Prophase I - The First Stage of Meiosis IBy Eugene M. McCarthy, Ph.D. Genetics - All rights reserved
Prophase I is the first stage of meiosis I (the remaining three are metaphase I, anaphase I, and telophase I). During this phase, the nuclear envelope (shown in white in the diagram at right) breaks up and disappears. The chromosomes (shown in red) become visible as they shorten, coil, and thicken (that is, as they "condense"). Also, the spindle apparatus (yellow strands in the diagram) begins to extend outward from the two centrosomes, which move to the opposite ends ("poles") of the cell. Each chromosome is composed of two sister chromatids containing identical genetic information. During most of meiosis (through the end of metaphase II) the sister chromatids remain attached to each other at the centromere.
During prophase I the two members of each homolog 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 substages 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. Crossing over. Normally, the two chromosomes making up a homolog pair are not genetically identical -- as is the case with sister chromatids -- because homologs are inherited from different parents. When the chromosomes synapse during prophase I, each gene in each chromosome is brought into contact with the same gene on that chromosome's homolog. During this process of synapsis the two chromosomes of each homolog pair exchange segments of DNA in a process known as crossing over. As a result, the gene combinations on a chromosome can be changed. For example, suppose one chromatid of a chromosome initially contained genes for brown eyes and brown hair. After crossing over, it could contain genes for blue eyes and brown hair, by taking the gene for blue eyes from one of the chromatids of that chromosome's homolog (see figure right). No crossing over occurs between sister chromatids. NEXT PAGE >>
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