CELL CYCLE AND DIVISION

The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle. Cell cycle includes three processes cell division, DNA replication and cell growth in coordinated way. Prevost and Dumas (1824) first to study cell division during the cleavage of zygote of frog.

Two processes take place during cell reproduction.

  • Cell growth: (Period of synthesis and duplication of various components of cell).

  • Cell division: (Mature cell divides into two cells).

Cell cycle includes three processes cell division, DNA replication and cell growth in coordinated way.

Duration of cell cycle can vary from organism to organism and also from cell type to cell type. (e.g., in Yeast cell cycle is of 90 minutes, in human 24 hrs.)











 

Interphase

  • It is divided into 3 further phases G1, S, and G2.

G1 phase (Gap 1 Phase)

  • Corresponds to the interval between mitosis and initiation of DNA replication.

  • During G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA.

S phase (synthesis phase)

  • period during which DNA synthesis or replication takes place.

  • During this time the amount of DNA per cell doubles. (only amount of DNA is doubled, no of chromosomes remain same.)

  • In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.

G2 phase (Gap 2 Phase)

  • Proteins are synthesised in preparation for mitosis while cell growth continues.

  • Cells in this phase are metabolically active but they do not divide unless they are called on to do so.

Mitosis or M phase

  • In animals, mitotic cell division is only seen in the diploid somatic cells while in the plants mitotic divisions can be seen in both haploid and diploid cells.

  • it is also called as equational division as the number of chromosomes in the parent and progeny cells are the same.

  • Mitosis is divided into the following four stages:

    • Prophase

    • Metaphase

    • Anaphase

    • Telophase

 

Prophase

  • It follows the S and G2 phases of interphase.

  • The centrioles now begin to move towards opposite poles of the cell.

  • In prophase Chromosomal material condenses to form compact mitotic chromosomes.

  • Initiation of the assembly of mitotic spindle with the help of the microtubules.

  • Cell organelles like Golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope disappear.

Metaphase

  • Start of metaphase is marked by the complete disintegration of the nuclear envelope.

  • The chromosomes are spread through the cytoplasm of the cell.

  • condensation of chromosomes is completed and they can be observed clearly under the microscope.

  • This is the stage at which morphology of chromosomes is most easily studied.

  • At this stage, metaphase chromosome is made up of two sister chromatids, which are held together by the centromere.

  • centromere serve as the sites of attachment of spindle fibres to the chromosomes.

  • chromosomes are moved into position at the centre of the cell.

  • the metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole.

  • The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate or equatorial plate.

Anaphase

  • At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids begin to move towards the two opposite poles.

  • As each chromosome moves away from the equatorial plate, the centromere of each chromosome is towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind

Telophase

  • At the beginning of telophase, the chromosomes at their respective poles decondense and form chromatin network.

  • Nuclear envelope assembles around the chromatin network.

  • Nucleolus, Golgi complex and ER etc cell organelles reform.

Cytokinesis

  • After karyokinesis the cell itself is divided into two daughter cells by a separate process called cytokinesis.

  • In an animal cell, this is achieved by the appearance of a furrow in the plasma membrane.

  • The furrow gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into two.

  • Plant cells undergo cytokinesis by cell plate method. In cell plate method wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls.

  • The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells.

  • At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells.

  • 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). (should be coenocytic)

It’s a wrap!! Hope all the steps were clear and helped you understand the process with ease!