Stages of meiosis

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MEIOSIS I
Prophase I
• The chromosomes begin to condense. They become shorter, thicker and clearly visible.
• The homologous chromosomes come together to form bivalent through a process called synapsis.
• Non-sister chromatids exchange segments of DNA in a process known as crossing over.
• Crossing over results in a new combination of genes on a chromosome.
• The points at which segments of chromatids cross over are called chiasmata.
• At the end of prophase I, the nucleolus and nuclear membrane disappear.
• The two pairs of centrioles migrate to the opposite poles of the cells.
• These features are similar to those of prophase during mitosis.
Metaphase I
• The chromosomes are lined up side by side as tetrads on the metaphase plate.
• The chromosomes are still in homologue pairs.
• The centromere does not divide.
Anaphase I
• The spindle fibres pull the homologous chromosomes away from one another and move them to the opposite poles of the cell.
• Each chromosome still consists of two sister chromatids which move as a single unit.
• Although the cell started with four chromosomes, only two chromosomes (each with two sister chromatids) move towards each pole.
Telophase I
• The chromosomes arrive at the poles.
• Each pole now has a haploid daughter nucleus because it contains only one set of chromosomes.
• The spindle fibres disappear.
• The nuclear membrane reappears to surround each set of chromosomes.
• The nucleolus then reappears in each nucleus.
Cytokinesis
• Cytokinesis usually occurs simultaneously with Telophase I, resulting in two haploid daughter cells, each receiving one chromosome from the homologous pair.
• Meiosis II follows immediately after cytokinesis, usually with no interphase between them.
• DNA replication does not occur and the chromosomes remain in a condensed state.
MEIOSIS II
Prophase II
• The nuclear membranes of the daughter cells disintegrate again.
• The spindle fibres reform in each daughter cell.
Metaphase II
• The chromosomes, each still made up of sister chromatids, are positioned randomly on the metaphase plate with the sister chromatids of each chromosome pointing towards the opposite poles.
• Each sister chromatid is attached to the spindle fibres at the centromere.
Anaphase II
• The centromere of the sister chromatids finally separate, and the sister chromatids of each chromosome are now individual chromosomes.
• The chromosomes move towards the opposite poles of the cell.
Telophase II
• Finally, the nucleolus and nuclear membranes reform. The spindle fibres break down.
• Cytokinesis follows and four haploid daughter cells are formed, each containing half the number of chromosomes and is genetically different from the parent diploid cell.
• These haploid cells (n) will develop into gametes.