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We have mostly talked about the celebrities of division, the chromosomes. But it turns out that chromosomes are not any different from puppets on a string. Without the string the puppets can't move, even though they are the stars of the show. In mitosis and meiosis, the string is the spindle, and without the spindle the chromosomes aren't going anywhere.
What is the mitotic spindle? The spindle itself comprises microtubules, which are organized in animal cells by special structures called centrosomes. When it's time for the cell to divide, the two centrosomes in the cell move around the nucleus such that they are on opposite sides. Only one centrosome makes it into each daughter cell at the end of cell division, so the cell needs to create a new centrosome each cell cycle. In meiosis, a similar spindle forms during each meiotic division to orchestrate the movement of the chromosomes.
Now that we know all of the key players of mitosis and meiosis, we can look at each process in detail. First, let's focus on mitosis.
Prior to mitosis, the chromosomal DNA is spread throughout the cell nucleus to facilitate transcription and replication. But this stretched out form is way too hard to segregate. Just think about how difficult it is to separate a whole plate of cooked spaghetti into individual spaghetti strands. Wouldn't it be so much easier if the spaghetti coiled itself up into neat little pasta spirals while it was cooking and then unwound on your plate again after you had separated it? Believe it or not, that is pretty much what your chromosomes do during mitosis.
During prophase, the first stage of mitosis, the DNA tightly compacts into rod-like structures that, similar to those pasta spirals, are much easier to separate. The sister chromatids are held tightly together—especially at their centers, a region called the centromere—in a structure that we know resembles an X. The nuclear envelope also begins to break down, which allows the microtubules to begin to interact with the chromosomes. The breakdown of the nuclear envelope is considered the starting point of the next stage of mitosis, prometaphase.
At the beginning of prometaphase, the condensed chromosomes are spread throughout the nuclear region. The microtubules of the mitotic spindle attach to the centromere of the chromosomes at a structure known as the kinetochore. Each sister chromatid must be attached to a mitotic spindle originating from one pole by special fibers known as kinetochore fibers. Other fibers, which run between the centrosomes, are known as polar fibers. The spindle pulls the chromosomes, resulting in their alignment directly in the center of the cell.
The cell is considered to be in metaphase once the chromosomes are all attached and aligned down the center (aka equator) of the cell.
The mitotic spindle then begins to pull the sister chromatids to opposite poles, a part of the cell cycle called anaphase.
Now in telophase, the chromosomes are at the opposite poles. The chromosomes begin to unwind (decondense) and return to their elongated forms, and the nuclear envelope reforms around the two individual chromosome groups. Finally, cytokinesis results in the physical pinching off of the cell into two separate daughter cells.
Just like before mitosis, a cell about to begin meiosis undergoes DNA replication. The first step of meiosis is prophase I. In prophase I, the chromosomes condense and pair with their homologous chromosome in a process known as synapsis, which makes it considerably different from the prophase of mitosis.
The homologous chromosomes are held together by the special meiotic complex called the synaptonemal complex and chiasmata, the connections generated by recombination. The paired chromosomes are known as bivalents (because there are two chromosomes) or tetrads (because there are four chromatids). Finally, in the last stage of meiotic prophase I, the meiotic spindle assembles and the nuclear envelope disassembles.
The cell then progresses into meiotic metaphase I, when the homologous chromosomes align in their pairs at the center of the cell.
Then in anaphase I, the pairs of homologs are pulled toward the poles. Remember that the sister chromatids stay attached to each other throughout this part of the process.
In telophase I, the nuclear envelope may or may not reform, and the chromosomes do not usually decondense. This completes the first division cycle of meiosis. The two cells created by meiosis I are haploid because they only have one member of each homologous chromosome. But they still have the same amount of DNA as a resting diploid cell because they contain replicated sister chromatids.
The cell proceeds to meiosis II, but without replicating its DNA again. In prophase II of meiosis, the nuclear envelope breaks down if it reassembled during telophase I of meiosis I. The sister chromatids attach to spindles emanating from opposite poles of the cell, as in mitosis. The sister chromatids align in the center of the cell during metaphase II, like in metaphase of mitosis. In many organisms, meiosis actually stops during metaphase II of egg production and doesn't resume until the egg is fertilized.
Then in anaphase II, the sister chromatids are pulled to the opposite sides of the cell. Finally in telophase II, the spindle dissembles, and the nuclear envelope reassembles. The result is a total of four haploid cells with half the amount of DNA of the original diploid cell.
We have broken mitosis and meiosis down into their individual stages. In the next two sections, we'll recap and look at each process again in all its glory.
Meiosis I can take a long time. In human females, for example, meiosis I begins in the egg around birth but doesn't stop until puberty, meaning it may not complete this stage for decades. In fact, if the egg produced doesn't get fertilized, it never finishes meiosis II either.
Here's an old-fashioned pregnancy test: inject a woman's urine into an African clawed frog, Xenopus laevis. If the woman is pregnant, the hormones in her urine will cause the frog to jump-start meiosis and produce eggs within 24 hours. Nowadays, we add those same hormones to monoclonal antibodies in urine dipsticks. Not only is this test much more accurate, it also means no more injections for the poor frogs.