Cell division: Organisms consist of new cells to grow, replace old, dead, and injured cells, also to form gametes which are necessary for reproduction. The process by which cell arises from pre- existing cells is called cell division.
A cell passes through phases of growth after which able to duplicate its chromosomes before they divide. These phases in the life of a cell constitute the sequences in which the cell duplicates its genome, synthesizes the other constituents of the cell, and eventually divides into two daughter cells, the term given as cell cycle.
In eukaryotes, two types of cell division occur:
- one parent cell divides into two daughter cells having an exact copy of genetic material in the parent cell called mitosis.
- In the other type of division, the genetic material precise half by which fertilization restores to be diploid is termed as meiosis.
Different Phases of the cell cycle
1. Interphase – Non-dividing period (Growth phase). Subdivided into three main periods – G1 phase, S phase, and G2 phase.
2.Dividing phase – Also called M-phase (M for mitosis or meiosis). Actual cell division or mitosis occurs
1.Interphase– It is the interval between two successive cell divisions, termed interphase in this phase the cell is not dividing. It is the longest period found in the cell cycle. It has subdivisions:
- G1 (Gap-1) Phase i.e. First phase of growth – This is the longest phase proteins and RNA are synthesized during this phase. In this phase, the cell is metabolically active and continuously grows but does not replicate its DNA.
- S or synthetic Phase – In this phase DNA is (synthesized). A chromosome contains a double helical strand of DNA molecule. After S-phase, each chromosome becomes longitudinally double except at the centromere, and thus, it has two molecules of DNA and two chromatids. Thus each chromatid contains one molecule of DNA. S or synthesis phase marks the period during which DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles. No increase in chromosomes takes place.
- G2 (GAP 2) phase – More proteins including the histones are synthesized for mitosis in this phase. Cytoplasmic organelles such as mitochondria and Golgi bodies get duplicated. Centriole also divides into two centrioles contained in a single centrosome.
- Quiescent stage– Some cells of animals and plants do not divide so they exit the division from the G1 phase, termed as the quiescent stage or G0 stage. For example-heart cells.
2. Dividing phase OR M phase-(mitosis). The number of chromosomes in the parent and progeny cells is the same, it is also called equational division. It takes place for the growth and replacement of older cells by new cells. Daughter cells are identical and similar to mother cells in all aspects. Mitotic cell division found in haploid as well as in diploid cells
Mitosis (Mitos = thread) – (The term mitosis given by Walther Flemming in 1882). Mitosis is divided into 4 phases or stages termed as:
Facts-Division of one nucleus to produce two daughter nuclei is called (karyokinesis). The division of the cytoplasm to give two daughter cells is called cytokinesis.
1.Prophase-initiation of condensation of chromosomal material takes place in prophase. Prophase goes through some sequences-
- Early prophase -Centriole divides and each of the two centrioles starts moving towards opposite poles of the nucleus of the dividing cell. Chromosomes appear as long threads, and coiling is started. The nucleus enlarges and becomes less distinct.
- Middle prophase– Chromosome condensation is complete and they become short and thick. Each chromatid contains a newly replicated daughter DNA molecule.
- Late prophase– Centrioles reach the opposite poles of the dividing cell. At the end of prophase, Golgi complexes, endoplasmic reticulum, nucleolus, and the nuclear envelope is not seen.
- chromosomes are brought towards the equator of the cell, with the help of spindle fibers.
- Each chromosome becomes attached to the two spindle fibers by the centromere. Whereas each centromere is joined to the opposite poles.
- The sister chromatids not yet separated as centromere not divided.
- Centromeres of chromosomes get divided and then each chromatid becomes a chromosome.
- Contraction of spindle fibers takes place, which pulls the centromeres to the opposite poles.
- Chromatids move to opposite poles
- Centromeres split and chromatids get separated.
- Chromosomes get clustered at opposite spindle poles and their identity is lost as discrete elements.
- A new nuclear membrane is formed around each of two new daughter nuclei.
- The nucleolus, Golgi complex, and ER reform in each daughter nuclei.
- in this process cytoplasm of a cell gets divided into two, initiated at beginning of telophase, and gets completed till the end of telophase.
- Cytokinesis is different in plant and animal cells.
- In an animal cell, invagination of the plasma membrane proceeds from the periphery of the cell towards the interior.
- In-plant cell phragmoplast (cell plate) begins to form in the center of cell and then expands towards the periphery.
In some organisms, karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of syncytium (e.g., liquid endosperm in coconut
Significance of mitosis
- This is equational division, and daughter cells are identical to parent cells in all aspects.
- The same number of chromosomes as daughter and mother cell.
- By this division growth in multicellular organisms takes place.
- Reproduction in unicellular organisms takes place by mitosis.
- Mitosis plays important role in regeneration and growth as cell division takes place constantly.
- For ex- regeneration of the tail of a lizard, replacement of skin cells, replacement of blood cells, etc.
But in some special cases, the uncontrolled division and growth of cells may cause cancer. so it should be in a controlled manner
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