Meiosis – Cell Division Ep 4

Meiosis is a little more complicated than mitosis, and has a different outcome. Meiosis is a method of cell division which only happens in the reproductive organs of organisms which reproduce by sexual reproduction. It produces genetically different gametes with half the number of chromosomes as the parent cell. These gametes (the egg cells and sperm cells in almost all animals) join at fertilisation to produce a zygote with a full set of chromosomes. In this article we will go over the two divisions in meiosis.

Firstly, let’s recap the term ‘homologous pair‘. A homologous pair of chromosomes is a pair consisting of one maternal chromosome and one paternal chromosome. They carry the same genes, but can have different alleles. Humans have 23 homologous pairs of chromosomes (46 chromosomes in total).

Before the divisions begin, meiosis starts with interphase (much like mitosis). The DNA replicates, so the chromosomes consist of two identical sister chromatids. Now we are ready to begin.

Meiosis I (the first cell division)

Meiosis I can be called a reduction division. This is because the total number of chromosomes halves. A cell with the full amount of chromosomes (2n, where n=23 in humans) is called diploid – the parent cell is diploid. But during interphase the DNA was replicated, so we start meiosis with 2 x 2n chromosomes. After meiosis I, we are left with two haploid daughter cells – cells with only half the number of chromosomes, but still two chromatids of each (2 x n). These are the steps needed to get there:

1. Prophase I – the chromosomes become condensed and compact, and arrange into homologous pairs (crossing over happens here – see next article). The chromosomes are ‘x’ shaped (double-armed) because there are two sister chromatids joined at the middle by the centromere. The nuclear envelope breaks down and the centrioles move to the opposite poles of the cell.
2. Metaphase I – the centrioles form the spindle fibres, and the chromosome pairs attach to the spindle fibres using the centromeres. They can be seen lined up along the centre of the cell.
3. Anaphase I – this is where things start to differ significantly from mitosis. Instead of the chromatids separating, the homologous pairs separate and are pulled to the opposite poles of the cell when the spindle fibres contract. This is when independent assortment happens – see next article.
4. Telophase I – the nuclear envelope forms around each set of chromosomes, but there are only half the number i.e. only one from each homologous pair. However, remember they are still ‘x’ shaped as the chromatids haven’t separated yet. The cytoplasm divides by cytokinesis to produce the two haploid daughter cells.

Meiosis II (the second cell division)

In meiosis II, the sister chromatids separate and we end up with the four haploid gametes. We start with 2 x n chromosomes, and end up with n chromosomes (half of the original 2n before DNA replication began). Here are the steps:

1. Prophase II – this is a brief stage because the chromosomes stay partly condensed from meiosis I. But the chromosomes condense further, and the nuclear envelope breaks down.
2. Metaphase II – this is pretty much the same as metaphase from mitosis. The centrioles form the spindle fibres, then the chromosomes attach to the spindle fibres by the centromere and line up along the centre of the cell.
3. Anaphase II – this time the centromeres divide and the sister chromatids separate. Again, this is very similar to anaphase in mitosis.
4. Telophase II – the nuclear envelope forms around the haploid sets of chromosomes. The chromosomes are now single-armed (they are no longer ‘x’ shaped). The cytoplasm divides by cytokinesis so we are left with four haploid daughter cells, which are the gametes.

Next time we will look at the different ways in which meiosis introduces genetic variation.

Summary

This is definitely a difficult topic, and some exam boards need more detail than others for A-Level biology. Here are some summary points:

• Meiosis occurs in reproductive organs to produce genetically different haploid gametes with half the number of chromosomes of a normal diploid cell.
• Meiosis I separates the homologous pairs so there is only one from each pair in the daughter cell. It is a reduction division.
• Meiosis II separates the sister chromatids so the chromosomes go from being double-armed (‘x’ shaped) to single-armed in the final gametes.
• The chromosome number begins as 2n (diploid), goes to 2 x 2n after DNA replication, 2 x n during meiosis I, then n (haploid) during meiosis II. In humans, n=23.

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