Meiosis is the process in which gametes (egg or sperm cells) are produced. A diploid cell (with two sets of chromosomes) divides to produce four haploid cells (each with just a single set of chromosomes).
Meiosis takes place in two stages – Meiosis I and Meiosis II. Before these stages, however, the cell must first undergo interphase in order to duplicate the genetic information in the cell.
As with mitosis, each stage of meiosis consists of 4 steps – prophase, metaphase, anaphase and telophase.
Meiosis I
Each diploid cell has 23 pairs of chromosomes. During the first phase of meiosis the 23 pairs must be separated.
IMPORTANT NOTE: Due to interphase each chromosome has two sister chromatids that are identical to each other.
Prophase I
Similarly to mitosis the chromosomes condense, the nuclear envelope breaks down and the centrosomes begin moving towards opposite ends of the cell forming the spindle (a network of protein fibres).
Unlike mitosis, however, the homologous pairs each pair up (i.e. both copies of chromosome 1 are together). Once they have aligned with each other the pairs undergo a process called crossing over which increases the genetic variation in the gametes.
The chromosomes begin to attach to the spindle.
Metaphase I
The chromosomes line up along the middle of the cell in their pairs. The orientation of the pairs is random – this is called independent assortment. For example, in the diagram the light red and light blue chromosomes could both be on the left.
Each chromosome from a pair connects to the spindle by fibres from opposite poles of the cell. This is different from mitosis in which each sister chromatid of a chromosome is attached to opposite poles.
Anaphase I
During anaphase the spindle contracts to separate the homologous pairs and pull them to opposite ends of the cell. The sister chromatids remain together and the cell begins to elongate.
Telophase I
The chromosomes reach the opposite poles of the cell and begin to decondense. A nuclear envelope forms around each set of chromosomes. The cell divides, a process called cytokinesis, to leave two haploid cells.
IMPORTANT NOTE: The cells at the end of meiosis I have half the number of chromosomes as the original cell. But each chromosome still has a pair of sister chromatids.
Meiosis II
Once meiosis I is complete the two cells that formed begin meiosis II. This stage is very similar to mitosis, with the main difference being each cell only has half the number of chromosomes as cells undergoing mitosis.
Prophase II
The chromosomes again condense and the nuclear envelopes break down. The centrosomes begin to move to opposite sides of the cell and the spindle starts to form.
IMPORTANT NOTE: The centrosomes usually duplicate between meiosis I and meiosis II.
Metaphase II
The chromosomes in each cell line up along the centre of the cell. Each sister chromatid is attached to the spindle by fibres from opposite centrosomes in the cell.
Anaphase II
The sister chromatids are separated and pulled to opposite poles of the cell. Once separated they become individual chromosomes. The cell also begins to elongate.
Telophase II
Once the chromosomes reach opposite sides of the cell they begin to decondense and a nuclear envelope forms around them. Cytokinesis takes place, separating both cells into two new cells.
This leaves 4 haploid cells that are all genetically different – meiosis is complete.
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