Wednesday, January 26, 2011

Cells: They Reproduce

Well...I'm back. It's been a while.

DISCLAIMER: This material is confusing. I've done my best to explain it in a clear fashion, but there might be a few places where readers may get lost. My apologies! 

Recently, I've been studying the various ways in which cells divide and reproduce. There are two primary methods through which this is accomplished: mitosis and meiosis. On the surface, the differences are somewhat slight: one produces two cells with two pair of chromosomes; the other, four cells with only twenty-three chromosomes each. 

Now, I decided that I was going to stop starting paragraphs with, "Let's take a deeper look at each." With that said, I will instead finish this paragraph with: Let's take a deeper look at each.

To understand mitosis, one must comprehend the cell cycle. After one series of mitosis has ended, the daughter cells enter what is known as "G1," in which all that happens is growth of the cell. Then, during the next period of time, the chromosomes duplicate, causing this phase to be known as "synthesis." Another phase of growth, this time, "G2," occurs. The previous three phases are collectively known as "interphase."

Now, the mitotic cycle starts. This is where it gets really interesting. The chromosomes become visible under a microscope and the nucleolus dissolves. This period of time is known as "prophase." Now, the chromosomes appear in an X shape because their duplicates formed during the synthesis phase are joined in the middle, along what is known as the centromere. Next, the sister chromatids line up along the middle of a cell, known as the metaphase plate, as this time is called "metaphase." Then, the sister chromatids are pulled apart along the centromeres by fibers emitted from the centrioles (poles at both ends of the cell), and the chromosomes head to opposite ends of the cell in "anaphase." Finally, the cell's membrane splits the cell into two distinct cells. This is known as "telophase."

Meiosis has a few differences. First off, the goal of meiosis is to produce a cell with only twenty-three chromosomes so that it can share its chromosomes with another cell in order to produce a cell with unique genes that is then capable of developing into a baby of the species. Therefore, the cells undergo one more division than they do in mitosis.

Now, meiosis starts out just like mitosis does, with the chromosomes replicating and then condensing. However, the first difference comes in what is known as "Prophase I." Here, the condensed chromosomes pair up with their corresponding chromosomes (remember, each cell has a two sets of chromosomes). While they are paired up, enzymes cut sections of DNA from each chromosome and exchanges it with the other. This allows genes (more on those later!) to be transferred between the strands.

Then, the centrioles attach to the pairs of chromosomes--fibers from both centrioles to 23 chromosomes. The centrioles pull the chromosomes (as in metaphase), but instead of lining up along the metaphase plate, the chromosomes line up so that the pairs of chromosomes are divided by the plate. The pairs of chromosomes are now separated as one member of each pair is pulled to both sides of the cell. The sister chromatids, however, are still attached. The sister chromatids arrive at opposite ends of the cell, and nuclei form around them. Telophase I occurs and the cell divides into two cells, each with one set of 23 chromosomes that were duplicated during the synthesis phase.

So, quick recap: Originally, there were two pairs of 23 chromosomes. Each pair duplicated, creating four pairs. Then, the cell divided, creating two cells, each with two pairs of 23 chromosomes.

Now, the two cells basically perform mitosis again. The chromosomes condense into chromatids, line up along the metaphase pate, divide along the centromere, and a new membrane forms. Because each cell (which had two set of 23 chromosomes) has now divided into two, there are now four cells with one set of 23 chromosomes--the original goal of meiosis.

Well, that's that! See you again soon! 

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