Cell division & sex determination

The cell cycle has two main phases:

1. Interphase: the cell grows, carries out its normal functions, replicates its chromosomes (each chromosome is copied so the cell has two identical sets), and respires to build up energy for division.
2. Mitosis + cytokinesis: the nucleus divides to produce two genetically identical diploid daughter cells, then the cytoplasm divides (cytokinesis).

Uses: growth, repair and replacement of cells, and asexual reproduction.

Cancer arises when cell-cycle checkpoints fail. Normally the cycle pauses to check DNA accuracy; cells with unrepaired errors are stopped or destroyed. If a checkpoint is missed, cells divide uncontrollably, forming a tumour.

Meiosis includes interphase then two divisions, producing four daughter cells, each with a single copy of each chromosome (haploid gametes). At fertilisation, two gametes fuse so the zygote is diploid again. Meiosis produces genetically different gametes, giving variation.

Asexual reproduction has one parent; offspring are genetically identical (clones) unless mutations occur. Sexual reproduction has two parents; offspring are genetically different, increasing variation. In most mammals (including humans) females are XX and males are XY; analyse genetic diagrams to find the sex and ratio of offspring.

Humans have 46 chromosomes in 23 pairs: 22 pairs of autosomes and 1 pair of sex chromosomes (XX = female, XY = male). Body cells are diploid (46 chromosomes); gametes are haploid (23). The Y chromosome is smaller than X and carries fewer genes, including those that direct male development.

Binary fission is how bacteria reproduce asexually: the circular DNA replicates → the cell elongates → a new cell wall forms across the middle → two identical daughter cells separate. It is much faster than mitosis.

• Mixing up the outcomes: mitosis → 2 identical diploid cells; meiosis → 4 different haploid cells.
• Saying asexual reproduction gives variation (it gives clones).