while sister chromatids remain associated at their centromeres; the chromatids separate only at anaphase II |
| Where it occurs | In somatic cells - in animals it is restricted to the diploid somatic cells | In diploid cells destined to form gametes - during gametogenesis in plants and animals |
| Genetic outcome | Diploid daughter cells with identical genetic complement | Genetically variable daughter cells, because of crossing over and recombination |
| Function | Growth, restoring the nucleo-cytoplasmic ratio, cell repair and replacement | Formation of haploid gametes and the introduction of genetic variability |
The row that is most often got wrong is number of DNA replications - the answer is one for each, not two for meiosis. Two divisions, one replication.
Question 34
Q. Distinguish cytokinesis from karyokinesis.
Answer.
| Feature | Karyokinesis | Cytokinesis |
|---|---|---|
| What divides | The nucleus - it is the nuclear division, corresponding to the separation of daughter chromosomes | The cytoplasm - the cell itself is divided into two daughter cells by the separation of cytoplasm |
| Where it sits in M phase | The M phase starts with karyokinesis | The M phase usually ends with cytokinesis |
| Stages | Four - prophase, metaphase, anaphase and telophase | No stages; a single event |
| What it produces | Two daughter nuclei inside one cell | Two separate daughter cells |
| Mechanism | Chromosome condensation, spindle formation, chromosome movement and nuclear envelope reformation | A furrow in the plasma membrane in animal cells; a cell-plate in plant cells |
| When it can occur alone | Yes - in some organisms karyokinesis is not followed by cytokinesis, giving a syncytium, as in the liquid endosperm in coconut | It does not occur without a preceding karyokinesis |
The one line that answers the question: karyokinesis is the division of the nucleus and cytokinesis is the division of the cytoplasm, and cell division gets completed at the end of cytokinesis.
Question 35
Q. How does cytokinesis in plant cells differ from that in animal cells?
Answer. The difference exists for a structural reason: plant cells are enclosed by a relatively inextensible cell wall, and therefore they undergo cytokinesis by a different mechanism.
| Feature | Animal cell | Plant cell |
|---|---|---|
| Reason for the difference | Flexible plasma membrane, no cell wall | Enclosed by a relatively inextensible cell wall |
| Structure formed | A furrow in the plasma membrane | A cell-plate, the precursor of the new cell wall |
| Direction of division | From the periphery inward - the furrow gradually deepens and ultimately joins in the centre, dividing the cell cytoplasm into two | From the centre outward - wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls |
| What the new structure becomes | The two separated cytoplasms | The cell-plate represents the middle lamella between the walls of two adjacent cells |
Answer it in one line by direction: animal cell, furrow, outside inward; plant cell, cell-plate, centre outward. In both cases organelles like mitochondria and plastids get distributed between the two daughter cells.
Question 36
Q. Distinguish anaphase of mitosis from anaphase I of meiosis.
Answer.
| Feature | Anaphase of mitosis | Anaphase I of meiosis |
|---|---|---|
| What splits | The centromere of each chromosome splits | The centromere does not split |
| What separates | Sister chromatids separate | Homologous chromosomes separate, while sister chromatids remain associated at their centromeres |
| What moves to each pole | Daughter chromatids, now referred to as daughter chromosomes, each with a single chromatid | Whole chromosomes, each still made of two chromatids held at the centromere |
| Chromosome number reaching each pole | The same as the parent cell, | Half that of the parent cell, |
| Type of division | Equational | Reductional |
| Preceded by pairing | No pairing of homologues | Yes - the bivalents formed at zygotene |
| N and C in the cell | , | , |
What does not differ: in both, chromosomes move towards opposite poles, and in both the kinetochores are attached to spindle fibres. The line to memorise is in anaphase I the homologous chromosomes separate while the sister chromatids remain associated at their centromeres - the standard distractor, "centromeres split and chromatids separate", is true of anaphase of mitosis and anaphase II and false of anaphase I.
