All living things are made up of cells, and depending on how complex the organism is, it may have one cell, like a bacterium, or trillions of cells, like humans. The question is, how does a fertilized human embryo with one cell develop into an adult with 5 trillion cells? The answer is in a process referred to as cell division. A single fertilized human egg cell will divide to produce two cells. The two cells have the same genetic information as each other and the parent cell. The cells will continue to divide until trillions of cells are produced. This is how a fertilized egg grows into a baby, and then an adult. This division of cells is referred to as mitosis.
During the cell cycle, a cell grows, prepares for division, and divides to create two daughter cells. The three main stages of the cell cycle are interphase, mitosis, and cytokinesis. Cells spend most of their lives, about 78 percent of the time, not dividing in a phase called interphase. During interphase the cells get larger, replicate their chromosomes for cell division, synthesize substances that are essential to cell division, and build materials for growth and development. Cells in interphase have an easily seen nucleus and nucleolus, and the chromosomes appear as short strands within the nucleus.
During interphase the chromosomes, which are composed of DNA, are replicated before the cell divides so an exact copy of every chromosome is ready for each daughter cell. During mitosis the chromosomes in a cell are split apart when the cell divides. Chromosomes contain the genetic information that is passed from one generation to the next. The genetic information is DNA which is like the blueprints for a cell. The DNA is identical in all cells of the body, but it is expressed differently in different types of cells. In hair cells the DNA codes for the color of the hair. In eye cells it codes for the color of your eyes. DNA also instructs each well when to divide, and how to function.
Mitosis occurs in four phases. The first phase is prophase. Cells in prophase have an enlarged nucleus, the nucleolus is no longer visible, and the chromosomes appear as short jumbled strands within the nucleus. Cells spend about 14 percent of the cell cycle in prophase. This is the next largest amount of time spent in a phase after interphase. This phase takes longer than the others because the nuclear envelope fragments and the microtubules have to attach to the chromosomes. Metaphase follows prophase. Cells in metaphase have the chromosomes, which appear as long thin strands under the microscope, lined up along the center of the cell. Metaphase takes about 4 percent of the time required for the completion of a cell cycle. This is one of the quickest phases because all that occurs is the lining up of the chromosomes at the center of the cell. The third phase is anaphase. This phase only takes about 0.8 percent of the cell cycle to complete, which is the shortest time required out of all the phases. During anaphase the sister chromatids, or the two identical parts of a chromosome, are pulled apart to opposite ends of the cell by the spindle fibers. This allows each daughter cell to have an identical copy of each of the original cell’s chromosomes. The last stage of mitosis is telophase. Telophase takes up about 3 percent of the cell cycle. This is also a short phase. During telophase the chromosomes appear at the opposite ends of the cell and a new nuclear membrane begins to form around the chromosomes in each half of the cell. Cytokinesis occurs at the end of mitosis as the cytoplasm divides and two distinct cells are formed. In plant cells, a cell plate forms halfway between the divided nuclei, and then a cell wall appears in the cell plate. In animal cells, the cell membrane moves inward until the cytoplasm in pinched in half. Each half has its own nucleus and cytoplasmic organelles.
Meiosis reduces the chromosomal number by half to form four haploid gametes, or reproductive cells, that are genetically different. When gametes combine during fertilization the cell becomes diploid. During the first half of meiosis, homologous chromosomes separate. During prophase I crossing-over occurs. This results in the exchange of genetic material between homologous chromosomes. During meiosis I the number of chromosomes is split in half and the resulting cells are haploid. During meiosis II the two chromatids of each chromosomes split and the resulting cells are haploid. At the end of meiosis there are four haploid cells. Meiosis increases genetic variation in organisms through independent assortment, crossing-over, and random fertilization.
