Somatic cells refer to all cells in the body, with the exception of the reproductive cells. Gametes are the reproductive cells, namely the sperm and egg cells.

Somatic cells each have 46 chromosomes in total, with 23 chromosomes from each parent. Each somatic cell contains 23 homologous pairs and is considered diploid. Gametes have 23 chromosomes in total, one copy from each homologous pair, and are considered haploid. When two gametes fuse to form a zygote, each gamete contributes one copy of each chromosome. This results in a diploid zygote with 46 chromosomes.

Mitosis is the period of division for somatic cells and results in two identical copies of the parent cell. In contrast, meiosis is the formation of unique gametes from germ cells. Mitosis is broken into prophase, metaphase, anaphase, and telophase. Meiosis is broken into meiosis I and meiosis II, each of which contains all four phases of division.

During prophase, the nuclear envelope dissolves and chromosomes condense. The chromosomes migrate to the center of the cell and align during metaphase. The mitotic spindle fibers attach during metaphase and retract during anaphase, pulling sister chromatids from each chromosome to separate poles of the cell. During telophase, the chromatids are localized at the poles of the cell and cytosolic cleavage begins.

The chromosome will align along the center of the cell in metaphase, ready to be pulled apart into separate chromatids.

Mitosis begins in prophase, when chromosomes begin to condense and the nuclear envelope begins to dissolve. Chromosomes assemble at the equatorial plate during metaphase and are separated by spindle fibers during anaphase. During telophase, the chromosomes are enclosed in new nuclear envelopes. Finally, cytokinesis completes the division of the cell cytoplasm. Interphase refers to the portion of the cell cycle between mitotic divisions.

The chromosomes must be allowed to align at the center of the cell in order for the sister chromatids to be separated. The chromosomes align during metaphase, and spindle fibers separate the sister chromatids during anaphase. In order for these processes to occur, the nuclear membrane must be dissolved during the earliest mitotic phase: prophase. During prophase, DNA condenses into chromosomes, the nuclear membrane dissolves, and the chromosomes enter the cytoplasm.

Mitosis is the process that results in two identical daughter cells. The separation of sister chromatids is essential to ensure that both daughter cells receive a copy of each chromosome. The maintenance of ploidy is a way of describing that the daughter cells will have the same number of each chromosome as the parent cells.

Crossing over, or recombination, is a process that only takes place during meiosis and helps promote genetic diversity. 

The correct order of the stages of mitosis is as follows: prophase, prometaphase, metaphase, anaphase, telophase. Prometaphase is the stage of mitosis in which the nuclear membrane breaks down and the mitotic spindles begin to interact with the kinetochore region of sister chromatids, beginning the process of lining up the chromosomes along central plane of the cell. Chromosomes condense in prophase. During metaphase, the chromosomes line up along the metaphase plate, and during anaphase, the sister chromatids are pulled apart to opposite poles of the cell. 

Centrioles are composed of nine triplets of microtubules arranged in a ring. A pair of centrioles makes up a centrosome, which is important in the formation of mitotic spindles in mitosis and meiosis and in the organization of microtubules in the cytoplasm. Cilia are also made of microtubules, but arranged in a different way. Cilia have nine pairs of microtubules around the edge of the cilium, and another pair of microtubules in the middle of the cilium.

Mitotic spindles attach to the kinetochore region of the centromere, the area of a chromosome that links sister chromatids. Centromeres are not always in the center of a chromosome, sister chromatids can be metacentric, acrocentric, or telocentric where the centromere is located in the middle, towards one end of a chromosome, or at the end of a chromosome arm, respectively. In eukaryotes, centromeres contain repeating DNA sequences in a highly compacted state. The kinetochore is a protein complex that assembles at the centromere that is able to bind to both the centromeric DNA and to mitotic spindles. Each sister chromatid has it’s own kinetochore.

During cytokinesis, the cell physically divides into two daughter cells. In animals, this is done through the formation of a contractile ring, composed of myosin and actin filaments, that forms at the cell equator. The filaments contract to form a cleavage furrow, where the cell membrane begins to invaginate and eventually pinches off to form two daughter cells. Note that the myosin found in the contractile ring in different from that in muscle cells. Also note that in animals cells, cytokinesis involves the formation of a cell plate, rather than a contractile ring.

Motor proteins are a class of proteins that use energy from ATP hydrolysis to move along a substrate. Two classes of motor proteins control movement along microtubules: kinesins and dyneins. Kinesins and dyneins differ from one another in the direction of their movement; kinesins “walk” towards the positive end of the microtubule and dyneins move towards the negative end. During mitosis, mitotic spindles originate from the centrosomes. The end of the microtubule at the centrosome is the negative end, while the end attached to the kinetochore is the positive end. Thus, to move a sister chromatid to a cell pole, towards the negative end of a microtubule, a dynein is involved.