Summary and Exam Tips

Chapter Summary

1. The Cell Cycle:

• Interphase (95% duration):
  • G1: Growth, metabolically active.
  • S: DNA replicates ($2C \rightarrow 4C$), Centriole duplicates. Chromosome number ($2n$) stays constant.
  • G2: Protein synthesis, preparation for mitosis.
• M Phase (<5% duration): Karyokinesis + Cytokinesis.

2. Mitosis (Equational Division):

• Prophase: Condensation, spindle forms, organelles disappear.
• Metaphase: Chromosomes align at equator (Metaphase plate).
• Anaphase: Centromeres split, chromatids move to poles.
• Telophase: Decondensation, nuclear envelope reforms.
• Cytokinesis: Furrow (Animals) vs. Cell Plate (Plants).

3. Meiosis (Reductional Division):

• Meiosis I: Reduces chromosome number ($2n \rightarrow n$).
  • Prophase I: Leptotene, Zygotene (Synapsis), Pachytene (Crossing Over), Diplotene (Chiasmata), Diakinesis.
  • Anaphase I: Homologous chromosomes separate; Centromere does NOT split.
• Meiosis II: Resembles Mitosis. Centromeres split.

High-Yield Exam Tips (NEET Focus)

1. The "S-Phase" Trap:

• Question: If a cell has 14 chromosomes and 2C DNA at G1, what does it have after S phase?
• Answer: 14 Chromosomes (Unchanged) and 4C DNA (Doubled). Don't get tricked into doubling the chromosomes!

2. Anaphase vs. Anaphase I:

• Splitting: If the question says "Centromere splits", it is Mitosis or Anaphase II.
• No Splitting: If the question says "Homologous chromosomes separate", it is Anaphase I.

3. Prophase I Sequence Mnemonic: Remember "LZPDD": Lazy Zebras Ponder Dull Days.

• Leptotene (Condense)
• Zygotene (Zip/Synapsis)
• Pachytene (Pack/Crossing Over)
• Diplotene (Dissolve/Chiasmata)
• Diakinesis (Depart/Terminalisation)

4. Calculation Hacks:

• Generations: To find the number of generations ($n$) for $X$ cells: $2^n = X$.
• Meiosis Count: To produce $N$ seeds: You need $N + (N/4)$ meiotic divisions. (e.g., 100 seeds = 100 egg meiosis + 25 pollen meiosis = 125).