Question 37
Q. Distinguish metaphase of mitosis from metaphase I of meiosis.
Answer.
| Feature | Metaphase of mitosis | Metaphase I of meiosis |
|---|---|---|
| What lies on the equator | Individual chromosomes, each of two sister chromatids | Bivalents, that is pairs of homologous chromosomes, aligned on the equatorial plate |
| What the spindle fibres attach to | The kinetochores of the two sister chromatids of one chromosome | The kinetochore of homologous chromosomes |
| Which way the attachment faces | One chromatid to one pole, its sister to the opposite pole | One whole homologue to one pole, its partner to the opposite pole |
| What follows | Centromeres split and chromatids separate | Homologues separate; the centromere does not split |
| N and C | , | , |
Both cells carry the same N and C, so the counts alone cannot separate them - the presence of bivalents is the clue the examiner supplies. The wrong option always says "sister chromatids" for metaphase I; that description belongs to metaphase of mitosis and to metaphase II.
Question 38
Q. Distinguish prophase of mitosis from prophase I of meiosis.
Answer. Prophase of the first meiotic division is typically longer and more complex when compared to prophase of mitosis.
| Feature | Prophase of mitosis | Prophase I of meiosis |
|---|---|---|
| Duration and complexity | Short and simple | Typically longer and more complex |
| Sub-stages | None | Five - leptotene, zygotene, pachytene, diplotene and diakinesis, based on chromosomal behaviour |
| Pairing of homologues | Absent | Present - synapsis at zygotene, with the synaptonemal complex, forming a bivalent or tetrad |
| Crossing over | Absent | Present - at pachytene, at the recombination nodules, between non-sister chromatids, mediated by recombinase |
| Chiasmata | Absent | Present - the X-shaped structures at diplotene, terminalised at diakinesis |
| Chromosome condensation | Chromosomal material condenses to form compact mitotic chromosomes, each of two chromatids attached at the centromere | Compaction runs through leptotene and the chromosomes are fully condensed by diakinesis |
| Nuclear envelope and nucleolus | Gone by the end of prophase, along with golgi complexes and endoplasmic reticulum | By the end of diakinesis the nucleolus disappears and the nuclear envelope also breaks down |
| Spindle | The centrosome moves to opposite poles, radiating asters; the two asters with spindle fibres form the mitotic apparatus | The meiotic spindle is assembled at diakinesis to prepare the homologous chromosomes for separation |
Question 39
Q. Distinguish meiosis I from meiosis II.
Answer.
| Feature | Meiosis I | Meiosis II |
|---|---|---|
| What it separates | Homologous chromosomes | Sister chromatids |
| Nature | Reductional - each pole receives half the chromosome number of the parent cell | Equational - the chromosome number does not change, stays |
| Prophase | Prophase I is long and complex, with five sub-stages | Prophase II is short, with no sub-stages; the nuclear membrane disappears by its end and the chromosomes again become compact |
| Pairing, crossing over, chiasmata | All present | All absent - the homologues were already separated at anaphase I |
| Centromere at anaphase | The centromere does not split | Simultaneous splitting of the centromere of each chromosome |
| Cells produced | A dyad - two cells, each , | A tetrad - four haploid daughter cells, each , |
| Resemblance | Unlike anything in mitosis | In contrast to meiosis I, meiosis II resembles a normal mitosis |
| What precedes it | S phase, in which the parental chromosomes replicate to produce identical sister chromatids | Interkinesis, in which there is no replication of DNA; meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated |
Question 40
Q. Distinguish a bivalent, a tetrad and a dyad.
Answer. Three words that sound like counting words, and only two of them count the same thing.
| Term | What it is | When it appears |
|---|---|---|
| Bivalent | The complex formed by a pair of synapsed homologous chromosomes - so two chromosomes, four chromatids | From zygotene, where synapsis is accompanied by the synaptonemal complex; clearly visible at pachytene |
| Tetrad, in the sense of chromatids | The same bivalent, named for its four chromatids - the four chromatids of each bivalent chromosome become distinct and clearly appear as tetrads | Pachytene |
| Tetrad, in the sense of cells | Four haploid daughter cells - cytokinesis follows, resulting in the formation of a tetrad of cells | The end of telophase II |
| Dyad | Two cells - the nuclear membrane and nucleolus reappear, cytokinesis follows, and this is called a dyad of cells | The end of telophase I |
Bivalent and tetrad mean the same object at pachytene. The trap is the word tetrad in its two senses - four chromatids at pachytene, four cells at the end of meiosis II - so read the stem to see whether it is talking about chromatids or cells. And keep the pair straight: dyad after telophase I, tetrad after telophase II.
