Mitosis (along with the step of cytokinesis) is the process of how a eukaryotic somatic cell, or body cell, divides into two identical diploid cells. Meiosis is a different type of cell division that begins with one cell that has the proper number of chromosomes and ends with four cells that have half the normal number of chromosomes (haploid cells). In a human, almost all cells undergo mitosis. The only cells in a human that are made by meiosis are gametes or sex cells (the egg or ovum for females and the sperm for males). Gametes only have half the number of chromosomes as a normal body cell because when gametes fuse during fertilization, the resulting cell (called a zygote) then has the correct number of chromosomes. This is why offspring are a mixture of genetics from the mother and the father (the father's gamete carries half of the chromosomes and the mother's gamete carries the other half) and why there is so much genetic diversity -- even within families.
Although there are very different results for mitosis and meiosis, the processes are pretty similar with just a few changes within the stages of each. Let's compare and contrast mitosis and meiosis to get a better idea of what each does and why.
Both processes start out after a cell goes through interphase and copies its DNA exactly in the S Phase (or Synthesis Phase). At this point, each chromosome is made up of sister chromatids that are held together by a centromere. The sister chromatids are identical to each other. During mitosis, the cell only undergoes the M Phase (or mitotic phase) once, ending in a total of two identical diploid cells. In meiosis, there will be a total of two rounds of the M Phase so the end result is four haploid cells that are not identical.
Stages of Mitosis and Meiosis
There are four stages of mitosis and a total of eight stages in meiosis (or the four stages repeated twice). Since meiosis undergoes two rounds of splitting, it is divided into meiosis I and meiosis II. Each stage of mitosis and meiosis have many changes going on in the cell, but they have very similar, if not identical, important events that happen that mark that stage. Comparing mitosis and meiosis is fairly easy if these most important events are taken into account.
The first stage is called prophase in mitosis and prophase I or prophase II in meiosis I and meiosis II. During prophase, the nucleus is getting ready to divide. This means the nuclear envelope has to disappear and the chromosomes start to condense. Also, the spindle starts to form within the centriole of the cell that will help with the division of chromosomes during a later stage. These are all things that happen in mitotic prophase, prophase I, and usually in prophase II. Sometimes, there is no nuclear envelope at the beginning of prophase II and most of the time, the chromosomes are already condensed still from meiosis I.
There are a couple of differences between mitotic prophase and prophase I. During prophase I, homologous chromosomes come together. Every chromosome has a matching chromosome that carries the same genes and is usually the same size and shape. Those pairs are called homologous pairs of chromosomes. One homologous chromosome came from the individual's father and the other came from the individual's mother. During prophase I, these homologous chromosomes pair up and sometimes intertwine. A process called crossing over can happen during prophase I. This is when homologous chromosomes overlap and exchange genetic material. Actual pieces of one of the sister chromatids breaks off and reattaches to the other homolog. The purpose of crossing over is to further increase genetic diversity since alleles for those genes are now on different chromosomes and can be placed into different gametes at the end of meiosis II.
In metaphase, the chromosomes are going to line up at the equator, or middle, of the cell and the newly formed spindle will attach to those chromosomes to prepare for pulling them apart. In mitotic metaphase and metaphase II, the spindles attach to each side of the centromeres that are holding the sister chromatids together. However, in metaphase I, the spindle attaches to the different homologous chromosomes at the centromere. Therefore, in mitotic metaphase and metaphase II, the spindles from each side of the cell are connected to the same chromosome. In metaphase I, only one spindle from one side of the cell is connected to a whole chromosome. The spindles from opposite sides of the cell are attached to different homologous chromosomes. This attachment and set up is essential for the next stage and there is a checkpoint at that time to make sure it was done correctly.
Anaphase is the stage in which the physical splitting occurs. In mitotic anaphase and anaphase II, the sister chromatids will be pulled apart and moved to opposite sides of the cell by the retraction and shortening of the spindle. Since the spindles attached at the centromere on both sides of the same chromosome during metaphase, it essentially rips apart the chromosome into two individual chromatids. Mitotic anaphase pulls apart the identical sister chromatids, so identical genetics will be in each cell. In anaphase I, the sister chromatids are most likely not identical copies since they probably had undergone crossing over during prophase I. In anaphase I, the sister chromatids stay together, but the homologous pairs of chromosomes are pulled apart and taken to opposite sides of the cell.
The final stage is called telophase. In mitotic telophase and telophase II, most of what was done during prophase will be undone. The spindle begins to break down and disappear, a nuclear envelope begins to reappear, chromosomes start to unravel, and cell prepares to split during cytokinesis. At this point, mitotic telophase will go into cytokinesis that will create a total of two identical diploid cells. Telophase II has already gone one division at the end of meiosis I, so it will go into cytokinesis to make a total of four haploid cells. Telophase I may or may not see these same sorts of things happening, depending on the cell type. The spindle will break down, but the nuclear envelope may not reappear and the chromosomes may stay tightly wound. Also, some cells will go straight into prophase II instead of splitting into two cells during a round of cytokinesis.
Mitosis and Meiosis in Evolution
Most of the time, mutations in the DNA of somatic cells that undergo mitosis will not be passed down to the offspring and therefore are not applicable to natural selection and do not contribute to the evolution of the species. However, mistakes in meiosis and the random mixing up of genes and chromosomes throughout the process do contribute to genetic diversity and drive evolution. Crossing over creates a new combination of genes that may code for a favorable adaptation. Also, the independent assortment of chromosomes during metaphase I also leads to genetic diversity. It is random how homologous chromosome pairs line up during that stage, so the mixing and matching of traits has many choices and contribute to the diversity. Finally, random fertilization also can increase genetic diversity. Since there are ideally four genetically different gametes at the end of meiosis II, which one is actually used during fertilization is random. As the available traits are mixed up and passed down, natural selection works on those and chooses the most favorable adaptations as the preferred phenotypes of individuals.