Meiosis includes 2 cell divisions with different characteristics.
We start with a diploid (2 n) cell, in which DNA duplication has occurred so that each DNA strand is double. These strands do not yet appear as chromosomes (condensed bodies); in fact, the chromatin strands have to be "relaxed" in order to duplicate.
***Meiosis I: (in which the nº of chromosomes decreases to half)
1. The chromatin (DNA strands) starts to condense and appears as small dense dots (the chromosomes) within the cell nucleus. These chromosomes will each have 2 "arms" (they look like an X or a V) because the DNA in each has been duplicated.
2. Now all these chromosomes align in homologous pairs, with the aid of a transparent structure formed by tiny intracellular fibers (the spindle) that has formed and will help them to move in orderly fashion within the nucleus.
3. While they keep condensing, each homologous chromosome contacts its "brother", and they exchange some fragments of their DNA ("crossing over"). Usually only one "arm" of each chromosome participates in crossing over.
4. In the meantime, the nuclear membrane is disintegrating...
5. The pairs of homologues, still united, align at the middle (equator) of the spindle...
6... and then each pair of homologues is divided, one chromosome of each pair goes toward each "pole" of the spindle.
7. New nuclear membranes form around the 2 groups of chromosomes. Each of these groups will only have ONE of each pair of homologues. Essentially the information in each new nucleus is the same, BUT has some differences because, as you recall, there has been crossing over i.e. partial exchange of DNA between homologues.
8. The original cell with the 2 nuclei divides and forms 2 cells, with half the nº of chromosomes of the original cell (haploid or n). Each of the chromosomes still has a duplicate DNA strand.
... the cell takes a short break (you can take a break too!)... but the chromatin does not duplicate this time (it's already done that)
***Meiosis II: (in which each double DNA strand becomes single again, this phase is similar to a normal mitosis).
1. The chromatin condenses again, so that it appears as chromosomes.
2. This time the spindle aligns the center of each chromosome at its equator.
3. The central part of the chromosome is duplicated and the sister "arms" (chromatids) separate, each migrating to opposite poles.
4. Now you have two groups of chromosomes, each group still has the haploid (n) nº of chromosomes (the same amount with which they started this 2nd meiosis) but now, each chromosome has a single DNA strand.
A new nuclear membrane forms around each of these groups of chromosomes.
5. The rest of the cell divides.
So, to sum up: you started with ONE diploid cell. You end with FOUR haploid cells. In animals, this process only happens during the production of gametes.
If we're talking about the formation of female gametes, then only ONE of the 4 resulting cells will become an ovule (and that only after more maturation), the other 3 cells form "polar bodies".
In contrast, when it's spermatozoids being produced, all FOUR resulting cells will become spermatozoids.
That's one of the reasons sperm is cheaper than ovules! There's just more of them in the entire animal kingdom.
Now, what is the purpose of this extremely complicated (the above is a very simplified version) process? The result will be cells that have half the chromosomes of the normal body cells. These cells cannot originate a new organism by themselves; they have to combine with another haploid cell to form a zygote. The zygote will have again pairs of homologous chromosomes, each pair formed by one chromosome from the mother and one from the father. In addition, thanks to the crossing over, not all the gametes will have the same information as the original parental cell.
This is an essential process to assure that there will be variability within a population (offspring will not be exactly like their parents, but not "half-and-half" either).
Mitosis: Mitosis happens in all parts of your body that are capable of growth and repair. The brain and nerve cells are the ones that do not regenerate well after an injury.
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