The Process of Mitosis and Meiosis

Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells – sperm in males, eggs in females.During the process of meiosis one cell divides two times to form four daughter cells.

These four daughter cells only have half the number of chromosomes of the parent cell which are called haploids. Meiosis produces our sex cells or gametes which are (eggs in females and sperm in males). Meiosis can be divided into nine stages. These are divided between the first time the cell divides (meiosis I) and the second time it divides (meiosis II): Meiosis I

1. Interphase: First, the DNA in the cell is copied resulting in two identical full sets of chromosomes. Then. outside of the nucleus are two centrosomes with each containing a pair of centrioles, these structures are critical for the process of cell division. During the process of interphase, microtubules extend from these centrosomes.
2. Prophase I: The copied chromosomes shortened into X-shaped structures that can be easily seen under a microscope. Then every other chromosome is composed of two sister chromatids having identical genetic information. The chromosomes pair up so that both copies of chromosome one are together, both copies of chromosome two are also together. The pairs of chromosomes may then interchange small parts of DNA in a process called recombination or crossing over. At the end of Prophase one, the membrane around the nucleus in the cell dissolves away, releasing the chromosomes. The meiotic spindle, consisting of microtubules and other proteins, extends across the cell between the centrioles.
3. Metaphase I: The chromosome pairs line up next to each other along the center which is the (equator) of the cell. The centrioles are now at opposites poles of the cell with the meiotic spindles increasing from them. Then the meiotic spindle fibers attach to one chromosome of each pair.
4. Anaphase I: Then the pair of chromosomes are pulled apart by the meiotic spindle, that pulls one chromosome to one pole of the cell and the other chromosome to the opposite pole. In meiosis one, the sister chromatids stay together which is different from what happens in mitosis and meiosis II.
5. Telophase I and cytokinesis: The chromosomes finished their move to the opposite poles of the cell. At each pole of the cell, a full set of chromosomes get together. A membrane forms around each set of chromosomes to create two new nuclei.Then single cell pinches in the middle to form two separate daughter cells in which each contains a full set of chromosomes within a nucleus. This process is known to be cytokinesis.Meiosis II
6. Prophase II: At this point, there are two daughter cells with 23 chromosomes each (23 pairs of chromatids). In every two daughter cells, the chromosomes condense again into visible X-shaped structures that can be easily seen under a microscope. The membrane around the nucleus in each daughter cell dissolves away releasing the chromosomes. Then the centrioles duplicate and the meiotic spindle forms again.
7. Metaphase II: In each of the two daughter cells the chromosomes (a pair of sister chromatids) line up end-to-end along the equator of the cell. Now the centrioles are at opposites poles in each of the daughter cells. Meiotic spindle fibers at each pole of the cell attach to each of the sister chromatids.
8. Anaphase II: The sister chromatids are then pulled to opposite poles due to the action of the meiotic spindle. The separated chromatids are then individual chromosomes.
9. Telophase II and cytokinesis: Finally the chromosomes complete their move to the opposite poles of the cell. At each pole of the cell, a full set of chromosomes get together. A membrane forms around each set of chromosomes to make two new cell nuclei. This is the last phase of meiosis, however, cell division is still not complete without another round of cytokinesis. Once cytokinesis is complete there are four granddaughter cells, each with half a set of chromosomes (haploid): in males, these four cells are all sperm cells in females, one of the cells is an egg cell while the other three are polar bodies which are (small cells that do not develop into eggs).

Mitosis: Mitosis is the process where a eukaryotic cell nucleus separates in two, followed by division of the parent cell into two daughter cells. The word “mitosis” means “threads,” and it refers to the thread-like appearance of chromosomes as the cell prepares to separate. These tubules, collectively known as the spindle, expand from structures called centrosomes, with one centrosome located at each of the opposite ends, or poles, of a cell. As mitosis starts progressing, the microtubules join to the chromosomes, which have already duplicate their DNA and have come together across the center of the cell. The spindle tubules then contract and move toward the poles of the cell. As they move, they pull the one copy of each chromosome with them to opposite poles of the cell. This process ensures that each daughter cell will contain one exact copy of the parent cell DNA.

