Cells divide in order to replace worn out or old cells, and to create reproductive cells, or gametes. Mitosis is the division of non-sex cells, or somatic cells. Meiosis is the process by which a parent cell can make gametes, or sex cells. In mitosis, a parent cell makes two identical daughter cells, which are called diploid (2n) cells because they have two copies of each chromosome. In meiosis, parent cells divide to make gametes that are haploid (n), with only one copy of each chromosome. Haploid gametes will unite in fertilization to make a diploid zygote which has the potential to become a new organism. – Learn meiosis vs. mitosis.
HUMAN CHROMOSOME NUMBER EXAMPLE
For example, in humans all somatic cells have 46 chromosomes, pairs of each of the 23 unique kinds of chromosomes. One set of the 23 chromosomes came from mom’s gamete and one set came from dad’s gamete. They united in fertilization to make the 46 pairs in the original zygotic cell that divided to give rise to the individual. In the individual’s body, all the somatic cells go through mitosis to make diploid somatic cells, such as liver cells, kidney cells, and skin cells. A small group of parent cells go through meiosis instead, to make haploid gametes for future reproduction.
Including Interphase and Cytokinesis, there are 6 stages in the mitotic cell cycle. In order, the three major portions are Interphase, Mitosis, and Cytokinesis, and Mitosis consists of 4 separate steps.
INTERPHASE
Interphase means “between phases” and is literally the stage between division of the cell and nucleus. Interphase is largely a time of cellular growth and development, where the cell performs its normal functions such as protein synthesis, photosynthesis, and cellular respiration, may increase in size, and gets itself ready to go through mitosis and cytokinesis. Interphase has 3 portions to it – G1, or Gap 1, where the cell largely concerns itself with growing, S which refers to Synthesis, where the cell duplicates its genetic material, and G2 or Gap 2, where the cell checks to see if it is ready to move onto Mitosis.
MITOSIS – PROPHASE
Mitosis begins with Prophase, or “first phase.” Prophase is when the nuclear membrane around the genetic material dissolves. The genetic material, or DNA, condenses from stringy chromatin into shorter, more compact chromosomes. These are X-shaped in this stage because the two identical copies from the S part of Interphase remain attached for now. The point at which they are attached is known as the Centromere. In prophase, organelles known as centrioles also begin migrating to the poles of the cell to begin growing the spindle fibers. The spindle fibers make up the spindle apparatus when complete, and are responsible for sorting and moving the chromosomes around the cell.
MITOSIS – METAPHASE
Metaphase, or “middle phase” happens next. Metaphase is easy to observe under the microscope, as all the chromosomes are arranged down the center, or equator, of the parent cell. Spindle fibers complete growing and attach to the centromere of each X-shaped chromosome.
MITOSIS – ANAPHASE
The third phase of Mitosis is Anaphase, or “apart phase.” In anaphase, the centromeres divide and chromosomes are no longer X-shaped. Single copies of each chromosome are pulled by the spindle fibers to the poles of the cells.
MITOSIS – TELOPHASE
Finally, mitosis is completed with Telophase, or the “end phase.” In Telophase, the two new nuclei have formed, each with a complete set of DNA, and a new nuclear membrane forms around each set.
CYTOKINESIS
The final stage of cell division is Cytokinesis, which literally translates to “cell splitting.” Cytokinesis can begin in Telophase, but is thought to be a separate step not included in mitosis. In animal cells, because of the lack of a cell wall, cytokinesis occurs when the cell membrane pinches in, forming a cleavage furrow. This cleavage furrow deepens until it pinches the parent cell in half, making two identical daughter cells. In plant cells, the parent cell performs cytokinesis when the new cell wall begins to grow across the center, dividing the cell in half, each half complete with a nucleus and DNA. This newly forming cell wall is referred to as a cell plate, and it grows completely across. The cell membrane is divided by this new wall, and the parent plant cell is divided into two identical daughter cells.
Cytokinesis completes the mitotic life cycle of a cell. One parent cell has grown, duplicated its DNA, readied itself for division, split the nuclear contents during mitosis, and split the cell itself through cytokinesis.
