Mitosis Mitosis is the process of formation of two identical cells (usually) for replication and division of chromosomes from the original resulting in a "copy" of it. Friday, January 12, 2007
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Mitosis Mitosis is the process of formation of two identical cells (usually) for replication and division of chromosomes from the original resulting in a "copy" of it.
eukaryotic cells have a higher number of chromosomes as the other hand are much larger than those of prokaryotes. Mitosis Mitosis is the process of formation of two identical cells (usually) for replication and division of chromosomes from the original resulting in a "copy" of it. The structure and condensed replicated chromosomes have several points of interest. The kinetochore is the point where "anchor" the spindle microtubules. Replicated chromosomes consist of two DNA molecules (along with their associated proteins: histones) that are known by the name of chromatids. The area where both chromatids are in contact is called the centromere, the kinetochore is on the outside of the centromere. It should be emphasized that the chromosomes are chromatin (histone DNA more) and noted the peculiarity that the ends of chromosomes (which are named after telomere) are repeated sequences of DNA.
Scheme of a chromosome. Modified from http://www.whfreeman.com/life/update/.
Depending on the position of the centromere of chromosomes are classified as:
A. telocentric, with the centromere at one end
B. acrocentric, one arm is very short
C. submetacentric, different lengths
D. metacentric, arms of equal length
Packaging
DNA DNA-associated proteins are collectively known by the name of histones. Relatively short polypeptides are positively charged (basic) and therefore are attracted to the negatively charged acid (DNA) Histones are synthesized in quantity during the S phase (S for synthesis) of the cell cycle. One of the functions of these proteins is related to DNA packaging in the form of chromosome: 2 meters of DNA from human cells are packaged in 46 chromosomes of a combined length of approximately 200 nm. The cell has about 90 million molecules of histones with the majority belonging to a type known as H1. Known five types of following histones (H1, H2A, H2B, H3 and H4, 8 molecules in total), with the exception of most H1 histones of eukaryotes are very similar.
Image modified from the University of Illinois' site DNA and Protein Synthesis.
The nucleosome is the fundamental unit of "bundling" of eukaryotic DNA. The "reel" ("core") of the same consists of two molecules of H2A, H2B, H3 and H4, around which DNA is wound twice (1). Histone 1 is outside the "reel." This level of packaging (packing) is known as "beads on a necklace" (2). The next level is called 30 nm fiber, whose details are not known organization completely. The fibers were subsequently condensed into loop domains of 300 nm (3). Domains are part of the condensed sections (4) (700 nm) of chromosomes (chromosome has a width of 1,400 nm at metaphase) (5).
During mitosis replicated chromosomes are positioned near the middle of the cell and then segregate so that each resulting cell receives a copy of each original chromosome (if you start with 46 chromosomes in the original cell ends up with 46 chromosomes resulting in 2 cells). To do this cells utilize microtubules (in this case together form the mitotic spindle) that "pull" of the chromosomes to bring to each future cell. Animal cells (except a group of worms known as the nematode) have centrioles. Plants and most other eukaryotes do not have centrioles and prokaryotes, of course, no spindle and centrioles, cell membrane in prokaryotes supplies this function to drag the chromosomes attached to it during the cytokinesis of binary fission. Cells that contain centrioles also have a "crown" of small microtubule aster, that extend from the centrioles to the nuclear membrane.
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phases of mitosis are really difficult to separate. Keep in mind that the process is not static described in the text, but dynamic and which can be followed in
Prophase Prophase is the first stage of mitosis. The chromatin condenses (remember that DNA is replicated chromatin in the interphase), so at this point there are two chromatids together. The nuclear membrane dissolves, centrioles (if present) are divided and the couple migrated to the poles, the mitotic spindle is formed. Centromeres (or primary constraints) become clearly visible, because they have associated protein plaques on both sides: the kinetochore. In the cytoplasm the endoplasmic reticulum and Golgi complex is fragmented into vesicles, the cytoskeleton is disrupted by As the cell loses its original shape and becomes spherical.
Modified: http://www.whfreeman.com/life/update/. Metaphase
The following metaphase prophase. The chromosomes (which at this point consist of two chromatids held together at the centromere) reach their maximum condensation and migrate to Ecuador from the cell where the spindle fibers are "glued" to the kinetochore fibers.
Anaphase Anaphase begins with the separation of the centromeres and the drag of the chromatids (we call chromosomes after the separation of centromeres) to opposite poles.
Modified: http://www.whfreeman.com/life/update/. Telophase
In telophase the chromosomes reach the poles of their respective zones, reconstituted nuclear membrane, the chromosomes uncoil and become the chromatin and the nucleolus, which disappeared in prophase reconstituting. Where once there was now two small cell with exactly the same genetic information and chromosome number. These cells can then differentiate into different forms during development.
Modified: http://www.whfreeman.com/life/update
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