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Meiosis

Organisms can reproduce asexually via mitosis. While asexual reproduction is very efficient, it leads to genetically identical offspring. This means the whole population has the same adaptations to the environment and resistance to disease. A genetically diverse population is better able to evolve and adapt to changing environments. In sexual reproduction, sex cells or gametes fuse during fertilization to create a new genetically unique individual. To ensure that the offspring receives the normal number of chromosomes (46, 23 pairs in humans), each of the gametes must only contain half the amount of genetic material as a normal somatic cell.

Meiosis is a process of cell division that gives rise to four daughter cells, each containing 1 copy of the genome (1n). The four daughter cells are known as haploid. Somatic cells containing 2n are called diploid. Meiosis consists of two rounds of division, meiosis I and meiosis II (Figure 1). At the start of meiosis I, all chromosomes are replicated, just like in mitosis. Each daughter cell inherits 2 sister chromatids (the replicated chromosome), but only 1 of the homologous pair (either the paternal or maternal chromosome). In the next round of division, meiosis II, no replication takes place. Now, the sister chromatids separate. Meiosis therefore consists of 2 rounds of cell division, but only one round of DNA replication.

Stages of meiosis. The cell starts the process at Interphase, where DNA is condensed, followed by Prophase one, where chromosomes are formed duplicated. Then it moves to Meiosis one that consists of Prometaphase one, where spindle fiber microtubules are attached to the centres of the chromosomes, Metaphase one, where chromosomes are arranged in the centre of the cell, Anaphase one, where chromosomes are pulled apart, and Telophase one, where chromosomes arrive at opposite poles of the cell. Meiosis one is finished with cytokinesis, resulting in two haploid cells. In Meiosis two, each cell goes through the same phases as in the case of meiosis one, but this time sister chromatids are separated in Anaphase two, and the whole process results in forming four unique haploid cells.

Figure 1: Overview of meiosis

Genetic rearrangement takes place in two ways. First, during early meiosis I, the homologous chromosomes exchange equivalent parts of the genome in a process called crossing over. Second, when the homologous chromosomes separate in late meiosis I, the maternal and paternal chromosomes are distributed randomly.