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Cell Division Cells are considered the “building blocks” of life. All living organisms, from the tiniest single­celled bacteria to the enormous whale shark, are made up of tiny units called ​
cells​
. All multicellular organisms are composed of eukaryotic cells. A ​
eukaryotic cell​
has an organized nucleus surrounded by a nuclear membrane. (A prokaryotic cell does not have a nucleus.) A cell’s control center is its nucleus. The nucleus controls the cell’s ability to grow and reproduce. The cell’s chromosomes, located in the nucleus, contain “directions” for performing these processes. When a cell is not dividing, its chromosomes are not visible. As a cell begins to divide, the chromatin in the nucleus condenses to form chromosomes. The ​
cell cycle​
is the recurring process by which cells grow and reproduce. It is called a cycle because there is no beginning or end. After cytokinesis occurs and a cell splits into two daughter cells, each new daughter cell (unless some problem is discovered during the cycle) goes through the cell cycle. The cell cycle involves three major phases: interphase, division of the nucleus (mitosis or meiosis), and cytokinesis. A cell’s life is not evenly divided between the three phases. In fact, cells spend 90 percent of their life cycle in a growth phase called ​
interphase​
. Interphase consists of three phases. The length of these phases depends on the type of cell involved in the cell cycle. During the ​
G1 (Gap 1) phase​
, the cell grows and carries out normal cell functions. During the ​
S (synthesis) phase​
, the cell continues to grow and its chromosomes are copied, doubling the cell's DNA. As a result, at this point the cell contains enough genetic material for two cells. The centrioles also duplicate. The ​
G2 (Gap 2) phase takes place after the DNA has been replicated but before the cell's nucleus begins to divide. During the G2 phase, the cell continues to grow, and it produces chemicals it will need in order to divide. The phase after interphase in the cell cycle for most cells is mitosis. During ​
mitosis​
nuclear division occurs as a “parent cell” begins to form two “daughter cells” that are identical to the parent cell. Mitosis involves four stages. (Some sources include a fifth stage called prometaphase, which is actually a combination of activities that occur at the end of prophase and at the beginning of metaphase.) During the first stage, ​
prophase​
, chromatin condenses, or thickens, and becomes more coiled to form chromosomes. Each chromosome consists of two identical rod­shaped sister chromatids joined together at the centromere. Structures called centrioles move toward opposite ends of the cell, and spindle fibers begin to form between them. Also, the nuclear membrane and nucleolus begin to disintegrate. During ​
metaphase​
the chromosomes, which are attached to the spindle fibers, line up in the middle of the cell. At this point the cell enters the stage of mitosis called anaphase. During anaphase​
the spindle fibers separate the chromatids, now considered “daughter chromosomes,” and pull them to opposite ends of the cell. During the final stage, ​
telophase​
, a nuclear membrane begins to form around the chromosomes at the opposite ends of the cell. The chromosomes start to uncoil into thin strands of chromatin. In order for two separate identical daughter cells to exist, one more step must occur in this cell cycle: cytokinesis. During ​
cytokinesis​
, the cell’s cytoplasm divides and the cell splits in two. In an animal cell, cytokinesis involves the pinching in of the cell membrane by a contractile ring until the cell splits. Because plant cells have a stiff cell wall, a cell plate forms across the center of the cell and eventually fuses with the cell membrane on both sides of the cell. A new cell membrane and cell wall form along the plate, and the two daughter cells are able to separate. Note that cytokinesis occurs after mitosis, but in meiosis it occurs after telophase I and again after telophase II. Meiosis​
is a process that reduces a parent cell’s number of chromosomes by half to produce sex cells (gametes). When a male and female of a species come together to produce offspring, they must pass the correct number of chromosomes on to the next generation. Sex cells are haploid cells, which means they contain half the number of chromosomes that a body cell contains. For example, human body cells (somatic cells) have 46 chromosomes, so the human sex cell has 23 chromosomes. Meiosis produces sperm cells and egg cells needed for reproduction. Like mitosis, meiosis follows interphase, during which the chromosomes and centrioles become doubled. There are two stages of meiosis: meiosis I and meiosis II. During meiosis I, a cell halves its number of chromosomes. Meiosis I begins with prophase I. During prophase I, homologous chromosomes (each from a different parent), known as tetrads, line up side by side and exchange genetic material. As a result, the attached sister chromatids are no longer identical. This material exchange is called crossing over, or recombination. Next, during metaphase I the homologous chromosomes line up along the cell’s equator. During anaphase I, the homologous chromosomes separate from each other and move to opposite poles. The sister chromatids are still linked at their centromeres, but each tetrad is pulled apart. This means each new cell formed will have only one chromosome from the original pairs. Finally, in telophase I nuclear membranes form around the genetic material. This phase is followed by cytokinesis and interphase. The two new cells are haploid, each having half the number of chromosomes the original cell had. Meiosis II follows meiosis I. During meiosis II, the nuclei in the two cells formed during meiosis I divide, resulting in a total of four. Meiosis II begins with prophase II, when the chromosomes form and move toward the center of the cells. During metaphase II, the chromosomes line up on the metaphase plate, and spindle fibers from each pole attach to the nearest chromatids. During anaphase II, the chromatids separate and the sister chromatids are pulled to opposite poles of the cell. Finally, during the last phase, telophase II, a nuclear membrane forms around each group of chromosomes. After telophase II, cytokinesis occurs again and each of the two cells divides to produce two daughter cells. Each daughter cell contains half the number of chromosomes the original cell had (before meiosis I) and contains different genetic material due to crossing over. Now when the organism reproduces it will have the correct number of chromosomes and will have genetic variation.