the production of haploid gametes from diploid cells by meiosis, including the significance of crossing over of chromatids in prophase I and independent assortment of homologous chromosomes in metaphase I for the generation of genetic diversity

What this dot point is asking

VCAA wants you to describe the two divisions of meiosis that produce haploid gametes from diploid parent cells, and explain the two main sources of genetic diversity in those gametes: crossing over in prophase I and independent assortment in metaphase I.

The answer

Why meiosis exists

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.

In humans, a diploid germ cell (2n = 46) undergoes meiosis to produce four haploid gametes (n = 23). When sperm meets egg, the zygote is again 2n = 46.

Meiosis also generates genetic variation, the raw material of evolution.

The two meiotic divisions

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

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

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

• Prophase I. Chromosomes condense. Homologous chromosomes pair up (synapsis) to form bivalents (also called tetrads, because each contains 4 chromatids). Crossing over occurs: non-sister chromatids exchange segments at points called chiasmata.
• Metaphase I. Bivalents line up at the equator. Each bivalent is oriented independently: maternal homologue may face either pole. This is independent assortment.
• Anaphase I. Spindle fibres pull each homologue to opposite poles. Sister chromatids remain joined at the centromere.
• Telophase I and cytokinesis. Two haploid daughter cells form. Each has n chromosomes, each chromosome still consisting of two sister chromatids.

Meiosis II (equational division, like mitosis). Separates sister chromatids. Does not change chromosome number.

• Prophase II. Chromosomes recondense. New spindles form.
• Metaphase II. Chromosomes line up at the equator of each cell.
• Anaphase II. Centromeres divide; sister chromatids are pulled to opposite poles.
• Telophase II and cytokinesis. Four haploid daughter cells in total, each with n chromosomes, each chromosome now a single chromatid.

End result: four genetically unique haploid gametes from one diploid parent.

Crossing over (prophase I)

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

The consequence: each chromatid ends up with a mosaic of maternal and paternal DNA. Genes that were linked on one parental chromosome can be recombined with alleles from the other parent.

Crossing over creates new allele combinations on each chromatid. Without crossing over, only two combinations would exist (pure maternal or pure paternal) for any whole chromosome.

The frequency of crossing over between two genes is roughly proportional to the distance between them on the chromosome. This is the basis for genetic mapping (the Unit 2 linked/unlinked genes dot point).

Independent assortment (metaphase I)

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.

For one pair, two possible orientations gives 2 combinations. For two pairs: 2 × 2 = 4. For three pairs: 8.

For humans (23 pairs): 2 to the power of 23 = 8,388,608 possible combinations of maternal and paternal chromosomes per gamete from independent assortment alone.

Genetic diversity: the three sources

1. Crossing over in prophase I (within chromosomes).
2. Independent assortment at metaphase I (between chromosomes).
3. Random fertilisation of any one of millions of possible sperm with any one of hundreds of possible eggs.

For humans: 8.4 million × 8.4 million × crossing over variation = effectively infinite combinations. Every human (except identical twins) is genetically unique.

This variation is the raw material on which natural selection acts.

Meiosis vs mitosis

Errors in meiosis

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).

The risk rises with maternal age because eggs are arrested in prophase I from before birth until ovulation, accumulating damage over decades.

Worked example

A human cell with diploid number 2n = 46 enters meiosis. After S phase, each of the 46 chromosomes consists of two sister chromatids (92 chromatids total). In prophase I, crossing over swaps segments between non-sister chromatids of each homologous pair. In metaphase I, the 23 homologous pairs line up independently. In anaphase I, homologues separate; each daughter cell receives 23 chromosomes (each still two chromatids). In meiosis II, sister chromatids separate; the four final cells each have 23 chromosomes, each a single chromatid. Each gamete is genetically unique because of crossing over and independent assortment. Fertilisation by any one of the partner's hundreds of millions of unique sperm produces a zygote that is one in trillions.

Common traps

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

Saying meiosis halves DNA in both divisions. 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.

Confusing chromosomes and chromatids. 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.

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

Calling crossing over "swapping chromosomes". It swaps segments of chromatids, not whole chromosomes.

Saying meiosis makes "two" gametes. 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.)

In one sentence

Meiosis is two consecutive nuclear divisions of a diploid cell that produces four haploid, genetically unique gametes, with diversity generated by crossing over of non-sister chromatids in prophase I and independent assortment of homologous pairs at metaphase I (giving 2 to the power of 23 combinations in humans), and reduction of chromosome number happening in meiosis I (homologues separate) while sister chromatids separate in meiosis II.