NEET-Pattern Practice Questions

How This Chapter Is Asked in NEET

Cell Cycle and Cell Division is almost pure stage-identification. There is nothing to calculate and nothing to derive. A question gives you an event and wants a stage, or gives you a stage and wants an event, and the whole mark turns on whether you learnt the wording exactly.

Every question in this chapter is one of four shapes:

1. Name the stage from an event. "Centromeres split and chromatids separate" - anaphase. "Pairing of homologous chromosomes" - zygotene.
2. State what happens at a named stage. "What happens in S phase" - DNA replication, and in animal cells the centriole duplicates in the cytoplasm.
3. Give the chromosome number or the DNA content at a stage. "Each cell of the dyad" - $n$ and $2C$.
4. Spot the odd one out or the incorrect statement. Four statements about meiosis, three of them lifted word for word from the chapter and one quietly altered.

The traps repeat, and in this chapter they are always the same ones:

• Interphase called the resting phase. It is called the resting phase, but it lasts more than 95 per cent of the duration of the cell cycle and is the busiest part of it - cell growth and DNA replication both happen there, while cell division proper lasts for only about an hour of the 24 hour human cell cycle. Any option that calls interphase inactive is wrong.
• Which phase cells exit to reach $\mathrm{G_0}$. Cells that do not divide further exit the $\mathrm{G_1}$ phase, not $\mathrm{G_2}$ and not M phase.
• What a $\mathrm{G_0}$ cell is doing. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so. Inactive with respect to division is not the same as metabolically inactive, and the wrong option always merges the two.
• S phase doubles the DNA but not the chromosome number. The amount of DNA per cell doubles, $2C$ to $4C$, and there is no increase in the chromosome number - a $2n$ cell is still $2n$ after S phase. Each chromosome simply gains a second chromatid.
• When the centriole duplicates. In animal cells, during the S phase, DNA replication begins in the nucleus and the centriole duplicates in the cytoplasm. The distractor says prophase, because prophase is when the centrosome begins to move towards opposite poles - moving is not duplicating.
• Where chromosome morphology is best studied. Metaphase. By then condensation of chromosomes is completed and they can be observed clearly under the microscope. Students guess prophase and lose the mark.
• Kinetochore against centromere. Kinetochores are small disc-shaped structures at the surface of the centromeres, and they serve as the sites of attachment of spindle fibres to the chromosomes. The spindle attaches to the kinetochore, not to the centromere itself.
• Anaphase of mitosis against anaphase I of meiosis. This is the single most examined contrast in the chapter. In anaphase of mitosis, each chromosome is split simultaneously, the centromeres split and the chromatids separate. In anaphase I, the homologous chromosomes separate, while sister chromatids remain associated at their centromeres - the centromere does not split. One is equational, the other reductional.
• The five sub-stages of prophase I, in order, and what is in each. Leptotene - the chromosomes become gradually visible under the light microscope. Zygotene - chromosomes start pairing, and this process of association is called synapsis, accompanied by the synaptonemal complex. Pachytene - the four chromatids of each bivalent appear as tetrads, recombination nodules appear, and crossing over occurs between non-sister chromatids. Diplotene - the synaptonemal complex dissolves and chiasmata appear. Diakinesis - terminalisation of chiasmata. Learn synapsis at zygotene and crossing over at pachytene as a pair; swapping them is the commonest error in the chapter.
• Chiasmata appear at diplotene and terminalise at diakinesis. Two stages, one term - and the paper sets them against each other.
• No DNA replication during interkinesis. The stage between the two meiotic divisions is called interkinesis, and it is generally short lived, and there is no replication of DNA during it. Any option giving interkinesis an S phase is wrong.
• Reductional against equational. Meiosis I is reductional, because each pole receives half the chromosome number of the parent cell. Meiosis II is equational, because meiosis II resembles a normal mitosis and $n$ stays $n$.
• Dyad against tetrad. Telophase I gives a dyad of cells - two cells. Telophase II gives a tetrad of cells - four haploid daughter cells. Options swap the two words freely.
• The other meaning of tetrad. At pachytene, the four chromatids of each bivalent chromosome become distinct and clearly appear as tetrads, and the complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. Same word, two meanings - read the stem to see whether it is talking about chromatids or cells.
• Plant cytokinesis against animal cytokinesis. In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls, beginning with a cell-plate that represents the middle lamella. In an animal cell a furrow appears in the plasma membrane, deepens and ultimately joins in the centre. Centre outward in plants, outside inward in animals.

