S phase

Date: 2015-10-07; view: 371.

Interphase

Before a cell can enter cell division, it needs to take in nutrients. All of the preparations are done during the interphase. Interphase proceeds in three stages, G₁, S, and G₂. Cell division operates in a cycle. Therefore, interphase is preceded by the previous cycle of mitosis and cytokinesis. Interphase is also known as preparatory phase. In this stage nucleus and cytosol division does not occur. The cell prepares for division.

G₁ phase

The first phase within interphase, from the end of the previous M phase until the beginning of DNA synthesis is called G₁ (G indicating gap). It is also called the growth phase. During this phase the biosynthetic activities of the cell, which had been considerably slowed down during M phase, resume at a high rate. This phase is marked by the use of 20 amino acids to form millions of proteins and later on enzymes that are required in S phase, mainly those needed for DNA replication. Duration of G₁ is highly variable, even among different cells of the same species. It is under the control of the p53 gene.

The ensuring S phase starts when DNA replication commences; when it is complete, all of the chromosomes have been replicated, i.e., each chromosome has two (sister) chromatids. Thus, during this phase, the amount of DNA in the cell has effectively doubled, though the ploidy of the cell remains the same. During this phase, synthesis is completed as quickly as possible due to the exposed base pairs being sensitive to external factors such as any drugs taken or any mutagens (such as nicotine).

G₂ phase

During the gap between DNA synthesis and mitosis, the cell will continue to grow. The G2 checkpoint control mechanism ensures that everything is ready to enter the M (mitosis) phase and divides.