Mitosis phases

Eucaryotic cells execute their reproduction in a cyclic process, in which at least two phases, a S phase and a M phase, can be differentiated on the basis of biochemical and morphological features. The biochemical characteristic of the S (synthesis) phase is the replication of nuclear DNA and thus doubling of the genetic information. In M (mitosis) phase, division of the chromosomes between the daughter cells is prepared and carried out. 

Because MIB-1 monoclonal antibody is used extensively to determine the cell proliferation index, its applications are discussed below. This antibody detects the nuclear antigen Ki-67 expressed in proliferating cells but not in resting cells. The antibody reacts with the nuclei of cells in mid-Gj (first gap), S (DNA synthesis), G2 (second gap), and M (mitosis) phases, but not in the G0 or quiescent phases. The use of MIB-1 antibody is one of the simplest and most reliable labeling techniques for assessing the rate of proliferation of a neoplastic cell population. Thus, the antibody can be used to assess the growth fraction (i.e., the number of cells in cell cycle) of normal, reactive, and neoplastic tissues. 

Kaposi s sarcoma-associated herpesvirus Bcl-2 homolog low molar mass DNA cell cycle mitosis phase... 

Of the four phases, the two critical ones are DNA replication, which occurs during Gi to S phase, and the physical process of cell division, which occurs during G2 to M (for mitosis) phase these are critical checkpoints . 

Hg. 6.1 Cell cycle and regulation of cell cycle progression by cyclin-dependent kinases [3]. Cl, Capi phase between M and S phases S, DNA synthesis phase G2 gap 2 phase between S and M phases and M, mitosis phase. 

Livers of one experimental group were extracted and fixed in formalin. After fixing, liver pieces were embedded in paraffin and then slices of up to 10 pm thickness were made. The slices were pigmented by hematoxylin-eosin. An average area of a single hepatocyte as well as cell quantity in various mitosis phases were calculated in terms of a thousand elements under microscopical investigation. 

Morphometric analysis demonstrated that the area of hepatocytes in reference group animals varied within the range of 450 20 pm, and in control groups 460 25 pm. In animals that received ES or IPG, the area of hepatocytes amounted to 650 40 pm in both cases, i.e. authentically increased (p < 0.01) both in regard to reference and control. Total cell quantity in different mitosis phases amounted to 20 3% of total cell quantity in reference rats, 23 4% in control animals, and among rats that received ES or IPG the total amount of fissionable cells increased by 36% on... 

During the M (mitosis) phase, cell division occurs. 

Free radicals can also attack the DNA molecule and block its replication locally. They can sometimes break the DNA molecule during the replication phase of the cell cycle, before the mitosis phase, or if photosensitizing molecules are used. Fortunately, human cells contain mechanisms for the repair of damaged DNA, involving DNA polymerase and other enzymes. 

The eukaryote cell cycle is composed of the following four phases M phase, mitosis phase (gap 1), between mitosis and initiation of DNA synthesis S phase, DNA synthesis and phase (gap 2), cell growth and macromolecule synthesis. 

S—DNA replication phase Gj—Resting phase Gj—Resting phase M—Mitosis phase... 

Cell biologists consider the cell cycle to be divided into five phases Gl, S, G2, M and GO. There are three principal checkpoints at which the cell cycle is stopped if defects are detected the Gl checkpoint, the G2 checkpoint and the spindle checkpoint during the mitosis phase. The tumour suppressors p53, the guardian angel protein , and Rh protein regulate progression from the Gl phase to the S phase. Mutations of p53 and Rb that fail to suppress progression result in uncontrolled cell division, i.e. tumour development. 

Gap (or growth) 2 phase (G2 phase). The cell grows in size, mitochondria replicate, chromosomes condense and proteins (e.g. microtubular proteins) needed for mitosis are made during this phase. The G2 checkpoint confirms the integrity of DNA and the cell volume. Providing the check is satisfactory, the mitosis phase is triggered by the active cdkl/cyclin B complex. 

Mitosis phase (M phase). The spindle checkpoint ensures that all the chromosomes are correctly aligned on the spindle fibre prior to... 

The Gl phase, S phase and G2 phase are collectively known as the interphase. The interphase takes typically 23 hours for completion but can be shorter in cancer and gut cells. This is considerably longer than the mitosis phase which typically takes only 1 hour. 

Figure 7. Phases of cell cycle, (resting) phase lasts about 24 hours. S (DNA synthesis) phase lasts about 8 hours, G2M (mitosis) phase lasts about 2-3 hours.

---