Mitotic cell division (Mitosis), its stages and Significance

What is cell division?

• Life of multicellular organisms starts from a single cell known as a zygote. An adult plant or animal contains millions of cells.
• With the growth of an organism, a large number of cells are formed from pre-existing cells. A single cell divides to form daughter cells, which in turn divide again and again forming large number of cells.
• Hence, cell division is a process by which new cells are formed from pre-existing cells.
• Cell division in plants is restricted to certain areas or regions called the meristematic regions. Animals have active centers of cell division in the skin, bone marrow and certain cells of the intestinal lining.
• In an adult animal, there is less or no division of nerve cells and these if lost cannot be replaced.
• In order to understand the process of cell division, the study of the structure of chromosome is very important because it plays a significant role in the process.
• In cell division, two major processes are involved;
  • Karyokinesis:
    • Involves the division of the nucleus or nuclear material (chromosomes). During the division of the nucleus, each chromosome splits longitudinally, forming two chromatids.
    • These chromatids remain attached in the region of centromere. The centromere appears as a constriction at one place of a chromosome.
    • The spindle fibers attach to the centromere of the chromosome in a dividing cell.
  • Cytokinesis: Involves the division of cytoplasm and cytoplasmic organelles.
• There are two types of cell division:
  • Mitosis
  • Meiosis

Also see: Differences between Mitosis and Meiosis

MITOSIS

• Mitosis is a common method of cell division for the growth and development or tissue repair in multicellular organisms.
• It occurs in somatic or vegetative cells.
• German biologist Walter Flemming was the first person to observe mitosis in living organisms in 1877. He named it mitosis meaning thread formation. This refers to the nuclear division where chromosomes appear as threads.
• It occurs in all types of living organisms. It is characterized by the division of the parent cell into daughter cells.
• The two daughter cells are qualitatively and quantitatively similar to each other. They contain the same number of chromosomes and a more or less equal number of cell organelles.
• Therefore, mitosis is also called equational division.
• Once the process of mitosis begins, it continues until two new cells are formed. For the sake of convenience, the sequence of events occurring during mitosis can be divided into different stages or phases, which are;
  • Interphase
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• These phases are characterized by the arrangement of chromosomes during nuclear division. i.e karyokinesis.

1. Interphase:

• Before the cell division starts, the cell is in the interphase. It is the period between two nuclear divisions.
• It is the non-dividing stage of the cell.
• It is the period of biochemical activities in which the DNA is replicated and coiled around a central core of protein to form a distinct chromosome.
• At this stage, the nucleus has distinct nuclear membrane. The nucleolus can be seen but the chromosomes are long, coiled and thread-like and are called chromatin.
• This is also called a preparatory phase as the cell is prepared to undergo cell division.

2. Prophase:
   • It is the longest stage in cell division. The nucleus undergoes the following set of changes in this phase.
     1. The chromatin threads gradually condense into short, thick and coil forming condensed chromosomes.
     2. As the chromosomes become prominent, the nucleolus decreases in size and disappears at the end of the prophase.
     3. The nuclear membrane breaks and dissolves into the cytoplasm at the end of the prophase. This makes the chromosomes unbound and free in the cytoplasm.
     4. Just before the chromosomal activity begins, the centrosome divides into two (now called centioles) and move away from each other into opposite poles of the cell and form ‘poles. Each centriole then radiates spindle fibers.
   • [Note: Centrioles are present in animal cells and some lower plant cells. In most of the higher plants, centrioles are absent and the poles of the divisions are determined by two clear zones in the cytoplasm of the cell.]

3. Metaphase:
   • The completion of the mitotic spindles (spindle fibers) begins the metaphase. It is much shorter than prophase and comprises of the following steps:
     1. The chromosomes become the shortest and the thickest and get themselves arranged on the equator plane of the cell with the spindle fibers. This enables equal distribution of the chromosomes in the daughter cells. By this time, it is apparent that each chromosome is double, consisting of two similar strands called chromatids.
     2. The chromatids are joined together at one point by a centromere. The centromere of each chromosome divides into two, so that each sister chromatid has its own centromere.
     3. The spindle fibers from the centrioles get attached to the centromeres.

4. Anaphase:
   • Anaphase begins as each sister chromatid with centromere separate from each other and moves towards the poles of the cell by the contraction of the spindle fibers.
   • The contraction of the spindle fibers causes the movement of the chromatids towards the opposite poles. These chromatids are now called chromosomes. The new chromosomes assume a characteristic U, V, J shapes depending on the position of the centromeres on the chromosomes.
   • The new chromosomes become shorter and thicker.
   • The chromosomes reaching the respective poles marks the end of anaphase.

5. Telophase:
   • It is the final stage of karyokinesis of mitosis. In this phase, a nucleus is reconstructed and the following changes occur.
     1. The chromosomes are at the poles of the cell and form two groups.
     2. These chromosomes become longer and slender by uncoiling process to form chromatin reticulum or network and begin to disperse.
     3. The nucleolus reappears by the reorganization of the nuclear materials which were scattered during the prophase. New nuclear membrane forms around each chromatin network and nucleolus. At the end of the telophase, two identical nuclei are formed in the parent cell.
     4. The spindle fibers disappear. The centriole duplicates and forms a centrosome at each pole.

Cytokinesis:

• After the division of nuclear materials (chromosomes) also called karyokinesis, the cell undergoes cytokinesis.
• Cytokinesis is the process of division and separation of cytoplasmic constituents. The process of cytokinesis differs in plant and animal cells.
• The end of telophase coincides with the division of cytoplasm. In animal cells, a constriction appears outside the cell at the equatorial plane. The constriction deepens from outside towards the center and divides the cytoplasm into two compartments forming two daughter cells.

• In plant cells, cytokinesis usually begins in the early telophase with the formation of a new cell plate at the equatorial spindle. The separation of the cytoplasm begins in the center of the cell and gradually extends outwards on each side of the plane, perpendicular to the axis of the spindle.
• Thus, two new cells, each with the same number of chromosomes as was with the parent cell are formed. Following the cell division process, the new cells grow in size until they appear to be approximately the same size of the parent cell.

Significance of mitosis:

• Growth and Tissue repair:
  • Mitosis is a common means of growth (increase in cell number) and tissue repair in multicellular organisms. The fertilized egg (zygote) develops into an embryo and finally into a fetus and baby by repeated mitotic division.
• Genetic stability:
  • It results in the division of the parent cells into two exactly similar daughter cells, each with the same number of chromosomes of the mother cell. So, it serves to keep the number of chromosomes equal in all the cells of an individual. For example, when a human cell with 46 chromosomes divides by mitosis, each daughter cell will also have 46 chromosomes. In this process, the number of genes remains the same. Thus, each daughter cell has the genetic uniformity with the mother cell and this helps maintain genetic stability in living organisms.
• Asexual reproduction:
  • It is a common method of asexual reproduction in unicellular organisms.
• Regeneration and replacement of cells:
  • In some animals like star fish, planaria, axolotl larva of salamander, the regeneration of lost body parts takes place by mitotic cell division. It also replaces the damaged or lost cells like healing of wounds.

Mitotic cell division (Mitosis), its stages and Significance