Major Events in Meiosis

Meiosis is reduction division that occurs only in germ cells where gametes are produced with half the chromosome number to that of the parent cell. This reduction is essential for maintenance of chromosome number in a species. It occurs only in germ cells to produce gametes.

We are 99.99% similar in genetic makeup according to human genome project. This diversity is contributed by just 0.01% of the genome. One of the major reasons for this variation is crossing over that takes place in germinal cells during gametogenesis.

In humans, gametes are formed by meiosis where the chromosome number is reduced to 23. therefore, egg (23 chromosomes) + sperm (23 chromosomes) = zygote (46 chromosomes).
Important events of meiosis are:
1) Two successive cell division without DNA replication
2) Pairing of homologous chromosomes during prophase I followed by formation of chaismata and crossing over that lead to genetic variation
3) Separation of homologous chromosomes in anaphase I of meiosis I
4) Separation of sister chromatids in anaphase II of meiosis II


Major events in each stage of meiosis: 

Major Events
Prophase I
Chromosomes condense , homologous chromosomes synapse, crossing over takes place, nuclear envelope break down and mitotic spindle forms.

Metaphase I
Homologous pairs of chromosomes line up on the metaphase plate.

Anaphase I
Two chromosomes of each with two chromatids of each homologous pair separate and move toward opposite poles.

Telophase I
Chromosomes arrives at the spindle poles

Cytoplasm divides to produce  two cells, each having half the original number of chromosomes

In some cells the spindle breaks down, chromosomes relax and a nuclear envelop re-forms, but no DNA synthesis is takes place.
Prophase II
Chromosomes condense, the spindle forms, and the nuclear envelop disintegrates.

Metaphase II
Individual Chromosomes line up on the  metaphase plate

Anaphase II
Sister chromatids separate, becoming individual chromosomes that migrate toward spindle poles

Telophase II
Chromosomes arrive at spindle poles, the spindle breaks down and a nuclear envelop re-forms

Cytoplasm divides

Homologous chromosomes: Each pair of a chromosome in a diploid cell has one chromosome derived from the male parent and the other from the female parent. They are morphologically and genetically similar and are called homologous chromosomes.
Crossing over (Genetic recombination): Reshuffling of the genes on chromosome that occurs during meiosis as a result of breakage and reunion of segments of homologous chromosomes
Synapsis: The process by which homologous become joined to one another during meiosis.
Synaptonemal complex (SC): A ladder like structure composed of three parallel bars with many cross fibers. The SC holds each pair of homologous chromosomes in the proper position to allow the continuation of genetic recombination between strands of DNA.
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