Mitosis is composed of five morphologically distinct phases: prophase, prometaphase, metaphase, anaphase, and telophase. Each one of these phases involves characteristic steps in the process of chromosome alignment and separation. Once mitosis is complete, the entire cell divides in two by way of the process called cytokinesisProphase which is the first stage in mitosis, occurring after the conclusion of the G2 portion of interphase. During prophase, the parent cell chromosomes, which were duplicated during S phase — condense and become thousands of times more compact than they were during interphase. Due to the fact that each duplicated chromosome consists of two identical sister chromatids joined at a point called the centromere, these structures now appear as X-shaped bodies when viewed under a microscope. Several DNA binding proteins catalyze the condensation process, including cohesin and condensin. Cohesin forms rings that hold together the sister chromatids, whereas condensin forms ring that coil the chromosomes into highly compact forms. Also, the mitotic spindle begins to develop during prophase. As the cell’s two centrosomes move against opposite poles, microtubules gradually assemble between them, forming the network that will later pull the duplicated chromosomes apart.

After prophase is complete, the cell enters prometaphase which is the second stage of mitosis. During prometaphase, phosphorylation of nuclear lamins by M-CDK causes the nuclear membrane to break down into numerous small vesicles. Therefore, the spindle microtubules now have direct access to the genetic material of the cell. Each microtubule is highly dynamic, growing outward from the centrosome and falling down backward as it tries to locate a chromosome. Ultimately, the microtubules find their targets and connect to each chromosome at its kinetochore, a complex of proteins positioned at the centromere. The actual number of microtubules that link to a kinetochore varies between species, but at least one microtubule from each pole link to the kinetochore of each chromosome. A tug-of-war then ensues as the chromosomes move back and forth between the two poles.

As prometaphase ends and metaphase begins, the chromosomes adjust along the cell equator. Every chromosome has at least two microtubules extending from its kinetochore and with at least one microtubule connected to each pole. At this point, the tension within the cell becomes balanced, and the chromosomes will no longer move back and forth. In addition, the spindle is now complete, and three groups of spindle microtubules are apparent. Kinetochore microtubules connect the chromosomes to the spindle pole; interpolar microtubules develop from the spindle pole across the equator, almost to the opposite spindle pole; and astral microtubules develop from the spindle pole to the cell membrane. Metaphase then leads to anaphase, during which each chromosome sister chromatids separate and move to opposite poles of the cell. Enzymatic breakdown of cohesin — which linked the sister chromatids together during prophase — causes this separation to occur. Upon separation, every chromatid becomes an independent chromosome. Changes in microtubule length provide the mechanism for chromosome movement. More specifically, in the first part of anaphase — sometimes called anaphase A — the kinetochore microtubules shorten and draw the chromosomes against the spindle poles. Then, in the second part of anaphase — at times called anaphase B — the astral microtubules that are anchored to the cell membrane pull the poles further apart and the interpolar microtubules slide past each other, exerting an additional pull on the chromosomes. During telophase process, the chromosomes arrive at the cell poles, the mitotic spindle disassembles, and the vesicles that include fragments of the original nuclear membrane assemble around the two sets of chromosomes. Phosphatases then dephosphorylate the lamins at every end of the cell. This dephosphorylation results in the development of a new nuclear membrane around each group of chromosomes.

Cytokinesis is the physical process that finally splits the parent cell into two identical daughter cells. During this process, the cell membrane pinches in at the cell equator, forming a cleft called the cleavage furrow. The position of the furrow will depend on the position of the astral and interpolar microtubules during anaphase. The cleavage furrow then forms because of the action of a contractile ring of overlapping actin and myosin filaments. As the actin and myosin filaments move passing each other, the contractile ring becomes smaller, akin to pulling a drawstring at the top of a purse. When the ring reaches its smallest point, the cleavage furrow completely bisects the cell at its center, resulting in two separate daughter cells of the same size.