Question 41
Q. Describe synapsis, crossing over and chiasmata as one connected sequence.
Answer. They are three steps of the same story, one sub-stage apart, and the marks come from putting them in order.
Step 1 - synapsis, at zygotene. Chromosomes start pairing together, and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Chromosome synapsis is accompanied by the formation of a complex structure called the synaptonemal complex, and the complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. Nothing has been exchanged yet - the two homologues have only been brought side by side, gene for gene.
Step 2 - crossing over, at pachytene. The four chromatids of each bivalent chromosome become distinct and clearly appear as tetrads. This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. It is an enzyme-mediated process, and the enzyme involved is called recombinase. Crossing over leads to recombination of genetic material on the two chromosomes, and recombination is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over. Note non-sister - two sister chromatids are identical copies, so exchanging between them would achieve nothing.
Step 3 - chiasmata, at diplotene. The beginning of diplotene is recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers, and these X-shaped structures are called chiasmata. So a chiasma is simply the place where a crossover happened, seen from outside: the homologues are trying to move apart and succeeding everywhere except where they swapped material.
The tail of the story - diakinesis. The final stage of meiotic prophase I is diakinesis, marked by terminalisation of chiasmata, after which the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation.
Put as one line for the answer script: synapsis at zygotene pairs the homologues into a bivalent, crossing over at pachytene exchanges material between non-sister chromatids at the recombination nodules, and the chiasmata seen at diplotene are the visible remains of those exchanges, terminalised at diakinesis.
Question 42
Q. Walk a cell that is , at through mitosis and then through meiosis, giving N and C at every stage.
Answer. Two rules run the whole answer. C doubles only in S phase and is halved at each cytokinesis. N rises only when centromeres split, and falls only when a cell divides. A chromosome counts as one whether it has one chromatid or two, so in practice count centromeres.
Through mitosis.
| Stage | N | C | Why |
|---|---|---|---|
| The reference state; the cell grows but does not replicate its DNA | |||
| S | to | The amount of DNA per cell doubles, but there is no increase in the chromosome number | |
| Only proteins are synthesised in preparation for mitosis | |||
| Prophase | Condensation changes shape, not number | ||
| Metaphase | The chromosomes only come to lie at the equator | ||
| Anaphase | The centromeres split, so each chromatid is now counted as a chromosome - but not one nucleotide of DNA was made | ||
| Telophase | in the cell, per nucleus | in the cell, per nucleus | Two daughter nuclei form |
| After cytokinesis | per cell | per cell | The cell divides in two, so both counts are shared out |
Since the number of chromosomes in the parent and progeny cells is the same, mitosis is called equational division.
Through meiosis. The cell enters at , , because meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase, and meiosis has only a single cycle of DNA replication.
| Stage | N | C | Why |
|---|---|---|---|
| Prophase I | Pairing and crossing over change neither count | ||
| Metaphase I | The bivalents align on the equatorial plate | ||
| Anaphase I | in the cell, to each pole | in the cell, to each pole | Homologous chromosomes separate but the centromere does not split |
| Telophase I and cytokinesis - each cell of the dyad | The chromosome number is halved here, so meiosis I is reductional; each chromosome still has two chromatids | ||
| Interkinesis | There is no replication of DNA during interkinesis | ||
| Prophase II | The chromosomes only become compact again | ||
| Metaphase II | Chromosomes align at the equator | ||
| Anaphase II | in the cell, to each pole | in the cell, to each pole | Simultaneous splitting of the centromere of each chromosome |
| Telophase II and cytokinesis - each cell of the tetrad | Four haploid daughter cells; meiosis II is equational |
Three traps in this walk. First, anaphase of mitosis is but still - splitting a centromere does not make DNA. Second, each cell of the dyad is , , not , - only N is halved at meiosis I, and C is halved at each of the two cytokineses, so it takes both divisions to bring down to . Third, S phase doubles the DNA but not the chromosome number.
