The purpose of mitosis is to create two genetically identical daughter cells (one preexisting cell to two cells). If mitosis goes correctly, a human cell will not only divide to create two daughter cells, each with 46 chromosomes, but each and every gene will be transferred to the new daughter cells. Mitosis is the process in which chromosomes are equally divided into each of two daughter cells. Mitosis is neither the doubling of DNA (happens during the S phase of the cell cycle) nor the creation of two new cells (finally occurs during cytokinesis, after mitosis).

Where does mitosis occur? In all somatic cells (body cells not involved with sexual reproduction). Meiosis is the process that creates germ cells (the gametes, or sperm and egg in humans).

What is divided during mitosis? DNA, which is the genetic material of living things. DNA is packaged in different forms during the life of a cell and it is necessary to keep these various terms separate. Learn these terms early and you will save yourself a great deal of frustration.

1. Chromosome (literally, "colored body") was named after they were first observed during early mitosis in the early 1900s. DNA coils itself around proteins to condense itself when it is not actively being expressed, and this trend towards condensing is extremely noticeable during mitosis. All of our human genes are contained within 46 "volumes" of information, similar to a set of encyclopedias. A chromosome is a single molecule of DNA, so the DNA within one of your cells (excluding sperm and egg) exists as 46 separate molecules.

2. Chromatin DNA, wrapped around its constituent proteins, is called chromatin.

3. Sister Chromatids Notice how this sounds like "kid," which is how I remember the term. When a chromosome is copied during the S phase of the cell cycle, the two copies of DNA ‘stick together’ at the centromere (an area, often near the center of the coiled chromosome, made of high condensed DNA and proteins). Although the X-looking figure often drawing during mitosis is really two identical copies of the same chromosome, by convention we say an "X" is one chromosome, made of two sister chromatids. When they separate during anaphase, we say they are no longer sister chromatids, but rather full chromosomes.

A note on human chromosome numbers: During this next quarter, we will often look at human genetics so it is best to get to know some specifics of the human genome. A ‘normal’ human has 46 chromosomes in their somatic cells. These come in 23 pairs - one set from mom, one set from dad. Our somatic cells (skin cells, for example) are said to be diploid ("di" for two) because we have two copies of each chromosome. Sperm and egg cells, in contrast, have only one set of chromosomes and are said to be haploid.

Figure 9.3 of your book illustrates the cell cycle of a general cell. Realize that this is not accurate for all cells; nerve cells spend most of their time in the G1 phase while as the cells lining your intestines are replaced every 5 days, so they spend more time in mitosis. The cell cycle has two general phases: interphase and mitosis (some texts also add a third phases, cytokinesis, as a distinct phase).

Interphase: Interphase is usually the longest portion of the cell cycle.

It starts with the G1 (growth 1) phase, during which cell undergo normal cell functions.

During the S (synthesis) phase, the cell replicates it DNA. Notice DNA is copied before

mitosis begins!

G2 phase occurs after the DNA is copied, it is a growth phase used to prepare the cell for division.

Mitosis: The four stages of mitosis will be covered in more detail in the following section.

Cytokinesis: At the end of mitosis, the cell still looks like a dumbbell. It is not divided until it leaves mitosis and completes cytokinesis, which is the division of the cell into two daughter cells. Mitosis only serves to divide all chromosomes equally into the two new cells, not the complete process of creating two new cells! In animal cells, cytokinesis involves the formation of a cleavage furrow, in which the cell membrane and cytoplasm pinch together to separate the two new cells. In plants, the cell wall will begin to grow across the gap, creating a cell plate that separates the two new nuclei.

Mitosis consists of four stages: prophase, metaphase, anaphase and telophase (PMAT). We will not spend too much time in class reviewing the stages, so if you don’t recall them be sure to review these two pages from the text. Know that spindle fibers are made from microtubules (during prophase, the existing cytoskeleton of the cell breaks down and the tubulin subunits are rearranged into spindle fibers which, along with motor proteins, are used to motor the process of mitosis); the differences between chromosomes, chromatin, chromatids and DNA(during mitosis, the chromosomes are visible because they are so condensed by coiling with their associated proteins, such as histones); and the terms centriole and centromere. Remember, mitosis ends with the two daughter cells still connected; they are not separated until cytokinesis, which occurs after mitosis!

In plant cells, the two daughter cells won’t split until a cell plate (a new cell wall) begins to form between the two cells. When looking at cells under a microscope, the existence of a full cell wall between the two cells indicates that telophase is over.

In animals, a cleavage furrow is noticeable as microfilaments (not larger microtubules used for the spindle fibers) begin to constrict under the cell membrane. Ultimately, the cells will pinch off, creating two genetically identical daughter cells.

For most cells, interphase is going to be the longest period of the cell cycle. For cells such as mature neurons, they will remain in the G1 (growth 1) phase for as long as they are alive. During G1, cells perform normal cell activities not associated with cell division. If a cell is going to divide, it must go through S phase and G2 in preparation for mitosis.

Eukaryotic DNA goes through a very specific dramatic shape conformation during cell replication. First, as the cell enters prophase, the DNA wraps tightly around proteins such as histones. These proteins act as spools around which DNA is wrapped. Each ‘spool,’ consisting of DNA and protein, is known as a nucleosome. As the DNA begins to coil, it looks like a beaded necklace. As the coiling continues, it will get more tightly coiled and will attain its characteristic ‘X’ shape late in prophase. Often near the center, a tightly wound area known as the centromere will be visible. At the centromere, a kinetochore will serve as the attachment location for the spindle fibers.

Once the spindle fibers are attached to the kinetochores, equal division of chromosomes can continue. The sister chromatids are separated by two methods: 1) the spindle fibers will shorten between the centrioles and the centromere because the tubulin subunits will be disassembled as the chromosome moves along the length of the spindle fiber, 2) some spindle fibers extend beyond the chromosomes and will actually push against each other, leading to a lengthening of the cell in anaphase and the ultimate separation of sister chromatids.

Clearly, the role of spindle fibers is very important in the process of cell division. Because cancer cells are going through rapid cell division, drugs that target the mechanism of spindle fiber formation can be effective in treating some forms of cancer (of course, these drugs will adversely affect any cells going through rapid mitosis, such as bone marrow cells, lining of the gut and hair follicles).