Quick questions on Meiosis, crossing over and genetic diversity: VCE Biology Unit 2

Sexual reproduction requires fusion of two gametes to make a zygote. To keep the chromosome number constant across generations, the gametes must contain half the normal chromosome number. Meiosis is the cell division that produces these haploid gametes from diploid cells.

Meiosis consists of two consecutive divisions after one round of DNA replication. It produces four haploid daughter cells from one diploid parent cell.

During prophase I, homologous chromosomes pair up tightly (synapsis). Non-sister chromatids cross each other at chiasmata and exchange DNA segments.

At metaphase I, each homologous pair lines up at the equator independently of every other pair. The maternal homologue may face the "top" pole or the "bottom" pole, with equal probability, and each pair makes that choice independently.

1. Crossing over in prophase I (within chromosomes). 2. Independent assortment at metaphase I (between chromosomes).

Non-disjunction: failure of homologous chromosomes (meiosis I) or sister chromatids (meiosis II) to separate. Produces gametes with the wrong chromosome number (n+1 or n-1). After fertilisation, this leads to aneuploidies like trisomy 21 (Down syndrome), XO (Turner) or XXY (Klinefelter).

DNA is replicated in S phase. Each chromosome now consists of two identical sister chromatids joined at the centromere.

Separates homologous chromosomes (one homologue to each daughter cell). Halves the chromosome number from 2n to n.

Separates sister chromatids. Does not change chromosome number.

Meiosis I separates homologues (reduction division). Meiosis II separates sister chromatids (equational, like mitosis).

Chromosome number is halved in meiosis I. DNA content per cell halves in both meiosis I and meiosis II, but the reduction in chromosome number happens only at meiosis I.

Before S phase: each chromosome = one chromatid. After S phase: each chromosome = two sister chromatids. After anaphase I or anaphase II: each chromatid becomes its own chromosome.

A common error in 3-mark "diversity" questions is to mention only independent assortment.

It swaps segments of chromatids, not whole chromosomes.

Meiosis makes four haploid cells. (In females, three are usually polar bodies and only one becomes the egg, but the cell-division process makes four.)