Life cycle of the cell is period from one cell reproduction to the next cell reproduction. It is usually 10 to 30 hours in human. There are two kinds of cell division:mitosisandmeiosis. Cells grow by increase in their mass, duplication of genetic material and division of cytoplasm so that each daughter cell receives equal complement of genetic material to ensure further perpetuation of cell lines.
MITOSIS: includes two processes:
Karyokinesis- it is nuclear division.
Cytokinesis- it is division of cell cytoplasm.
In mitosis, the genetic material of the cell is doubled. Then, the cell divides into two halves. One-half of the genetic material goes into each of the two daughter cells. In this manner, the two new cells each have the same genetic composition as the original cell. Mitosis is essentially a duplication process. It produces two genetically identical "daughter" cells from a single "parent" cell. We grew from a single embryonic cell to the person we are now through mitosis. Even after we are grown, mitosis replaces cells lost through everyday wear and tear. The constant replenishment of our skin cells, for example, occurs through mitosis. Mitosis takes place in cells in all parts of our body, keeping our tissues and organs in good working order.
MEIOSIS: Meiosis is a process of sexual reproduction by which chromosomes of diploid (2n) cell are reduced to haploid or halved (n) for the formation of gamete. The process is accomplished by two successive cell divisions with replication of chromosomes only once. First meiotic division (reductionalequational division) involves further division of two haploid cells into four similar haploid gamete cells. In other words it shuffles the genetic deck, generating daughter cells that are distinct from one another and from the original parent cell. Although virtually all of our cells can undergo mitosis, only a few special cells called germ cells or gametes are capable of meiosis- those that will become eggs in females and sperm in males. So, basically, mitosis is for growth and maintenance, while meiosis is for sexual reproduction. division) involves division of diploid cell into two haploid daughter cells whereas second division (
Phases of Mitosis
Mitosis is responsible for growth and development, as well as for replacing injured or worn out cells throughout our body. For simplicity, we have illustrated cells with only six chromosomes.
Interphase: Chromosomes duplicate, and the copies remain attached to each other.
Prophase: during this phase the cell nucleus becomes spheroid, and there is increase viscosity of cytoplasm. The chromosomes shorten and thicken and become stainable. The double stranded nature of chromosomes now becomes visible. With the progress of prophase, the chromosomes which were essentially distributed linearly during prophase migrate towards the nuclear membrane, leaving the central area clear. The centrosome, which had undergone duplication during interphase, now begins to move towards opposite poles of the cell. The spindle begins to form.
Prometaphase: beginning of this phase is marked by the disappearance of the nuclear membrane. When nuclear membrane dissolves there is no distinction between cytoplasm and nucleoplasm. The chromosomes are attached to the spindle through their centromeres. When the nuclear membrane dissolves, a fluid area is observed in the centre of the cell. The chromosomes move freely through this area as they proceed toward the equator.
Metaphase: at this phase the chromosomes usually are lined up in one plane to form the equatorial plate or metaphasic plate. Occasionally, only the centromeres lie on the equatorial plane, while the centrosome arms are directed away from the equator. Smaller chromosomes are usually central in position whereas the larger ones are peripheral.
Anaphase: chromosomes are arranged on the equatorial plate for a short period only. The sister chromatids begin to separate from each other, starting at their centromeres. These are now called daughter chromosomes. They now repel each other and the two sets of chromosomes migrate towards the poles. The chromosome movement is brought about by the shortening of spindle fibres attached to the centromeres.
Telophase: Nuclear membranes form around each of the two sets of chromosomes, the chromosomes begin to spread out, and the spindle begins to disappear. Telophase changes are associated with the restoration of interphase condition. In a way, telophase is the reverse of prophase. Each daughter cell gets the same complement of chromosomes and nucleoli as of the mother cell. The chromosomes gradually uncoil and become less compact. They eventually lose their staining ability.
Cytokinesis: In animal cells a cleavage furrow appears at the beginning of telophase. This furrow or constriction becomes progressively deeper as the spindle breaks down. Eventually, the in growing constrictions join and separate the two daughter cells. This division of cytoplasm is called cytokinesis. When nuclear division takes place without the cytoplasmic division it results in the formation of syncytium, which is a condition where a large number of nuclei are present in a single nuclei.
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