One habit pays more than any other here. In a "which is NOT" question, the wrong option is almost never invented - it is a real fact borrowed from a neighbouring stage. Centromere splitting offered for anaphase I, crossing over offered for prophase II, the cell-plate offered for an animal cell. Ask "which stage does this actually belong to" rather than "have I heard of this".

Before You Start

Give yourself 45 minutes for 45 questions and look nothing up. Mark every question where you were choosing between two options rather than certain - those marked questions, not only the wrong ones, are your revision list.

The facts most likely to decide your score, by topic:

The cell cycle. 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. The distribution of replicated chromosomes to daughter nuclei is under genetic control. Human cells in culture divide once in approximately every 24 hours; yeast can progress through the cell cycle in about 90 minutes. Two basic phases - interphase and M phase. M phase is the actual cell division; interphase is the phase between two successive M phases. Cell division proper lasts for only about an hour; interphase lasts more than 95 per cent of the duration of the cell cycle. M phase starts with karyokinesis, the separation of daughter chromosomes, and usually ends with cytokinesis, the division of the cytoplasm.

Interphase. $\mathrm{G_1}$ - the interval between mitosis and initiation of DNA replication; the cell is metabolically active and continuously grows but does not replicate its DNA. S phase - DNA synthesis or replication; the amount of DNA per cell doubles from $2C$ to $4C$, with no increase in the chromosome number, so $2n$ stays $2n$. In animal cells, during S phase, DNA replication begins in the nucleus and the centriole duplicates in the cytoplasm. $\mathrm{G_2}$ - proteins are synthesised in preparation for mitosis while cell growth continues. $\mathrm{G_0}$, the quiescent stage - cells that do not divide further exit the $\mathrm{G_1}$ phase; they remain metabolically active but no longer proliferate unless called on to do so; heart cells are the example. In animals mitotic cell division is only seen in the diploid somatic cells, with a few exceptions such as male honey bees; plants show mitosis in both haploid and diploid cells.

Prophase and metaphase of mitosis. M phase is the most dramatic period of the cell cycle, involving a major reorganisation of virtually all components of the cell. Mitosis is called equational division because the number of chromosomes in the parent and progeny cells is the same. Karyokinesis has four stages - prophase, metaphase, anaphase, telophase - and division is a progressive process, so very clear-cut lines cannot be drawn between them. Prophase - chromosomal material condenses to form compact mitotic chromosomes, each of two chromatids attached at the centromere; the centrosome, duplicated during interphase, begins to move towards opposite poles; each centrosome radiates microtubules called asters; the two asters together with spindle fibres form the mitotic apparatus. At the end of prophase, cells do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope. Metaphase begins with the complete disintegration of the nuclear envelope, so the chromosomes are spread through the cytoplasm. Metaphase is the stage at which the morphology of chromosomes is most easily studied. Kinetochores are small disc-shaped structures at the surface of the centromeres and are the sites of attachment of spindle fibres. One chromatid is connected by its kinetochore to spindle fibres from one pole and its sister chromatid to fibres from the opposite pole. The plane of alignment is the metaphase plate.

Anaphase, telophase and cytokinesis. 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, begin their migration towards the two opposite poles. The centromere remains directed towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind. Telophase - the chromosomes decondense and lose their individuality; a nuclear envelope develops around the chromosome clusters at each pole; nucleolus, golgi complex and ER reform. Cytokinesis - in an animal cell a furrow appears in the plasma membrane, deepens and ultimately joins in the centre; in a plant cell, enclosed by a relatively inextensible cell wall, wall formation starts in the centre and grows outward to meet the existing lateral walls, beginning with a cell-plate that represents the middle lamella. Mitochondria and plastids get distributed between the two daughter cells. Karyokinesis without cytokinesis gives a syncytium - the liquid endosperm in coconut.