Question 43
Q. What is the significance of meiosis, and why does it matter for evolution?
Answer. Meiosis does two jobs, and a full-mark answer gives both.
Job one - conservation of the chromosome number. Meiosis is the mechanism by which conservation of the specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half. Read the paradox as the answer, not as a contradiction. Meiosis ensures the production of the haploid phase in the life cycle of sexually reproducing organisms, whereas fertilisation restores the diploid phase. The production of offspring by sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes, so when the two gametes fuse the chromosome number is restored to the value in the parent. If gametes were made by mitosis and stayed , the zygote would be and the next generation - the reduction at meiosis exactly cancels the doubling at fertilisation.
Job two - variation, and this is the link to evolution. Meiosis also increases the genetic variability in the population of organisms from one generation to the next, and variations are very important for the process of evolution.
Where that variability comes from is stated earlier in the chapter. Meiosis involves pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes, occurring at pachytene, at the recombination nodules, mediated by recombinase, and it leads to recombination of genetic material on the two chromosomes. The result is that the four cells of the tetrad are not copies of one another and are not copies of the parent cell. Set that against mitosis, which usually results in the production of diploid daughter cells with identical genetic complement and therefore contributes nothing to variation.
Why evolution needs it. Evolution works on differences between individuals. A population whose members were all genetically identical would have nothing for selection to act on. Meiosis, by recombining the genetic material of the two homologues in every gamete, sends a different combination into each gamete, and fertilisation then combines two such gametes. So the same process that halves the chromosome number is also the process that keeps supplying the population with new combinations, which is why the chapter closes on variations are very important for the process of evolution.
The Chapter-End Exercises
All sixteen exercises at the end of this chapter are answered somewhere in this chapter. The table shows where each one is worked out, so you can check your own attempt against a full answer. Fifteen of them are answered inside the teaching sections, and the one activity-type exercise is worked out below. Exercise 6 has four sub-parts, and each sub-part is answered in the section that covers its stage.
| Exercise | Answered as |
|---|---|
| 1. What is the average cell cycle span for a mammalian cell | Question 2 of the The Cell Cycle and Its Phases section |
| 2. Distinguish cytokinesis from karyokinesis | Question 5 of the The Cell Cycle and Its Phases section |
| 3. Describe the events taking place during interphase | Question 7 of the The Cell Cycle and Its Phases section |
| 4. What is (quiescent phase) of the cell cycle | Question 10 of the The Cell Cycle and Its Phases section |
| 5. Why is mitosis called equational division | Question 1 of the Mitosis - Prophase and Metaphase section |
| 6 (i). Chromosomes are moved to spindle equator | Question 14 of the Mitosis - Prophase and Metaphase section |
| 6 (ii). Centromere splits and chromatids separate | Question 5 of the Mitosis - Anaphase, Telophase and Cytokinesis section |
| 6 (iii). Pairing between homologous chromosomes takes place | Question 14 of the Meiosis - Key Features and Prophase I section |
| 6 (iv). Crossing over between homologous chromosomes takes place | Question 15 of the Meiosis - Key Features and Prophase I section |
| 7. Describe synapsis, bivalent and chiasmata, with a diagram | Question 11 of the Meiosis - Key Features and Prophase I section |
| 8. How does cytokinesis in plant cells differ from that in animal cells | Question 12 of the Mitosis - Anaphase, Telophase and Cytokinesis section |
| 9. Examples where the four daughter cells from meiosis are equal and unequal in size | Question 11 of the Meiosis II section |
| 10. Distinguish anaphase of mitosis from anaphase I of meiosis | Question 3 of the Meiosis I - Metaphase I to Telophase I section |
| 11. List the main differences between mitosis and meiosis | Question 2 of the Significance of Meiosis, and Mitosis Compared with Meiosis section |
| 12. What is the significance of meiosis | Question 1 of the Significance of Meiosis, and Mitosis Compared with Meiosis section |
| 13. Haploid insects and lower plants where cell-division occurs, and haploid cells in higher plants where cell-division does not occur | Question 44 below, since it is not covered anywhere else |
| 14. Can there be mitosis without DNA replication in S phase | Question 8 of the Chromosome Number and DNA Content Through the Cycle section |
| 15. Can there be DNA replication without cell division | Question 9 of the Chromosome Number and DNA Content Through the Cycle section |
| 16. Analyse how N and C change during every stage of the cell cycle | Question 10 of the Chromosome Number and DNA Content Through the Cycle section |
Question 44
Q. Discuss with your teacher about (i) haploid insects and lower plants where cell-division occurs, and (ii) some haploid cells in higher plants where cell-division does not occur. This is one of the chapter-end exercises.
