Cell Division

New cells are required for replacement of worn out cells, for repair of cuts and injuries, and for growth and for reproduction. New cells are obtained through cell division.

Types of Cell Division

There are two types of cell division:

  1. Mitosis: In mitosis, a cell gives rise to two identical daughter cells. Mitosis is needed for growth, and repair of worn out parts.

  2. Meiosis: Cell division involved in production of sex cells which give rise to the egg in female and sperm in male.

Major events of both kinds of cell division are largely similar in both animal and plant cells.

Mitosis

Mitosis is an equational division in which the two daughter cells are identical to each other and to their parent cell.

  1. The chromosomal material (chromatin network) inside the nucleus condenses to form chromosomes.

  2. The nuclear membrane disappears.

  3. The centrosome (in animal cell) divides into two equal parts called centrioles, each of which migrates to opposite sides to orient the spindle which forms in the cytoplasm.

  4. A spindle of fibres appears between the centrioles.

  5. Each chromosome consists of two chromatids which are held by a centromere. The chromosomes arrange in the middle or equator of the spindle.

  6. Centromere splits. The chromatids (daughter chromosomes) of each chromosome now have their own centromere. The chromatids, now termed chromosomes separate from each other and subsequently, move to the opposite poles of the spindle.

  7. Chromosomes lose their identity, and turn into a network of chromatin threads at the two poles.

  8. Nuclear membrane reappears around each of the two new clusters of the chromatin material, formed at the poles.

  9. In the middle of the cell, at the two sides a furrows appear in the cell membrane. The furrows deepen to divide the parent cell into two new identical daughter cells.

Two main differences in mitosis in a plant cell and an animal cell

  1. In plant cells, there is no centrosome but a spindle forms in the cytoplasm.

  2. Upon the completion of mitosis, the cytoplasm in plant cell does not constrict (furrow is not formed). Instead, a cell plate or a new cell wall is laid down in the cytoplasm in the middle of the cell. It divides the original cell into two daughter cells.

Significance of Mitosis

  1. The daughter cells receive the same number of chromosomes as the parent cell. In other words mitosis is an equational division in which the two daughter cells are identical to each other and to their parent cell.

  2. Mitosis helps in wound healing and replacement of cells lost during wear and tear.

  3. It is responsible for the growth of an organism by addition of new cells.

  4. It is the method of asexual reproduction in single celled organisms like amoeba.

Meiosis

Meiosis is necessary for sexual reproduction. In animals, meiosis takes place in reproductive organs, such as the testis and the ovary, that produce eggs and sperms; and also in flowering plants it occurs in the anthers and ovary to produce pollen grains and the ovule, respectively.

Broadly, meiosis is completed in two phases.

Phase I: Two cells with half the number of chromosomes in each are formed at the end phase I. This is, therefore, a reduction division.

Phase II: The second division is equational as in mitosis and produces four cells at the end, each with half the number of chromosomes.

Sequence of Events

  1. Chromatin fibres condense into chromosomes.

  2. The chromosomes arrange in matching (or homologous) pairs. A matching pair means one chromosome having been received from the mother and the corresponding one received from the father. Both chromosomes of a pair bear same genes, but not necessarily the same alleles.

  3. Each chromosome in such a pair is made of two chromatids as duplication of chromosomes occurs before cell division begins. Thus, each pair of chromosomes is now a group of four chromatids.

  4. The nuclear membrane disappears, the homologous chromosomes which had paired now begin to separate and move apart.

  5. The cytoplasm divides into two cells, each of which has half the number of chromosomes originally present in the cell. Each chromosome is still made up of two chromatids as centromere has not divided.

  6. Meiosis II begins. it is exactly like mitosis.

  7. At the end of meiosis II, four cells form, each with half the number of chromosomes of the parent cell.

Significance of Meiosis

  1. During meiosis, the number of the chromosomes is halved in the resulting sex cells so that when the male cell and the female cell combine during fertilization, the normal number of chromosomes in the species is restored.

  2. Also during meiosis, new combination of genes are obtained in the gametes that result from meiosis.