Core Formulas
| Formula | Application |
|---|---|
| DNA after S phase = 2 × (pre-S DNA) | S phase doubles DNA content |
| Chromosome number after S phase = same | Sister chromatids joined = 1 chromosome |
| Chromatids after S phase = Chromosomes × 2 | Each chromosome → 2 sister chromatids |
| After meiosis I: n = ; DNA = = 2C | Homologue separation halves both |
| After meiosis II: n = n; DNA = = 1C | Chromatid separation halves DNA |
| Gametes from independent assortment = 2^n | n = number of homologous pairs |
| Recombinant gametes per crossover = 50% | 2 of 4 chromatids are recombinant |
Quick Reference Table: DNA Content at Every Stage
| Cell / Stage | DNA Content | Chromosome Number | Chromatids |
|---|---|---|---|
| G1 (pre-S) | 2C | 2n | 2n (1 per chr) |
| G2 / Early M | 4C | 2n | 4n (2 per chr) |
| Mid-Anaphase mitosis | 4C | 4n (transient) | 4n (1 per chr) |
| Daughter (post-mitosis) | 2C | 2n | 2n |
| Primary spermatocyte (G2) | 4C | 2n | 4n |
| After meiosis I | 2C | n | 2n |
| After meiosis II (gamete) | 1C | n | n |
NEET Trap: 4C DNA ≠ 4n Chromosomes
After S phase: DNA = 4C but chromosomes = 2n (not 4n!). Reason: Sister chromatids share one centromere → counted as ONE chromosome.