Model the processes involved in cell replication, including but not limited to: mitosis and meiosis, the role of meiosis and gamete formation in maintaining the chromosome number across generations

What this dot point is asking

NESA wants you to model meiosis (the cell division that produces gametes), distinguish it from mitosis, and explain how the alternation between meiosis (halving) and fertilisation (doubling) maintains the chromosome number across generations.

The answer

Meiosis is the cell division that produces gametes (sperm and eggs). It involves two consecutive divisions, Meiosis I and Meiosis II, from a single diploid (2n) parent cell. The result is four haploid (n) daughter cells, each genetically unique.

Meiosis I (reductive division)

Homologous chromosomes are separated.

1. Prophase I. Chromosomes condense. Homologous pairs (one from each parent) align and undergo crossing over at the chiasmata, exchanging segments of DNA.
2. Metaphase I. Homologous pairs line up at the equator. Independent assortment randomises which member of each pair goes to which pole.
3. Anaphase I. Homologous chromosomes are pulled to opposite poles. The chromosome number is halved here.
4. Telophase I and cytokinesis. Two haploid daughter cells form, each with one chromosome from each homologous pair.

Meiosis II (equational division)

Sister chromatids are separated, similar to mitosis but with haploid starting cells.

1. Prophase II. Chromosomes recondense.
2. Metaphase II. Chromosomes line up at the equator.
3. Anaphase II. Sister chromatids are pulled to opposite poles.
4. Telophase II. Four haploid daughter cells form, each genetically unique.

Sources of genetic variation in meiosis

1. Crossing over (Prophase I). Homologous chromosomes exchange segments, recombining maternal and paternal alleles.
2. Independent assortment (Metaphase I). Each homologous pair sorts independently. For humans with 23 pairs, this produces $2^{23}$ possible gamete combinations.
3. Random fertilisation. Any of the $\sim 2^{23}$ possible egg combinations can fuse with any of the $\sim 2^{23}$ possible sperm combinations, producing roughly $2^{46}$ possible offspring per pair of human parents.

How chromosome number is maintained

In humans, somatic cells are diploid (2n = 46). Gametes are haploid (n = 23). At fertilisation, the haploid sperm and haploid egg fuse to form a diploid zygote (2n = 46).

Meiosis halves the chromosome number in gamete formation. Fertilisation restores it. The alternation maintains the species-specific chromosome number across generations.

Meiosis vs mitosis comparison

Common traps

Confusing Meiosis I and II. Meiosis I separates homologous pairs (reductive). Meiosis II separates sister chromatids (similar to mitosis). Both happen in sequence from the same starting cell.

Forgetting independent assortment as a source of variation. Many students mention crossing over but skip independent assortment. Top responses cover both.

Mixing up haploid and diploid. Haploid = n = 23 (gametes). Diploid = 2n = 46 (somatic cells in humans). Get this wrong and the whole answer collapses.

In one sentence

Meiosis is two consecutive divisions from a diploid parent cell producing four genetically unique haploid gametes via crossing over and independent assortment, and the halving-then-fertilisation cycle maintains the species-specific chromosome number across generations.