Answer. The exercise is pointing at one rule and its exceptions. The rule is: in animals, mitotic cell division is only seen in the diploid somatic cells, and mitosis, the equational division, is usually restricted to the diploid cells only. The word doing the work is usually. Here is what this chapter and the earlier chapters of this book actually supply on either side of it.
(i) Haploid cells that do divide.
Insects. There are a few exceptions where haploid cells divide by mitosis - for example, male honey bees. This is the example the chapter itself gives, and it is the one to write down. A drone develops from an unfertilised egg, so it is haploid throughout its life, and yet its body cells go on dividing by mitosis to build and maintain the insect. The chapter states the same point more generally as well: in some lower plants and in some social insects, haploid cells also divide by mitosis - and the honey bee is a social insect.
Lower plants. Plants can show mitotic divisions in both haploid and diploid cells, and in some lower plants haploid cells also divide by mitosis. The haploid body that does this is the gametophyte. In a moss or a fern, the plant passes through a haploid gametophyte generation and a diploid sporophyte generation, and the haploid gametophyte grows by mitosis - it is built cell by cell from a single haploid spore. That is the whole reason alternation of generations is possible at all: a haploid cell must be able to divide mitotically to make a haploid multicellular body, and only then does meiosis appear at a different point in the cycle, in the sporophyte.
So the honest summary of part (i) is: the chapter states male honey bees as the animal exception, and states that in some lower plants and in some social insects haploid cells also divide by mitosis, and that plants show mitotic divisions in both haploid and diploid cells. The gametophyte of a moss or fern is the standard worked example of that statement.
(ii) Haploid cells in higher plants where cell division does not occur.
The clearest examples are the haploid gametes themselves - the male gamete and the egg. The production of offspring by sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes, and those gametes are the end products of meiosis: four haploid cells are formed at the end of meiosis II. A gamete does not go on to divide. Its next event is fertilisation, in which the two gametes fuse and the diploid phase is restored - meiosis ensures the production of the haploid phase in the life cycle of sexually reproducing organisms, whereas fertilisation restores the diploid phase. Division starts again only in the diploid zygote.
Be honest about the limits of what this chapter carries. This chapter states plainly that plants show mitotic divisions in both haploid and diploid cells, so it does not claim that haploid cells in higher plants generally fail to divide. What it does supply is the gamete, which is haploid and does not divide. The general principle the exercise is pointing at is the wider one: in a higher plant the haploid phase is very much reduced, and any haploid cell that has reached the end of its line - a gamete waiting to fuse - simply stops dividing. The detailed account of which haploid cells in a flowering plant divide and which do not belongs to the chapters on reproduction in flowering plants, not to this one, so do not invent detail here.
The one-line answer if the question is set for two marks. (i) Male honey bees, which are haploid throughout and whose cells divide by mitosis, and the haploid gametophytes of lower plants such as mosses and ferns, which grow by mitosis. (ii) The haploid gametes of higher plants - the male gamete and the egg - which are the end products of meiosis and do not divide further, but fuse at fertilisation to restore the diploid phase.