The Nitty Gritty of Cell Division
What is the mitotic spindle? The spindle itself comprises microtubules, which are organized in animal cells by special structures in the cell called centrosomes. When its 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; 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 out 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? Well, believe it or not, that is pretty much what your chromosomes do during mitosis.
In prophase, the first stage of mitosis, the DNA compacts into tight compact rod-like structures that, like the pasta spirals, is much easier to separate. The sister chromatids are held tightly together, especially at their centers, 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 the 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.
Once the chromosomes are all attached and aligned in a straight line down the center (equator) of the cell, the cell is considered to be in metaphase.
The mitotic spindle then begins to pull the sister chromatids to opposite poles, a part of the cell cycle called anaphase.
Finally 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 prophase of mitosis because of this process. 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 is assembled and the nuclear envelope disassembles.
The cell then progresses into meiotic metaphase I, when the homologous chromosomes align in their pairs in 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 then proceeds to meiosis II, but without replicating its DNA again. In prophase II of meiosis, the nuclear envelope breaks down if it reassembled in 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 4 haploid cells with half the amount of DNA of the original diploid cell.
We have broken mitosis and meiosis down into their individual stages, so to recap, let's look at them again in all their glory:
- Meiosis I can take a long time. In human females for example, meiosis I begins in the egg around birth but then stops until puberty, meaning it may not complete this stage for decades. In fact, if the egg produced doesn't get fertilized, it never actually finishes meiosis II, either…
- An old fashioned pregnancy test: The urine of a woman was injected into the African clawed toad, Xenopus laevis. If the woman was pregnant, the hormones in her urine would cause the frog to jumpstart meiosis and produce eggs within 24 hours. Nowadays, we use monoclonal antibodies to those same hormones in urine dipsticks. Not only is this test much more accurate, it also means no more odd injections for the poor toads.