Cell cycle

From ArticleWorld

The cell cycle is the series of events that are followed in eukaryotic cells in leading to cell division. There are three major phases which are interphase, mitosis and eventually cell division. Interphase is divided into the G1 and G2 phases where the chromosomes are not visible as they are uncoiled. This is followed by the stages in mitosis.

The first mitotic stage is prophase where the chromosomes coil, nuclear membrane disintegrates and spindle apparatus forms. Metaphase follows where the chromosomes become aligned. Anaphase is next and this is where the chromatids separate resulting in a doubling of the chromosomes. Telophase then occurs as the nuclear membrane reappears, the chromosomes uncoil, the spindle breaks apart and cell division occurs. The cycle then repeats with the G1 phase where chromosomes have one chromatid and the G2 phase where chromosomes have two chromatids.

Mitosis

This consists of four phases. These are as follows:

• Prophase
• Metaphase
• Anaphase
• Telophase

In prophase, the chromosomes start condensing and forming as DNA coils. The nucleolus then disappears and the nuclear membrane fragments and starts to disappear. Microtubule formation is initiated from the centrosomes, as this is necessary to the movement of the chromosomes. The single centrosome present in non-dividing cells duplicates on the start of mitosis. The centrosomes move apart in this phase and the microtubules are moved as well and these are now referred to as spindle fibers. During metaphase the chromosomes move towards the center of the cell. Metaphase is complete when the chromosomes divide and result in a doubling in numbers. At anaphase the division of the cytoplasm begins. This process is called cytokinesis. This results in the formation of two cells eventually. In this phase the chromosomes migrate to the poles of the cell. Telophase starts as the chromosomes reach the poles of the daughter cells. Then a process reverse to prophase occurs where there is uncoiling of the chromosomes, the nuclear membranes appear, the spindle is decomposed and the nucleolus reappears. The completion of cytokinesis occurs in telophase.

Interphase

Interphase is the non-dividing phase of the cycle. In this phase the nucleus is visible and chromosomes are invisible. Interphase consists of G1, S and G2. These are as follows:

• G1 – each chromosome has one chromatid, cell size increases and organelle synthesis occurs.
• S – DNA synthesis occurs.

G2 – each chromosome has two chromatids and synthesis of proteins like enzymes occurs for mitosis.