A Type of Cell Division That Occurs Within Sex Cells
This is Meiosis
(Don’t worry if you don’t understand this now, you will soon.)
Learn about step one when we learn the stages of meiosis.
Before meiosis can take place, a cell goes through interphase. This is a three-step process: G1, S, and G2.
- The G stands for “growth” and the S stands for “synthesis.”
- During G1, the cell grows and prepares for DNA Replication.
- During S, DNA synthesis occurs. In other words, DNA replication and chromatid duplication take place.
- During G2, the cell grows yet again.
DNA Replication & Chromatid Duplication
Even while DNA Replication and Chromatid Duplication are apart of Interphase, the topics are enough for their own subject. In animal cells, outside the nucleus are two centrosomes, each of which contain a pair of centrioles. The centrosomes serve as microtubule organizing centers (MTOCs). The chromatin from within the nucleolus is condensed to form a chromosome. Each chromosome is made up of two identical chromatids, known as sister chromatids. Combined, the two chromatids contain 100% DNA (2n) each, resulting in 200% (4n). The two chromatids that resemble 2n can also each be referred to as diploid. Because of the 4 in 4n, the result is a tetrad.
During Phophase I, the chromosomes become shorter and thicker. They are now visible under a microscope. The duplicated chromosomes now pair, and crossing-over occurs. Each pair is visible now as a tetrad, each consisting of two sister chromatids. The sites of crossing-over are called chaismata. The contact point at which two sister chromatin come into contact is referred to as a centromere.
During some point in prophase I, the nucleolus vanishes.
The meiotic spindle forms, which consists of microtubles and other proteins, between the two pairs of centrioles as they migrate to opposing points of the cell.
The nuclear envelope disappears at the end of prophase I, which allows for the spindle to enter the nucleus.
Prophase I is the longest phase of meiosis, typically consuming 90% of the time for the two divisions.
The centrioles are at opposite positions of the cell. The tetrad of chromosomes, now at their climax of condensation, become arranged on a plane in the middle of the two poles called the metaphase plate. Spindle fibers from one pole of the cell attach to one chromosome of each pair, and spindle fibers from the opposite pole attach to the homologous chromosome.
Anaphase I begins with the two chromosomes of each tetrad seperate and start moving toward opposite poles of the cell as a result of the action of the spindle. A key difference between mitosis and meiosis is that sister chromatids remain joined after metaphase I in meiosis, however in mitosis they seperate.
The homologous chromosome pairs complete their migration to the two poles as a result of the action of the spindle. A haploid set of chromosomes is now at each pole, with each chromosome still having two chromatids.
A nuclear envelope reforms around each chromosome set, the spindle disappears, and cytokinesis occurs. In animal cells, cytokinesis results in the slicing of the cell into two cells. After cytokinesis, each of the two progeny cells has a nucleus with a haploid set of replicated chromosomes.
Many cells that undergo rapid meiosis do not decondense the chromosomes at the end of telophase I.
You have made it this far, brave soul? Good, let us recap where we are quickly.
Our cell has just gone through cytokinesis which resulted in two progeny cells that each have a nucleus with a haploid set of replicated chromosomes. This means that we now have two cells!
Even though chromosome duplication took place in interphase, no new chromosome replication occurs until after the checkpoint. From here, the centrioles duplicate. This process is accomplished by the seperation of the two members of the pair, and then the formation of a daughter centriole perpendicular to each original centriole. The two pairs of centrioles seperate into two centrosomes. The nuclear envelope breaks down, and the spindle apparatus forms.
Each of the two daughter cells completes the formation of a spindle apparatus. Chromosomes align one-by-one on the metaphase plate, almost the same as chromosomes do in mitosis. This is done in contrast to metaphase I, in which homologous pairs of chromosomes align on the metaphase plate. Now, only one chromosome aligns on the metaphase plate at a time. For each chromosome, the kinetochores of the sister chromatids face the opposite poles, and each is attached to a kinetochore microtubule coming from that pole. In other words, the chromosomes are attached to the now fully formed spindle.
The centromeres seperate, and the two chromatids of each chromosome move to opposite poles on the spindle. The seperated chromatids are now referred to as individual chromosomes.
A nuclear envelope forms around each set of chromosomes. Cytokinesis takes place, in which four daughter cells (gametes) are formed, each with a haploid set of chromosomes. Each contain one n of 4n from prophase I, which means each contain 50% of the DNA. This is explained by crossing-over, in which some chromosomes are seen to have recombined segments of the original parental chromosomes.
The Final Product
At the end of meiosis, a group of four haploid gamete cells (sex cells that contain one complete set of chromosomes each) is formed.
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