Mitosis and Meiosis

Mitosis and meiosis are both processes of cell division, where old cells divide to make new cells. New cells are constantly being made to help the body grow and to replace old or damaged tissue. Every time new cells are made, a person's genetic information has to be copied so that each cell has the exact right amount of genetic information. The copying of information is done through the replication of the chromosomes. Each chromosome is a single, long structure. When a chromosome replicates itself, the end result is two long structures joined at the center. These are called sister chromatids and they behave as one unit for the cell division process:

Only two pairs of chromosomes (green pair and yellow pair) are seen in each of these cells. In a typical cell, one would find 23 pairs of chromosomes. After replication, there would be 23 pairs of sister chromatids.

Once replication has take place, cell division (either mitosis or meiosis) can begin.

Mitosis

Mitosis occurs in both haploid and diploid cells. These cells begin with 23 and 46 chromosomes, respectively. By the end of mitosis, the chromosomes have replicated and moved into new cells. The new cells will have the exact same number of chromosomes as the old cell. All of this happens in 4 distinct, but continuous, stages: prophase, metaphase, anaphase, and telophase.

Prophase : During prophase, the chromosomes condense, meaning they get shorter and fatter. The cellular structures that will help divide the cell later on form. The chromosomes may look something like this in the cell:

This picture represents only 2 of the 23 pairs of chromosomes. Each chromosome of the pair is made up of 2 sister chromatids. There are also two structures called centrioles in this picture (purple structures). The centrioles will become very important later during mitosis.

Metaphase: During metaphase, all of the chromosomes line up in the middle of the cell between the two centrioles. The end result looks something like this:

The chromosomes are now lined up in the middle, though in no particular order; they line up at random. The lines stretching between the centrioles and the chromosomes are called spindle fibers. The spindle fibers are attached to each of the sister chromatids. The fibers initially appear very late in prophase, but are easier to see during metaphase.

Anaphase: During anaphase, the sister chromatids separate from each other and move towards the centrioles. During this stage, the cell may look like this:

The spindle fibers pull the sister chromatids apart, towards the two centrioles. Each chromosome is now called a daughter chromosome and is returned to the single structure state it was in prior to replication.

Telophase: During telophase, the actual division of the cell begins. The cell narrows and finally “pinches off� right down the middle between the centrioles. The spindle fibers dissolve and a full 46 chromosomes (23 pairs) end up in each of the two new cells:

When mitosis is complete, the result is two daughter cells which are each exactly like each other and the parent cell from which they were created. The daughter cells look exactly like the parent cell before replication of the chromosomes took place. Both daughter cells have 46 daughter chromosomes, or 23 pairs, with the exact same genetic information as the parent cell.

Meiosis

Meiosis is the process by which human sex cells (the egg and sperm) are created. A separate process is needed for the creation of sex cells because these cells differ from other cells in a very important way: they have only 23 chromosomes instead of the usual 46. Thus, in the creation of a sex cell, the chromosome number must be reduced by half.

Just as in mitosis, the replication of chromosomes to create a pair of sister chromatids has already taken place before the process begins:

Unlike in mitosis, though, the homologous chromosomes (those belonging to the same pair) come together before meiosis begins, forming a quartet-type of structure:

In this cell, only 2 of the 23 pairs of homologous chromosomes are represented.

Meiosis is divided into two phases of separate cell divisions: Meiosis I and Meiosis II. The first division separates the homologous chromosomes and reduces the number of chromosomes from 46 to 23. The second division separates the sister chromatids. Each of the phases of division has 4 stages like those in mitosis: prophase, metaphase, anaphase, and telophase.

Prophase I: During prophase I of meiosis I, the chromosomes become thicker and more visible. It is also during this stage that a phenomenon called crossing over occurs. Crossing over involves the equal exchange of genetic information between homologous chromosomes. This can occur at more than one location, and can involve any amount of genetic information (large or small portion of the chromosome). The end result is that each chromosome in the quartet structure is slightly different from one another:

In addition to the crossing over between homologous chromosomes during prophase, we also see the appearance of centrioles (the purple structures).

Metaphase I : During metaphase I, the chromosomes line up at the center, much as in metaphase of mitosis. Also as in mitosis, the spindle fibers are seen attached to the chromosomes:

Anaphase I: During anaphase I, the homologous chromosomes are pulled apart by the spindle fibers and move towards the centrioles:

Telophase I : During telophase I, the chromosomes are divided into two separate cells. The centrioles and spindle fibers disappear. At this point, there are now two cells. Each cell got one homologous chromosome of the pair. The chromosomes themselves are still replicated and paired with their sister chromatids:

Each cell contains 23 chromosomes, one from each homologous pair. Each of those 23 chromosomes, however, is paired with its sister chromatid. So, essentially, there is 46 chromosomes' worth of genetic material in each cell.

Meiosis II : Prophase II, Metaphase II, Anaphase II, and Telophase II proceed much as their respective stages in mitosis. The stages occur at the same time in both new cells:

The end result of meiosis is 4 new daughter cells, each with only 23 chromosomes (one chromosome from each of the homologous pairs). As a result of crossing over, none of the cells are exactly alike. Their chromosome number is exactly the same, but the genetic information that is located on those chromosomes is variable from cell to cell.

 

Since these are sex cells, these are the cells that could potentially be used in fertilization. These are the cells that could pass genetic information on to the next generation; and each of them is slightly different. So even though each of these cells would come from the same parent, the genetic information being passed on would be different depending on which cell is used to conceive a child. This is why crossover during prophase is so important -- it allows for genetic variation in a single individual's egg or sperm cells. It is partly due to crossover events that all human beings are so unique, even those born to the same parents.