Meiosis

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Crossing over- The two chromosomes making up a homolog pair are not genetically identical because homologs are inherited from different parents. When the chromosomes synapse during prophase 1, each gene in each chromosome is brought into contact with the same gene on that chromosome’s homolog. During this process of synapses the two chromosomes of each homolog pair exchange segments of DNA in a process called crossing over. The gene combinations on a chromosome can be changed. For example, suppose one chromatid of a chromosome initially contained genes for blue eyes and brown hair, by taking the gene for blue eyes from one of the chromatids of that chromosome’s homolog.

Prophase I- During prophase I the two members of each pair become intimately associated along their entire lengths (they "synapse") to form a structure known as a tetrad (or bivalent). In the upper diagram two tetrads are represented as two x-shaped chromosomes associated side by side. This is to show that the two chromosomes of each homolog pair adhere to each other during this phase of meiosis. But the appearance of a tetrad changes in the different sub stages of prophase I Early on in this phase the two chromosomes of which a tetrad is composed fuse together so tightly that they cannot be distinguished from each other. Only later do they become separately visible.

Anaphase 1- During this stage of meiosis, the cell starts to lengthen. The two homologs of each chromosome pair separate and move toward opposite poles, drawn by the microtubules of the spindle apparatus. This contrasts with, where the sister chromatids of each homolog separate and move towards opposite poles. Here, the chromatids remain together as one complete, replicated chromosome. Telophase 1- At each pole, there is a complete haploid set of chromosomes (but each chromosome still has two sister chromatids). A cleavage furrow appears and by the end of this stage the parent cell has divided into two daughter...
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