Biology: the cell cycle

Tutoring biology (aka life science), one encounters the cell cycle. The tutor summarizes it.

The cell cycle is a top-down view of how a cell spends its time. It’s divided into two main phases, interphase (preparing to divide) and mitosis (cell division). Interphase is subdivided into G1, S, and G2; about 90% of the time, a cell will be in interphase.

G1 is an accumulation and growth phase: the cell is building up energy resources, as well as its stock of proteins and DNA components, to prepare for replication of its chromosomes.

During the S phase, the chromosomes are replicated; each copy is stuck to its original by a centromere. One might refer to the two identical chromosomes in such a pair as sister chromatids. Some people refer to them as a single chromosome, perhaps because they contain information from only one; they’re just two copies of it. Also during the S phase, the centrosome is copied. This is also preparation towards mitosis.

Next, in the G2 phase, the cell restores the energy reserves drawn down during the S phase. As well, more proteins are synthesized that will be used during mitosis. Some of the cell’s specific structures (organelles) are copied. Moreover, the cytoskeleton, a network of fibres used to cause movement within the cell, is taken apart; its parts will be repurposed as the spindle, which carries out separation of sister chromatids into two sets during mitosis.

Finally, mitosis occurs: the cell divides into two identical sister cells. After mitosis, the cell re-enters G1.

Comments:

• During G1 and G2 the cell grows; hence “G” might stand for “Growth.”
• During S, DNA is produced, or Synthesized.
• There is a G0 phase that some cells enter. However, the main ideas about the cell cycle are expressed as G1,S,G2, and Mitosis (aka Division).

Source:

bio.libretexts.org

bio.libretexts.org