Meiosis is a little more complicated than mitosis, and has a different outcome. Meiosis is a method of nuclear 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 of meiosis.
Firstly, let’s recap the term ‘homologous chromosomes‘. Homologous chromosomes are a pair consisting of one maternal chromosome and one paternal chromosome. They carry the same genes, but can have different alleles. Humans have 23 pairs of homologous 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 joined at the centromere. Now we are ready to begin.
Meiosis I (the first division)
Meiosis I can be called a reduction division. This is because the number of chromosomes halves. A cell that contains two of each chromosome (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 (one from each pair), but still two chromatids of each (2 x n). These are the steps needed to get there:
- Prophase I – the chromosomes become condensed and compact, and arrange into homologous pairs (crossing over can happen here – see next article). The chromosomes are ‘x’ shaped (double-armed) because there are two chromatids joined at the middle by the centromere. The nuclear envelope breaks down and the centrioles move to the opposite poles of the cell to form the spindle.
- Metaphase I – the chromosomes attach to the spindle using the centromeres. They can be seen lined up at the equator of the cell in their homologous pairs. Independent assortment begins here (see this article).
- Anaphase I – the homologous pairs separate and are pulled to the opposite poles of the cell when the spindle contracts. Due to independent assortment, it is random which chromosome from each pair will end up at each pole.
- Telophase I – two nuclear envelopes form 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 have not been separated yet. The cytoplasm divides by cytokinesis to produce the two haploid daughter cells.
Meiosis II (the second division)
In meiosis II, the chromatids are separated and we end up with 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:
- 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.
- Metaphase II – this is pretty much the same as metaphase from mitosis. The centrioles form the spindle, then the chromosomes attach to the spindle using the centromeres and line up along the equator of the cell.
- Anaphase II – this time the centromeres divide and the chromatids separate. Again, this is very similar to anaphase in mitosis. However, the chromatids are not necessarily genetically identical if crossing over occurred in prophase I. Therefore there can be independent assortment of genetically different chromatids.
- Telophase II – two nuclear envelopes form 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 into offspring.
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 chromosomes so there is only one from each pair in the daughter cell. It is a reduction division.
- Meiosis II separates the 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.
