1The words, pinned down

• Chromosome: one DNA molecule with its proteins. Count chromosomes by counting centromeres. A chromosome that has been replicated is still one chromosome; it just has two chromatids.
• Sister chromatids: the two identical copies made in S phase, held together at the centromere. Identical because one was copied from the other.
• Homologous chromosomes (homologues): the two chromosomes of a pair, one from each parent. They carry the same genes in the same order, but not necessarily the same versions (alleles) of those genes. They are similar, not identical.
• Diploid (2n): two of each chromosome. Haploid (n): one of each. Humans have 2n = 46, so a human gamete has n = 23.
• Bivalent: a pair of homologues lying side by side in prophase I, four chromatids in all.
• Synapsis: the pairing up of homologues along their length.
• Chiasma (plural chiasmata): a point where chromatids of two homologues have crossed and exchanged pieces (5.2).
• Kinetochore: the protein structure at the centromere to which spindle microtubules attach.

Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids.

2Before meiosis begins: one round of replication

The cell that will divide by meiosis first passes through interphase, and in S phase every chromosome is replicated. So at the start of meiosis a cell with 2n = 4 still has four chromosomes, but each now has two sister chromatids: eight chromatids in all. There is no second round of replication before meiosis II. That single fact is why the divisions halve the chromosome number: DNA is copied once and the cell divides twice.

3Meiosis I: homologues are separated

Figure 1 follows a cell with 2n = 4: a long pair and a short pair. The teal chromosome in each pair came from one parent and the amber from the other.

Prophase I. The chromosomes condense and become visible. Each pair of homologous chromosomes comes together and lines up gene for gene: synapsis, forming a bivalent. While they are paired, chiasmata may form where non-sister chromatids exchange segments (crossing over, which 5.2 deals with). Meanwhile the meiotic spindle begins to form, the centrosomes move to opposite poles of the cell, and the nuclear envelope breaks down.

Metaphase I. Spindle fibres line the homologous pairs up along the metaphase plate, the equator of the cell. The key picture: the chromosomes sit at the plate in pairs, one homologue on each side. Each homologue is attached to fibres from one pole only. Look at the top cell of Figure 1: two bivalents, each made of a teal and an amber chromosome facing each other across the dashed line.

Anaphase I. The homologues of each pair are pulled to opposite poles. The sister chromatids stay attached to each other at their centromeres. So each pole receives one chromosome from every pair, and each of those chromosomes is still a two-chromatid chromosome.

Telophase I. The spindle breaks down, a nuclear envelope forms around each group of chromosomes, and cytokinesis divides the cytoplasm: by a cleavage furrow in an animal cell, or a cell plate in a plant cell. The result is two haploid cells. In Figure 1, each middle cell has one long and one short chromosome: n = 2. The cells are haploid even though each chromosome still has two chromatids, because haploid means one of each chromosome, not one chromatid.

4Meiosis II: sister chromatids are separated

Each of the two haploid cells now divides again, without replicating its DNA. Meiosis II looks like mitosis in a haploid cell.

Prophase II. A new meiotic spindle forms. The chromosomes, each still two sister chromatids joined at the centromere, attach to the spindle.

Metaphase II. The chromosomes line up along the metaphase plate, this time in single file, not in pairs. The kinetochore of each sister chromatid is attached to a microtubule from the opposite pole.

Anaphase II. The proteins holding the sister chromatids together at the centromere break down, and the chromatids are pulled apart towards opposite poles. From this moment each chromatid counts as a chromosome in its own right.

Telophase II. The spindle breaks down, a nuclear envelope forms, the chromosomes begin to decondense, and cytokinesis divides each cell. The result of the whole process is four haploid cells, each with one unduplicated copy of every chromosome: the bottom row of Figure 1.

Notice that the four cells in Figure 1 are not all alike. Two carry the teal long chromosome and two the amber one. Meiosis has already started to mix the parents' chromosomes, and 5.2 explains how much mixing it does.

5Counting: chromosomes, chromatids and DNA

Questions give the diploid number and ask what a cell holds at a stated moment. Three rules: count centromeres to count chromosomes; a replicated chromosome has two chromatids; DNA follows chromatids, not chromosomes.

A worked example, with our own organism: an invented beetle with 2n = 8. Let the DNA in one unreplicated haploid set be 1 unit.

Read the table down the DNA column: 2, then 4, then 2, then 1. On a graph of DNA per cell against time that gives a step up (replication), then two steps down (the two divisions), ending at half the starting amount. The first step down is where the cell becomes haploid.

6Mitosis and meiosis compared

Both divisions use a spindle apparatus of microtubules, attached at kinetochores, to move chromosomes, and both end in cytokinesis. They differ in how many cells they make and what those cells contain.

In a drawing, pairs at the plate mean metaphase I; a set of differently sized chromosomes with no pairs means a haploid cell in meiosis II.