Mitosis and growth
For an organism to grow, the number of cells must increase. An increase in the number of cells of an organism is brought about by a process known as mitosis.
Mitosis is the process of cell division whereby the cell nucleus and cytoplasm divide to produce two identical daughter cells each with similar sets of chromosomes as the parental cell.
Mitosis involves the division of body cells called somatic cells. The somatic cells are diploid, meaning that they have two sets of chromosomes (2n). Chromosomes consist of two parallel strands called chromatids joined at a centromere as shown in Figure 1.2.
Centromere is a region where two sister chromatids are held together after the replication of a chromosome. During cell division, microtubule-like structures known as spindle fibres are formed from the centrioles. The fibres are attached to a point known as kinetochore.
Mitosis facilitates the growth and replacement of damaged cells.
Stages of mitosis
Mitotic cell division involves a series of consecutive events that occur between one cell division and the next. This sequence of events makes up the cell cycle.
The cell cycle is divided into three major phases:
- Interphase (preparatory phase),
- Mitotic phase (dividing phase), and
Interphase involves the process of cell growth and DNA replication whereas
Mitotic phase involves mitosis.
Cytokinesis phace involves the division of the cytoplasm to give two daughter cells.
Mitosis is a continuous process involving four stages which are prophase, metaphase, anaphase, and telophase. The interphase stages precede these stages.
Interphase; This is a non-dividing stage or phase that prepares cells for division. During the interphase stage, a cell is engaged in metabolic activities that prepare it for the mitotic division phase.
In this phase, there is an intensive synthesis of cell organelles and the growth of the cell.
Chloroplasts and mitochondria, if present, also replicate in this stage. It also involves the process of DNA replication to create two copies of DNA strands. At this stage, chromosomes are not clearly seen in the nucleus as seen in Figure 1.4. Each chromosome exists as a pair of chromatids held together by a centromere as shown in Figure 1.5.
Prophase is the longest stage of mitotic cell division occurring after the interphase stage. At this stage, the chromatids tend to shorten, thicken and become visible. Centrioles move to the opposite poles of the cell as shown in Figure 1.5. Microtubules called spindle fibres develop from the centrioles.
At the end of the prophase stage, nucleolus and nuclear membrane disintegrate and release chromosomes into the cell. The spindle fibres extend across the cell from one pole to another and are attached to the chromosomes. The role of the spindle fibres is to organise the chromosomes and move them during mitosis.