Significance of mitosis. Mitosis is usually restricted to the diploid cells only, but in some lower plants and in some social insects haploid cells also divide by mitosis. It results in diploid daughter cells with identical genetic complement. The growth of multicellular organisms is due to mitosis. Cell growth disturbs the ratio between nucleus and cytoplasm, so the cell must 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, give plants continuous growth throughout their life.

Meiosis, key features and prophase I. 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. Prophase I is typically longer and more complex than prophase of mitosis and has five sub-stages - leptotene, zygotene, pachytene, diplotene and diakinesis. Leptotene - chromosomes become gradually visible under the light microscope and compaction continues. Zygotene - chromosomes start pairing, a process called synapsis, accompanied by the synaptonemal complex; the complex formed by a pair of synapsed homologous chromosomes is a bivalent or a tetrad. Pachytene - the four chromatids of each bivalent appear as tetrads; recombination nodules appear, the sites at which crossing over occurs between non-sister chromatids; crossing over is enzyme-mediated and the enzyme is recombinase; recombination is completed by the end of pachytene. Diplotene - the synaptonemal complex dissolves and the recombined homologues separate except at the sites of crossovers; these X-shaped structures are chiasmata; in oocytes of some vertebrates diplotene can last for months or years. Diakinesis - terminalisation of chiasmata; chromosomes fully condensed; meiotic spindle assembled; nucleolus disappears and nuclear envelope breaks down; transition to metaphase.

Meiosis I, 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; each pole receives half the chromosome number of the parent cell. Telophase I - the nuclear membrane and nucleolus reappear, cytokinesis follows, and this is called a dyad of cells; the chromosomes undergo some dispersion but do not reach the extremely extended state of the interphase nucleus. Interkinesis - the stage between the two meiotic divisions, generally short lived, with no replication of DNA; it is followed by prophase II.

Meiosis II. Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated, and in contrast to meiosis I it resembles a normal mitosis. Prophase II - the nuclear membrane disappears by the end of prophase II and the chromosomes again become compact; there is no synapsis, no crossing over and no chiasmata. Metaphase II - the chromosomes align at the equator and the microtubules from opposite poles get attached to the kinetochores of sister chromatids. Anaphase II - the simultaneous splitting of the centromere of each chromosome, which was holding the sister chromatids together, allows them to move toward opposite poles by shortening of microtubules attached to kinetochores. Telophase II - the two groups of chromosomes get enclosed by a nuclear envelope, cytokinesis follows, and a tetrad of cells is formed, that is four haploid daughter cells.

N and C. $\mathrm{G_1}$ is $2n$, $2C$. S phase takes the DNA to $4C$ with the number unchanged at $2n$. $\mathrm{G_2}$, prophase and metaphase are $2n$, $4C$. Anaphase of mitosis is $4n$ in the cell with the DNA still $4C$. After cytokinesis each daughter cell is $2n$, $2C$. Meiosis starts at $2n$, $4C$. Prophase I, metaphase I and anaphase I are all $2n$, $4C$ in the cell. Each cell of the dyad is $n$, $2C$ - the number is halved here. Interkinesis, prophase II and metaphase II are $n$, $2C$. Anaphase II is $2n$ in the cell with $2C$ of DNA. Each cell of the tetrad is $n$, $C$. 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.

Significance of meiosis and the comparison. 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 ensures the production of the haploid phase, whereas fertilisation restores the diploid phase. 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 - one division, two daughter cells, $2n$, equational, no pairing, no crossing over, identical genetic complement, for growth and repair. Meiosis - two divisions on a single cycle of DNA replication, four daughter cells, $n$, reductional overall with meiosis II equational, pairing at zygotene, crossing over at pachytene, chiasmata from diplotene, genetically variable cells, for gamete formation and variation.