6.2 - Cell Cycle and Mitosis

Cell cycle is a 4-stage process that takes place in a cell as the cell grows and divides. Each complete cell division will result in two daughter cells. The four stages of cell division are Gap 1 ( G₁ phase ), Synthesis (S phase), Gap 2 ( G₂ phase ) and mitosis.

Figure 3 - The G₁, S and G₂ phases are also known as interphase. The process of mitosis and cytokinesis are also called M phase.

Mitosis

Mitosis is a process where a single cell divides to form two new identical daughter cells. The two daughter cells contain the same number of chromosomes and genetic content as their parent cell. Mitosis can be divided into 4 phases:

• Prophase
• Metaphase
• Anaphase
• Telophase

Figure 4 - 1. During prophase, chromatin condenses and becomes tightly coiled to form chromosome structure. Each chromosome consists of two sister chromatids joined together at the centromere. At the end of prophase, nucleolus disappear and nuclear membrane disintegrates.

Figure 5 - 2. Metaphase follows after prophase. Centrioles migrate to the opposite poles of the cell. The mitotic spindle/spindle fibres are fully formed. All the chromosomes are lined up randomly at the metaphase/equatorial plate. Metaphase ends when the centromeres divide.

Figure 6 - 3. Anaphase begins when the sister chromatids separate at the centromere. Shortening of the spindle fibres pull the sister chromatids apart to the opposite poles. The separated chromatids are referred to as daughter chromosomes. Anaphase ends when the chromosomes reach the poles of the cell.

Figure 7 - 4. Telophase begins when the sets of chromosomes reach the opposite poles of the cell respectively. The chromosomes then start to uncoil to become their original fine chromatin threads again. Nucleoli and nuclear membrane are reformed. Spindle fibres disappear and the process of mitosis is now complete. Cytokinesis follows after the telophase stage.

The Differences between Mitosis and Cytokinesis in Animal and Plant Cells

In plant cells, the spindle fibres form even though they do not have centrioles. The differences in the cytokinesis process in animal and plant cells can be seen in the figures below.

Figure 8 - 1. In animal cells, the microfilaments in the cytoplasm contract pull a ring of plasma membrane inwards. The cytoplasm is constricted in the middle of the cell between two nuclei.

Figure 9 - 2. The formation of cleavage furrow pinches at the equator of the cell.

Figure 10 - 3. The cleavage furrow deepens progressively until the cell separates, forming two daughter cells.

Figure 11 - 1. In plant cells, membrane-enclosed vesicles form and gather at the equator of the cell.

Figure 12. - 2. The vesicles join together to form a cell plate.

Figure 13 - 3. The cell plate divides the cell into two daughter cells. Cellulose produced by the cell strengthens the newly formed cell wall.

The Necessity of Mitosis

Controlled mitosis is important because the genetic information carried by the chromosomes is necessary for proper functioning of an organism. Mitosis is important in cell repair and regeneration. Here are some examples where mitosis plays an important role:

• Lizards grow new tail if their tails break
• Starfish replaces lost arms by mitosis
• Liver cells divide to replace damaged and injured tissues
• Stem cells culture to produce meats
• Animal cloning (Dolly, the sheep)