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As discussed in part one of this series, meiosis is a two-part cell division process in organisms that sexually reproduce which results in gametes with one half the number of chromosomes of the parent cell. Let's look at an overview of the first cell division, meiosis I.
Meiosis I
Meiosis I encompasses four stages: prophase I, metaphase I, anaphase I and telophase I.
Prophase I
As previously mentioned, the largest differences between mitosis and meiosis occur in prophase I. Prophase I is usually longer in duration when compared to prophase in mitosis and it is usually much more complex. It can take days for prophase I to complete. It is estimated that prophase I accounts for some 85 - 95 percent of the total time for meiosis.
The following changes occur:
- Chromosomes condense and attach to the nuclear envelope.
- Synapsis occurs (a pair of homologous chromosomes lines up closely together) and a tetrad is formed. Each tetrad is composed of four chromatids.
- Crossing over may occur.
- Chromosomes thicken and detach from the nuclear envelope.
- Similar to mitosis, the centrioles migrate away from one another and both the nuclear envelope and nucleoli break down.
- Likewise, the chromosomes begin their migration to the metaphase plate.
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(Click on metaphase I image below to view full size.)
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Metaphase I
Metaphase I is of much shorter duration and complexity when compared to prophase I.
The following changes occur:
- Tetrads align at the metaphase plate.
- Note that the centromeres of homologous chromosomes are oriented toward the opposite cell poles.
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(Click on metaphase I image below to view full size.)
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Next page > Anaphase I/Telophase I
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