The Cell Cycle

In the cell cycle, cell division functions in reproduction, growth, and repair. The two forms of cell division are mitosis and meiosis. Mitosis requires the distribution of identical DNA to two daughter cells. Meiosis yields four nonidentical daughter cells, each with half the chromosomes of the parent. The purpose of meiosis is to produce cells for reproduction, and as you want the offspring to have the same number of chromosomes as the parents’ sex cells need to have half the number of chromosomes as the parents. A cell duplicates its DNA, moves the two copies to opposite ends of the cell, and then divides into two cells.  This genetic information that makes up the cell’s genetic information is called its genome.

The genome is made up chromosomes. Every eukaryotic species has a characteristic number of chromosomes (packaged DNA) in each cell nucleus. Eukaryotic chromosomes are made of chromatin which is a complex of DNA and protein. When a cell is not dividing chromosomes are chromatin fiber in the nucleus. Each duplicated chromosome consists of two sister chromatids, which appear when cell divisions are about to take place. The chromatids are attached at what is called the centromere. After the chromatids divide mitosis is followed by division of the cytoplasm or cytokinesis.

The mitotic phase alternates with interphase in the cell cycle. During interphase, the cell grows by producing proteins and cytoplasmic organelles, copies its chromosomes, and prepares for cell division. Interphase has three subphases: the G1 phase (“first gap”), the S phase (“synthesis”), and the G2 phase (“second gap”). Chromosomes are duplicated only during the S phase.

cell_cycle_simple

Mitosis itself broken into five subphases: prophase, prometaphase, metaphase, anaphase, and telophase. In prophase, the chromosomes are tightly coiled from the previous cell division. During prometaphase, the nuclear envelope fragments, and microtubules from the spindle interact with the chromosomes. The microtubules are what end up pulling the chromosomes to opposite ends of the cell. Each of the two chromatids of a chromosome has a kinetochore (a specialized protein structure located at the centromere). Kinetochore microtubules from each pole attach to one of two kinetochores. The spindle fibers push the sister chromatids until they are in a line at the metaphase plate, this is metaphase. At telophase, nuclei begin to form at the two poles. Cytokinesis, the division of the cytoplasm, is usually underway by late telophase. In animal cells, cytokinesis involves the formation of a cleavage furrow, which pinches the cell in two. In plant cells, vesicles from the Golgi apparatus make a cell plate in the middle of the cell.

mitosis

During interphase, the single centrosome forms two centrosomes and by the end of prometaphase, these are at opposite ends of the cell. This marks the start of mitosis. Centrosomes are organelles that organize the cell’s microtubules. Microtubules (called asters) extend from each centrosome and attached to the kinetochores. By metaphase, the microtubules of the asters have grown and are in contact with the plasma membrane. Anaphase ends when the proteins holding the sister chromatids together are inactivated so that they can separate. The cell is also lengthened by microtubules from the centromeres that are not attached to chromatids. The last step in mitosis is telophase in which the chromatids move to opposite ends of the cell and start to form two nuclei. During telophase in plants, vesicles form a cell plate, the plate enlarges until its membranes fuse with the plasma membrane. After mitosis cytokinesis follows in which the cytoplasm divides into two cells. In animal cells, cytokinesis occurs by a process called cleavage of which the first sign is, the appearance of a cleavage furrow.

interphase_mitosis

Mitosis in eukaryotes may have evolved from binary fission in bacteria. Prokaryotes reproduce by binary fission, not mitosis, which is very similar but bacteria don’t have membrane-bound organelles including a nucleus that they have to copy. The mechanism behind the movement of the bacterial chromosome is becoming clearer but is still not fully understood. As eukaryotes evolved, the ancestral process of binary fission gave rise to mitosis.

The frequency of cell division varies with cell type. Nerve and muscle cells do not divide after maturity. Fusion of a cell in mitosis (M phase) with one in interphase (even G1 phase) causes the second cell to enter mitosis. A checkpoint in the cell cycle is a critical control point where stop and go-ahead signals regulate the cycle. The signals are transmitted within the cell by signal transduction pathways. Three major checkpoints are found in the G1, G2, and M phases. If the cell receives a go-ahead signal at the G1 checkpoint, it completes the cell cycle and divides but, If it does not receive a go-ahead signal, the cell exits the cycle and switches to a nondividing state, the G0 phase. The M phase checkpoint ensures that all the chromosomes are properly attached to the spindle at the metaphase plate before anaphase which, ensures that daughter cells do not end up with missing or extra chromosomes.

Cancer cells have escaped from cell cycle controls. Cancer cells divide excessively and invade other tissues because they are free of the body’s control mechanisms. The abnormal behavior of cancer cells begins when a single cell in a tissue undergoes a transformation that converts it from a normal cell to a cancer cell. The immune system recognizes and destroys transformed cells but, cells that escape destruction reproduce to form a tumor (a mass of abnormal cells). If the abnormal cells remain at the originating site, the lump is called a benign tumor but if the cells don’t it’s a malignant tumor. Cancer cells often lose attachment to nearby cells, are carried by the blood and lymph system to other tissues, and start more tumors in an event called metastasis. Treatments for metastasizing cancers include high-energy radiation and chemotherapy with toxic drugs.

 

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