Meiosis & genetic diversity

Diploid, haploid and homologous pairs

Your chromosomes come in matching pairs: one of each pair came from your mother and one from your father. The two members of a pair are called homologous chromosomes (or homologues). They carry the same genes in the same order, but not necessarily the same alleles.

A cell with both sets is diploid, written 2n. In humans 2n = 46, so n = 23. A cell with just one set, one chromosome from each pair, is haploid (n). Meiosis turns one diploid cell into four haploid cells. In animals these become eggs or sperm; in plants and fungi they become spores.

Before meiosis: copying the DNA

Just like before mitosis, the cell copies its DNA during the S phase of interphase. Each chromosome now consists of two identical sister chromatids, held together along their length by ring-shaped proteins called cohesins, most tightly at the centromere.

To track DNA content, biologists use the C value: 1C is the amount of DNA in one haploid set. A diploid cell before S phase is 2C; after S phase it is 4C.

Meiosis I: separating the homologues

The first division is the one that makes meiosis special. Its job is to pull the two homologues of each pair into different cells.

1. 1Prophase I. Chromosomes condense, and each homologue finds its partner and zips up alongside it along its whole length. This pairing is called synapsis, and it's held in place by a protein scaffold called the synaptonemal complex. A paired set of homologues (four chromatids in all) is called a bivalent or tetrad.
2. 2Still in prophase I, crossing over happens: non-sister chromatids of the two homologues break and rejoin, swapping matching stretches of DNA. Later the synaptonemal complex comes apart, but the homologues stay linked at the crossover points, which show up as X-shaped chiasmata (singular chiasma). In most organisms, each pair needs at least one chiasma to separate properly.
3. 3Metaphase I. The bivalents line up at the middle of the cell, as pairs. Spindle fibres from one pole attach to one homologue, and fibres from the other pole attach to its partner. Which way each pair faces is random and independent of the other pairs.
4. 4Anaphase I. The cohesin along the chromosome arms is broken down, releasing the chiasmata, and the homologues are pulled to opposite poles. The cohesin at the centromeres is protected by a protein called shugoshin (Japanese for "guardian spirit"), so sister chromatids stay together.
5. 5Telophase I and cytokinesis. Two cells form. Each has 23 chromosomes, but every chromosome still has two chromatids.

Meiosis II: separating the sister chromatids

Meiosis II looks a lot like mitosis, just in haploid cells. There's no DNA copying in between.

1. 1Prophase II and metaphase II. A new spindle forms in each cell, and the 23 chromosomes line up individually at the middle, with sister chromatids attached to opposite poles.
2. 2Anaphase II. The remaining centromeric cohesin is finally cut, and the sister chromatids are pulled apart. Each chromatid is now counted as a chromosome in its own right.
3. 3Telophase II and cytokinesis. You end with four haploid cells, each with 23 single-chromatid chromosomes and 1C of DNA.

In human females the divisions are lopsided: nearly all the cytoplasm goes to one cell, which becomes the egg, while the small leftover cells (polar bodies) break down, so one meiosis gives just one egg. In males all four cells become sperm.

Keeping count through meiosis

Here's a human cell tracked from start to finish. Watch how chromosome number and DNA content change at different moments.

• G1 (before copying): 46 chromosomes, 46 chromatids, 2C.
• After S phase, through prophase I and metaphase I: 46 chromosomes, 92 chromatids, 4C.
• Each cell after meiosis I (and in metaphase II): 23 chromosomes, 46 chromatids, 2C.
• Each cell after meiosis II: 23 chromosomes, 23 chromatids, 1C.

So the chromosome number halves once, in meiosis I. The DNA content halves twice, once in each division, after having doubled in S phase.

Mitosis versus meiosis

• Purpose: mitosis makes identical cells for growth and repair; meiosis makes haploid gametes or spores.
• Divisions: mitosis has one; meiosis has two, after a single round of DNA copying.
• Pairing: homologues never pair in mitosis; in meiosis I they synapse and cross over.
• Metaphase line-up: in mitosis chromosomes line up individually; in metaphase I they line up in pairs.
• What separates: mitosis and meiosis II separate sister chromatids; meiosis I separates homologues.
• Result: mitosis gives two cells genetically identical to the parent (2n → 2n); meiosis gives four cells that differ from the parent and from each other (2n → n).

Three sources of variation

Brothers and sisters (other than identical twins) differ because meiosis shuffles genes in three independent ways.

Crossing over makes recombinant chromatids that mix maternal and paternal alleles on the same chromosome. After crossing over, even sister chromatids are no longer identical, so all four products of one meiosis can differ.

Independent assortment comes from the random way each pair faces in metaphase I. With n pairs, there are 2ⁿ possible combinations of maternal and paternal chromosomes in a gamete. For humans that's 2²³, about 8.4 million, before crossing over is even counted.

Random fertilisation multiplies it again: any one of roughly 8.4 million kinds of egg can meet any one of roughly 8.4 million kinds of sperm, giving about 70 trillion combinations. Crossing over pushes the real number far higher still.

Key terms

Meiosis

Two divisions after one round of DNA copying, turning one diploid cell into four haploid cells.

Diploid / haploid

Two sets of chromosomes (2n) / one set (n).

Homologous chromosomes

A matching pair, one from each parent, carrying the same genes in the same order.

Sister chromatids

The two identical copies of a chromosome made in S phase, joined by cohesin.

Synapsis / synaptonemal complex

The close pairing of homologues in prophase I / the protein scaffold that holds them together.

Crossing over / chiasma

Exchange of DNA between non-sister chromatids / the visible point where homologues stay joined afterwards.

Independent assortment

Each homologous pair orients randomly in metaphase I, giving 2ⁿ chromosome combinations.

C value

DNA content measured in haploid genomes: 1C is one set's worth of DNA.

Check yourself

Try answering in your head before you open each answer.

• 1.A cell from an organism with 2n = 6 has 3 chromosomes, each made of two chromatids, lined up singly at the middle of the cell. Which stage is it in, and how do you know it isn't mitosis?Show answerHide

  Metaphase II. A cell in mitotic metaphase of this organism would have all 6 chromosomes. Having only 3 duplicated chromosomes means the homologues have already been separated, which only happens in meiosis I.

• 2.Suppose a mutation destroyed shugoshin, so the centromeric cohesin was broken down in anaphase I along with the rest. What would go wrong?Show answerHide

  Sister chromatids would lose their connection early and could separate at random during meiosis I or drift apart before meiosis II. Chromatids would then be distributed unevenly, so many gametes would end up with a missing or extra chromosome.

• 3.A species has 2n = 20. Ignoring crossing over, how many different chromosome combinations can one individual's gametes carry? Why do two children of the same parents almost never share the same combination?Show answerHide

  n = 10, so 2¹⁰ = 1,024 combinations per parent. A child combines one egg and one sperm, so there are 1,024 × 1,024 ≈ 1 million combinations, and crossing over multiplies that enormously. The chance of two siblings matching is tiny.