MCAT - Cell Reproduction and Mitosis - Info. For Those Seeking Entry into Medical School

The MCAT is the exam taken by those seeking entry into medical school. This exam is long and grueling, requiring a lot of information. I took the exam two years ago and earned a score of 41 out of 45. Knowing what you should know for the exam and in what detail you should know these things is very important. I wanted to share some information with those of you who will be taking this exam. The following information is what I felt I needed to know to answer questions regarding the cell reproductive cycle and mitosis. Again, knowing the following was what I found helpful. In writing this, I assume you have knowledge of certain terms, as those of you already thinking about taking this exam must have had a biology class or two.

Cells reproduce themselves by doubling everything in their cytoplasm, doubling their genome, and then splitting in half. Though cell reproduction is continuous, the cell cycle can be thought of as having four phases. The S phase, or "synthesis" phase, is the part of the cell cycle during which the cell replicates its genome. At the end of the S phase, the cell has two complete copies of its genome (or 2n x 2 chromosomes).

The M phase is the part of the cell cycle that includes mitosis and cytokinesis. Mitosis refers to the partitioning of cellular components into two equal halves, each half with its own set of chromosomes, organelles, and cytoplasm. Cytokinesis refers to the physical process of cell division, including division of the cell cytoplasm.

There are two phases, G1 and G2, between the M phase and the S phase. The cell undergoes growth during these two "growth" phases. They are also referred to as "gap" phases.

The two gap phases and the S phase are together referred to as "interphase." Interphase is the part of the cell cycle between cell divisions. Cells actually spend most of their time in interphase. Here, they are growing - synthesizing needed molecules. Furthermore, there are some cells that are in the interphase stage of the cell cycle permanently. Examples of such cells include neurons and red blood cells. More specialized cells such as these lose the ability to reproduce themselves.

Mitosis is the part of a cell's reproductive cycle that requires the most detailed knowledge. Mitosis itself is divided into four phases. Those phases are prophase, metaphase, anaphase, and telophase.

The visibility of the genome under a light microscope is a sign that prophase has begun. The genome is now visible because it has condensed into densely packed chromosomes. It is no longer the diffuse chromatin that is not visible via a light microscope. At the beginning of prophase, a human cell can be seen to have 46 chromosomes. Each chromosome consists of two identical parts. These two parts are joined at the centromere. The two identical parts are actually two copies of the same chromosome. There are two copies of each chromosome due to the replication of the DNA in the S phase. Each copy of the same chromosome is called a sister chromatid. Each chromosome has one homologue. Homologous chromosomes are non-identical but equivalent pieces of the genome. Thus, the 46 chromosome pairs seen are actually 23 homologous pairs of identical sister chromatid pairs.

The first phase of mitosis is prophase. The nucleolus disappears during prophase I. The nuclear envelope is broken down into tiny vesicles. The spindle and kinetochore fibers appear. The pairs of centrioles begin to move to opposite ends of the cell. The cell has to two asters, which is the term for the microtubule-organizing center.

The second phase of mitosis is metaphase. In metaphase, all the chromosomes form the metaphase plate by lining up at the center of the cell. The chromosomes align thusly because the kinetochore of each sister chromatid is attached to spindle fibers, which are attached to the asters at each end of the cell.

Anaphase is the third phase mitosis. During anaphase, the spindle fibers shorten, and the centromeres of each sister chromatid pair are pulled apart. The cell also elongates. A cleavage furrow indicates the beginning of cytokinesis. A cleavage furrow is the result of a ring of microfilaments encircling the cell and contracting.

The last phase of mitosis is telophase. During telophase, a nuclear membrane reforms around both sets of chromosomes at either end of the cell. The chromosomes de-condense. A nucleolus at each end of the cell becomes visible. Cytokinesis is completed as the cell (and the cytoplasm within) is split in two equal halves. Each (daughter) cell has a single full complement of DNA (2n chromosomes).