Cell cycle and meiosis: study guide

Key ideas

Cells grow, copy their DNA and divide in a regulated cycle. Mitosis makes two identical diploid cells for growth and repair, and meiosis makes four different haploid cells for sexual reproduction.

• Interphase is G₁, S (DNA replication) and G₂. Mitosis follows in four stages, prophase, metaphase, anaphase and telophase, and then the cytoplasm divides.
• Mitosis keeps the chromosome number: 2n → 2n. Meiosis has two divisions and halves it: 2n → n, with crossing over and independent assortment giving variation.
• DNA doubles in S phase, but the chromosome number only doubles in the sense of chromatids. The number of chromosomes halves in meiosis I.
• With n chromosome pairs, independent assortment alone gives 2ⁿ different gametes.

Key equations

• mitotic index = cells in mitosis ÷ total cells × 100
• time in phase = (cells in phase ÷ total) × cycle time
• gamete combinations = 2ⁿ

Worked example

Of 200 cells counted, 30 are in mitosis, so the mitotic index is 30 ÷ 200 = 15 %. If the cell cycle lasts 24 h, mitosis takes 0.15 × 24 = 3.6 h. In humans (2n = 46) a cell in G₂ has 46 chromosomes and 92 chromatids, and independent assortment alone allows 2²³ ≈ 8.4 million different gametes.

Common mistakes

• Saying the chromosome number doubles in S phase. The DNA content doubles, but each chromosome now has two chromatids.
• Confusing the stages: sister chromatids separate in anaphase of mitosis and in anaphase II of meiosis, but homologous chromosomes separate in anaphase I.
• Calling the products of meiosis genetically identical. Crossing over and independent assortment make them different.

Practice

Test yourself with practice questions on this topic: Cell cycle and meiosis