Mitosis is a form of eukaryotic cell division cell that possess a clearly define nucleus) that produces two daughter cells with the same genetic component as the parent cell. Chromosomes replicated during the S phase are divided in such a way as to ensure that each daughter cell receives a copy of every chromosome. In actively dividing animal cells, the whole process takes about one hour.
Cell Cycle: is the series of stage eukaryote cell pass through during division. Two gap phases (G1 and G2) and S Phase (Synthesis stage), in which the genetic material is doubled and in M phase in which mitosis partitions the genetic material and the cell divides.
G1 phase. Metabolic changes prepare the cell for division. At a certain point - the restriction point - the cell is committed to division and moves into the next phase.
S phase. DNA synthesis replicates the genetic material. Each chromosome now consists of two sister chromatids.
G2 phase. Metabolic changes assemble the cytoplasmic materials necessary for mitosis and cytokinesis(plant substance that promote cell division).
M phase. A nuclear division (mitosis) followed by a cell division (cytokinesis).
The period between that is, G1, S and G2 - i.e is mitotic division is known as interphase.
Mitosis is a form of eukaryotic cell division cell that possess a clearly define nucleus) that produces two daughter cells with the same genetic component as the parent cell.
Chromosomes replicated during the S phase are divided in such a way as to ensure that each daughter cell receives a copy of every chromosome.
Time Taken for the Process
In actively dividing animal cells, the whole process takes about one hour.
Stages in Mitotic Division
· Interphase
· Prophase
· Metaphase
· Anaphase
INTERPHASE
During interphase the cell has the same appearance as any non-dividing cell. The chromosomes are not visible as distinct bodies either under the light microscope or the electron microscope. At this stage they are strung out in the form of long chromatin threads swollen at intervals into visible chromatin granules. Not until prophase do the chromatin threads condense to form visible chromosomes. To describe interphase as a resting stage is a complete misnomer. Far from being inactive the cell is growing and preparing for division. During interphase two things happen, both of which are essential if the cell is to divide. Firstly, the genetic material (DNA) replicates, i.e. doubles itself, so that sufficient DNA’s made available for each of the two daughter cells. A cell never divides until this new genetic material has been formed. Secondly, the cell builds up a sufficiently large store of energy, a kind of 'energy reservoir', to carry the process through. That this accumulation of energy takes place during interphase rather than during division itself can be shown by inhibiting respiration at different stages in mitosis. If a cell is treated with a metabolic poison such as cyanide during interphase, mitosis fails to take place. On the other hand if the cell is treated with poison after mitosis has started, the process will still go through to completion.
A third important event that takes place during interphase is the formation 'new cytoplasmic organelles - mitochondria, ribosomes, chloroplasts, and the like. If this did not happen successive cell divisions would result in a steady depletion of the cells' contents. Formation of new organelles takes place either by the production of new materials in the cytoplasm, or by the duplication of existing organelles. Either way, the synthetic activity involved necessitates the expenditure of much energy, and a high metabolic rate is typical of cells preparing for mitosis.
PROPHASE
If interphase is concerned with preparing the cell for division, prophase can be described as 'mobilization for action'. Certain clearly visible events can be seen under the microscope. The most they gradually become shorter and fatter. As the chromosomes shorten it becomes increasingly clear that each consists of a pair of bodies lying close to each other. These are called chromatids. They tend to lie parallel along most of their length but are particularly closely associated in the vicinity of a specialized region called the centromere. We shall see later that this represents the centre of movement of the chromosome. The chromatids of one chromosome are usually referred to as sister chromatids. It is essential to appreciate what these chromatids represent. Initially (i.e. in early interphase) each chromosome consists of a single, highly attenuated thread. During replication of the genetic material an exact duplicate of this single thread is produced. The two threads together (the original one and its newly-formed duplicate) are completely invisible until prophase when they make their appearance as the two chromatids. The two chromatids making up a chromosome are identical: They contain exactly the same genetic material point for point along their length.
METAPHASE
When the nuclear membrane disrupts the chromosomes migrate to the central plane of the cell and arrange themselves round the equator of the spindle. This is achieved through the organization of a series of fibres which run from the centromere of each chromosome to one of the two poles. At this moment the chromatids of each chromosome move slightly apart at the centromere region, the sister chromatids being orientated towards opposite poles.
ANAPHASE
Suddenly the chromatids belonging to each chromosome part company and move towards opposite poles of the spindle.. In recent years there has been much debate on the possible mechanism by which this migration of chromatids might take place. It used to be thought that the centromeres repel each other, causing the chromatids to move apart, but there is now evidence that the chromatids are pulled to the poles of the spindle. Observations on the spindle apparatus of live cells in the polarizing microscope have revealed two types of spindle. One type runs right across the cell from one pole to the other; the other type runs from the poles to the centromere of each chromatid.
TELOPHASE
The chromatids are destined to become the chromosomes of the daughter cells. On reaching the polar ends of the spindle they become densely packed together. Meanwhile the cell divides into two. In animal cells this takes place by means of a constriction of the plasma membrane which cuts across the equator of the spindle. In plants a new wall, the cell plate, grows across the middle of the cell: in root tip cells, where the spindle is about the full width of the cell, the plate is deposited as scattered droplets and vesicles which coalesce; in larger cells this is followed by expansion of the cell plate outwards until it fuses with the wall. While this is happening the spindle apparatus breaks down and the nuclear membrane is re-formed. The nucleolus reappears and the chromosomes gradually uncoil and return their original thread-like form. Mitosis is now complete; the daughter cells enter interphase and prepare for the
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