The cell cycle and mitosis, the significance of meiosis, and how mitosis and meiosis differ.

What this dot point is asking

WJEC wants you to describe the cell cycle and the stages of mitosis, explain the significance of mitosis and meiosis, state how meiosis produces genetic variation, and set out how meiosis differs from mitosis. You should be able to calculate the number of gamete combinations produced by independent assortment.

The cell cycle and mitosis

The cell cycle is a long interphase followed by a short mitotic phase. Interphase has three stages: G1 (the cell grows and makes organelles and proteins), S phase (DNA is replicated so each chromosome becomes two identical sister chromatids), and G2 (further growth and checking of the replicated DNA). DNA replicates during interphase, not during mitosis, which is why each chromosome already consists of two chromatids when mitosis begins.

Mitosis produces two genetically identical diploid daughter cells. It is the basis of growth, tissue repair and replacement, and asexual reproduction. Checkpoints during the cycle ensure DNA is undamaged and fully replicated before division; failure of these controls can lead to uncontrolled division and cancer.

Meiosis and variation

In meiosis I, homologous chromosomes (one from each parent, carrying the same genes) pair up. Two processes create variation. Crossing over in prophase I: homologous chromosomes form bivalents and exchange sections of chromatids at points called chiasmata, producing recombinant chromosomes with new allele combinations. Independent assortment in metaphase I: each homologous pair lines up at the equator independently of the others, so each gamete receives a random mix of maternal and paternal chromosomes. The number of combinations from independent assortment alone is $2^n$, where $n$ is the haploid number. Together with the random fertilisation of one gamete by another, these processes make each gamete and each offspring genetically unique.

Mitosis versus meiosis

Mitosis is one division giving two diploid, genetically identical cells; meiosis is two divisions giving four haploid, genetically varied cells. Homologous chromosomes pair and cross over only in meiosis, and the chromosome number is halved in meiosis I but not in mitosis.

Examples in context

Example 1. Skin and gut repair by mitosis. The lining of the small intestine is replaced every few days by mitosis of stem cells in the crypts. Because mitosis produces genetically identical cells, the new lining cells are exact copies, preserving the tissue's function. Rapidly dividing tissues like these are also why chemotherapy, which targets dividing cells, causes side effects such as hair loss and gut damage.

Example 2. Human gamete variation. Humans have $2n = 46$, so $n = 23$. Independent assortment alone gives $2^{23}$, over 8 million, possible chromosome combinations per gamete. Add crossing over and the random fusion of any sperm with any egg, and the number of genetically distinct offspring a couple could produce is astronomically large, which is why siblings differ.

Try this

Q1. State the products of mitosis in terms of number and chromosome content. [1 mark]

• Cue. Two genetically identical diploid daughter cells.

Q2. Explain why meiosis is essential for sexual reproduction. [2 marks]

• Cue. It halves the chromosome number to make haploid gametes, so fertilisation restores the diploid number; it also generates variation.

Q3. A species has a haploid number of $n = 5$. Calculate the number of gamete combinations from independent assortment. [1 mark]

• Cue. $2^5 = 32$.