Get the answer of: How Many Stages are in Mitosis ?
The DNA in the diffuse chromatin of the resting nucleus in interphase has been duplicated in S phase preceding this cell division. The extended state of interphase chromatin allows transcription and replication of DNA, but is not suitable for division into two daughter cells. Therefore, prophase, the first stage of cell division shows contraction and condensation of chromatin into shorter, thicker fibres.
By mid-prophase, the nucleolus starts to disappears and nuclear membrane breaks down, so that chromatin lies free in cytoplasmic space. Shortening of chromatin fibres continues to yield thicker, somewhat rod-like chromosomes. The breakdown of the nuclear envelope involves the enzyme Cdk kinase which is activated just before initiation of mitosis, in G2 phase of cell cycle.
The inner face of the nuclear envelope is lined by a layer of fibrillar proteins of the cytoskeleton (called intermediate filaments), termed nuclear lamina. The enzyme Cdk kinase phosphorylates the lamin filament molecules, causing disassembly of nuclear lamina.
The entire nuclear envelope that surrounds the condensing chromatin then breaks up into small vesicles which disperse into the cytoplasm. The nuclear pore complexes in the nuclear envelope also dismantle during fragmentation of the nuclear envelope.
The dissolution of the nuclear envelope at the end of prophase is described by some authors as the prometaphase stage. At metaphase, maximum condensation of chromatin fibres has been achieved giving rise to distinct rod-like chromosomes. Metaphase represents the most condensed state of chromatin in a cell.
The molecular mechanisms responsible for chromosome condensation are still poorly understood. The process seems to involve the DNA untangling enzyme topoisomerase II. The thick rod-like chromosomes begin to align themselves in the centre of the cell on what is conventionally referred to as the equatorial plate or the metaphase plate.
Structure of Metaphase Chromosome:
Each chromosome consists of two chromatids and a region of central constriction called centromere or primary constriction. Evidence has established that each chromatid consists of a single duplex DNA molecule. The centromere region contains many copies of highly repeated DNA sequences. A small nodule-like structure called the kinetochore is present at the outer surface of the centromere in each chromatid.
The kinetochore functions as the site of attachment of microtubules, a bundle of fibres making up a spindle fibre, all the fibres together constituting the mitotic spindle apparatus. Microtubules attached to the kinetochore are called chromosomal or kinetochore microtubules. The centromere divides longitudinally, and the two centromeres pull the chromatids apart to the two poles by means of spindle fibres.
The division of the centromere into two at metaphase of mitosis is a key event that segregates accurately, half the genetic material for one daughter cell and half for the other daughter cell. This is ensured by the fact that the two chromatids contain duplicated DNA acquired from DNA synthesis in S phase of cell cycle preceding mitosis.
Alignment of metaphase chromosomes in the equatorial region of the cell is followed by beginning of separation of chromatids to opposite poles. Misalignment of chromosomes in equatorial region arrests cells at metaphase and failure to segregate genetic material to the two daughter cells.
The protein encoded by a gene MAD2 is normally localised at the kinetochores of prometaphase chromosomes and misaligned metaphase chromosomes, but is not present on chromosomes that have become properly aligned at the metaphase plate. Cells that possess mutant copies of a gene MAD2 fail to become arrested at metaphase when their chromosomes are misaligned.
The presence of MAD2 at the kinetochores seems to provide a “wait” signal that delays progression into anaphase. As each chromosome becomes aligned at the metaphase plate, its kinetochore loses all of its MAD2 molecules. It is only after MAD2 protein is absent from all of the chromosomes that anaphase can begin.
Microtubule Organisation and Centrosome:
In most cells microtubules extend outward from a microtubule-organising center, which in animal cells is called the centrosome. Centrosomes are absent in plant cells. During mitosis, microtubules extend out from the duplicated centrosomes to form spindle fibres. Thus the centrosome seems to play a key role in determining the intracellular organisation of microtubules, but details of its function are not known.
The centrosomes contain a pair of centrioles, oriented perpendicular to each other, and surrounded by pericentriolar material. Centrioles are cylindrical structures consisting of nine triplets of microtubules, similar to the basal bodies of cilia and flagella.
During spindle formation, microtubules emerge from the pericentriolar material around a centrosome and form a star-like aster. These are referred to as astral microtubules. The pericentriolar material acts as the nucleating site for microtubules of the aster. The fast growing ends of the microtubules, denoted plus ends, are away from the centrosome. The microtubule initiating activity appears to be stimulated by the Cdk protein.
Following aster formation, the two centrosomes separate and move to opposite poles, while microtubules stretching between them increase in number and elongate. These are called polar microtubules. The two centrosomes establish two poles of the mitotic spindle. After mitosis, one centrosome is distributed to each daughter cell. Centrosomes are not essential components of mitotic spindles in all cells.
Many animal cell types do not have centrosomes, nor do higher plants. The minus ends of the microtubules are thought to be gathered into a cluster at each spindle pole through the activity of motor proteins described later.
The two sister chromatids of each chromosome split apart and start moving towards opposite poles. There is rapid degradation of an inhibitory protein that acts a proteinaceous “glue” holding the two chromatids together, that facilitates the onset of anaphase. The degradation of anaphase inhibitory proteins occurs in response to activity of the mitotic Cdks.
The separation of sister chromatids requires activity of topoisomerase II. The chromatids (actually chromosomes now) start moving towards the poles, accompanied by shortening of microtubules attached to their kinetochore. Shortening of the microtubule results from the loss of subunits at the kinetochore (minus end) during anaphase.
When the chromosomes reach the poles, there is elongation of the mitotic spindle, resulting in a simultaneous movement of the spindle poles further away from each other. This is accompanied by the addition of tubulin subunits to the plus ends of the polar microtubules. The movement of chromosomes to the poles has been observed to be completed within 2 to 60 minutes.
Chromosomes at the poles organise into a mass of chromatin at each pole that marks the beginning of telophase. The process of formation of nuclear envelope begins when membranous vesicles start fusing with one another to produce a double-membrane envelop surrounding the chromatin.
The nucleolus reappears. Thus, two daughter nuclei that are identical in genetic constitution to the parent nucleus are formed. Cytoplasmic membranous organelles such as Golgi complex, endoplasmic reticulum reform in each daughter cell.
In animal cells division of the cytoplasm of the parent cell is initiated at late anaphase as an indentation of the cell surface that appears as a band around the cell. The band deepens to form a furrow, the plane of the furrow lying in the same plane as the equatorial or metaphase plate on which chromosomes were aligned. Further deepening of the furrow splits the parent cell into two daughter cells.
In plants a cell plate consisting of polysaccharides starts depositing in the central region of the parent cell. This gives rise to the first wall or middle lamella between the two cells. Later on a primary wall is deposited towards the inside of the middle lamella of each daughter cell.