Lawrence C. Brody, Ph. Featured Content. Introduction to Genomics. The phases are listed below, along with the major events that occur during each phase. The cell is engaged in metabolic activity and performing its duty as part of a tissue.
The DNA duplicates during interphase to prepare for mitosis the next four phases that lead up to and include nuclear division. Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. The membranous organelles such as the Golgi apparatus and endoplasmic reticulum fragment and disperse toward the periphery of the cell.
The nucleolus disappears and the centrosomes begin to move to opposite poles of the cell. Microtubules that will eventually form the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen. The sister chromatids begin to coil more tightly with the aid of condensin proteins and become visible under a light microscope.
The remnants of the nuclear envelope fragment. The mitotic spindle continues to develop as more microtubules assemble and stretch across the length of the former nuclear area. Chromosomes become more condensed and discrete. Each sister chromatid develops a protein structure called a kinetochore in the centromeric region.
The proteins of the kinetochore attract and bind mitotic spindle microtubules. Kinetochore and Mitotic Spindle : During prometaphase, mitotic spindle microtubules from opposite poles attach to each sister chromatid at the kinetochore.
In anaphase, the connection between the sister chromatids breaks down and the microtubules pull the chromosomes toward opposite poles. The sister chromatids are still tightly attached to each other by cohesin proteins. At this time, the chromosomes are maximally condensed. Each chromatid, now called a chromosome, is pulled rapidly toward the centrosome to which its microtubule is attached. The cell becomes visibly elongated oval shaped as the polar microtubules slide against each other at the metaphase plate where they overlap.
The mitotic spindles are depolymerized into tubulin monomers that will be used to assemble cytoskeletal components for each daughter cell. Nuclear envelopes form around the chromosomes and nucleosomes appear within the nuclear area.
Division is not complete until the cell components have been apportioned and completely separated into the two daughter cells. Although the stages of mitosis are similar for most eukaryotes, the process of cytokinesis is quite different for eukaryotes that have cell walls, such as plant cells.
In cells such as animal cells, which lack cell walls, cytokinesis follows the onset of anaphase. A contractile ring composed of actin filaments forms just inside the plasma membrane at the former metaphase plate. The actin filaments pull the equator of the cell inward, forming a fissure.
The furrow deepens as the actin ring contracts; eventually the membrane is cleaved in two. The number of chromosomes in human cells is 46 with 22 autosomal pairs one of each type contributed by the mother and one of each type from the father and 2 sex chromosomes - 2 X chromosomes for females one from father and one from mother or an X and a Y chromosome for males the X from the mother and the Y from the father.
The characteristic banding of chromosomes you will see in pictures here is obtained by staining with various dyes. The banding of chromosomes by using dyes was discovered in the late 's and before that cytogeneticists depended on chromosome length and position of a constriction to identify the individual chromosomes.
The band width and the order of bands is characteristic of a particular chromosome - a trained cytogeneticist can identify each chromosome 1,2,
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