Cell, a unit of life, has to obtain energy from outside source for the process of reproduction. There are great varieties of cells and the process of reproduction varies according to the type of cell. Cells are mainly of two types:
• Somatic cells: somatic cells are also known as body cells since they make up skin, hair and muscles. These cells are divided by mitosis.
• Sex cells: sperm and ova include the sex cells. These cells reproduce by the process of meiosis in a special tissue.
The majority of cells in our body are somatic and they divide asexually. These cells do not acquire or exchange new information at the time of reproduction. A bacterium is the common example of asexually reproducing cells. Bacteria reproduce by the process of binary fission in which hereditary information is duplicated and then segregated. After the duplication of hereditary information cytoplasm divides and results into two daughter cells.
This process of asexual cellular division is most common in multicellular eukaryotes. The steps of mitotic division include:
• Firstly Cell grows
• Secondly Genomes are duplicated in S phase of mitotic cell cycle
• Duplicated chromosome sets are separated into nuclei at opposite poles of the cells
• Finally cytoplasm division takes place which results into progeny cells.
This process of division is divided into six phases which include:
Interphase: it is the stage between division of cell and nucleus. In this phase cellular growth begins and cell performs its normal functions like respiration, protein synthesis and photosynthesis. DNA replication occurs at this stage but DNA is not condensed into thread like structure called chromosome. They are in the form of loosely coiled structure known as chromatin.
Prophase: this is also known as the "first phase" of mitotic cell cycle. Following changes occur at this stage:
• Nuclear membrane around the genetic material dissolves
• DNA condenses into shorter and more compact chromosomes. Chromosomes are of X shape.
• Centrioles start migrating towards the pole of the cell to begin growing the spindle fibers. The movement of chromosome in the cell is just because of the spindle fibers.
Metaphase: also known as the "middle phase" of mitotic cell cycle. This phase is easy to observer under the microscope. In this phase:
• Spindle fibers attach to the centromeres of each X shaped chromosome.
• Chromosomes are aligned at the equatorial plate.
Anaphase: this third phase is also known as the "Apart phase" of cell cycle. In anaphase:
• Chromosomes are no longer "X" shaped
• Centromeres divide and spindle fibers get shorten
• Spindle fibers pull the single copy of each chromosome towards the pole of the cell.
Telophase: also known as "End Phase" as the mitosis is about to complete. At Telophase:
• Two new nuclei are formed with complete set of DNA
• Chromosome uncoil and nuclear envelop is reformed.
• Spindle fibers disintegrate.
This is the final stage of cell division. Cytokinesis refers to "cell splitting". In this stage the formed daughter cells are split apart.
Cytokinesis in animal cell: Cytokinesis in animal cell is entirely different than that of plant cell due to the lack of cell wall. Cytokinesis occurs when the cell membrane pinches in, forming a deep furrow. This furrow results in two identical daughter cells.
Cytokinesis in plant cell: in plant cells Cytokinesis begins when new cell wall is formed at the centre. This divides the cell into two halves. Each halve contains its nucleus and DNA. Cell membrane is also divided by the new cell wall. This finally results into two identical daughter cells. Cytokinesis thus completes the life cycle of eukaryotic cells.
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