The way in which meiosis proceeds is essentially the same in any eukaryotic cell that undergoes meiosis. The principle difference is the number of chromosomes involved. That number of chromosomes depends on the species. The number of different kinds of chromosomes is represented by the variable, N, which is known as the haploid number. This is because a haploid cell contains just one of each type of chromosome. A diploid cell is characterized as 2N, because a diploid cell has two of each type of chromosome (one from each sexual parent). Meiosis allows one diploid cell to become four haploid cells. Each haploid cell is not only genetically different from the original diploid cell, but each is also genetically different from the other three haploid cells produced. The stages of meiosis are illustrated below for a species with N=2. The original, diploid cell in this example (2N) therefore has 2×2=4 overall chromosomes. Each of the four cells produced has N=2 overall chromosomes (they are haploid). Whatever the value of N, during metaphase of meiosis I, N pairs of homologous, replicated chromosomes line up, and during metaphase of meiosis II, N individual, replicated chromosomes line up.
As you study this figure and the accompanying text in this section, you should be able to draw what the stages look like for any other value of N.
Mitosis and meiosis are two alternative processes that can be involved in eukaryotic cell division.
Here are key similarities:
Here are key differences:
Although there are several similarities between mitosis and meiosis, there are crucially important differences, allowing the two processes to serve separate purposes.
The life cycle of any sexual species features fertilization, which is the fusion of unicellular gametes (one male gamete and one female gamete) to produce a unicellular zygote. The unicellular zygote produced by fertilization carries the chromosomes from both gametes. Therefore, the ploidy of the zygote is double the ploidy of the gametes. If fertilization were the only process occurring each generation, then the ploidy would double each generation (tetraploid, then octaploid, etc.), and the zygote would not be able to contain the DNA. To prevent the ploidy from doubling each generation, a separate process is needed to cut the ploidy in half. That process is meiosis. Specifically, the reduction of ploidy occurs in meiosis I, when homologs separate and go to distinct daughter cells. Since a diploid cell that undergoes meiosis will produce haploid cells (gametes), when those haploid gametes fuse (in fertilization), the zygote will be diploid. By alternating meiosis and fertilization each generation, the ploidy simply goes back and forth between haploidy and diploidy (rather than continually increasing). Another important purpose of meiosis is to drastically increase the genetic variability of the gametes produced. That increase in genetic variability comes in the forms of crossing over (during prophase of meiosis I) and independent assortment (during metaphase of meiosis I). As you study the phases of meiosis, appreciate that crossing over and independent assortment produce new genetic combinations, and separation of homologs reduces the ploidy.
This vocabulary list includes terms that might help you with the review items above and some terms you should be familiar with to be successful in completing the final exam for the course.
Try to think of the reason why each term is included.