3.2 The Typical Cell Cycle

The Cell Cycle is the repeating pattern of cell growth (increase in size), followed by nuclear and then cytoplasmic division (splitting of one cell to produce identical daughter cells in mitosis, or to produce unique gametes in meiosis). The cycle is divided into four (4) main stages or phases: Gap 1 (G₁), Synthesis (S), Gap 2 (G₂), and either Mitosis or Meiosis (M). G₁, S and G₂ are collectively called Interphase (Figure 3.2.1).

The first stage of interphase is a lag period, and is called Gap 1 (G₁). It is the first part of interphase. This is where the cell does its normal cellular functions and grows in size — particularly after mitosis when the daughters are half the size of the mother cell. This stage ends with the onset of the DNA synthesis (S) phase, during which each chromosome is replicated.  Though the chromosomes are not condensed yet, because S phase is still part of interphase, they are replicated as two sister chromatids attached at the centromere (Figure 3.2.2). Still in interphase and following replication, there is another lag phase, called Gap 2 (G₂). In G₂, the cell continues to grow and acquire the proteins necessary for cell division. There are various checkpoint stages, as shown in Figure 1, which are controlled by cyclins. Cyclins are a family of proteins that control the progression of a cell through the cell cycle by activating cyclin-dependent kinase enzymes, or group of enzymes, required for synthesis of cell cycle. If there are any problems with replication or acquiring the needed proteins, the cell cycle will arrest, until it can fix itself or die. The final stage is mitosis (M), where the cell undergoes cell division.

Many variants of this generalized cell cycle also exist. Cells undergoing meiosis do not usually have a G₂ phase. Cells, like hematopoietic stem cells, which are found in the bone marrow and produce all the other blood cells, will consistently go through these phases as they are constantly replicating. Other cells, as in the nervous system, will no longer divide. These cells never leave G₁ phase, and are said to enter a permanent, non-dividing stage called G₀. On the other hand, some cells, like the larval tissues in Drosophila, undergo many rounds of DNA synthesis (S) without any mitosis or cell division, leading to endoreduplication. Understanding the control of the cell cycle is an active area of research, particularly because of the relationship between cell division and cancer.

Media Attributions

• Figure 3.2.1 Cell Cycle with Cyclins and Checkpoints, by Betts et al. (2013), OpenStax, CC BY 4.0, via Wikimedia Commons
• Figure 3.2.2 Sister Chromatids by Rye et al. (2016), CNX OpenStax, CC BY 4.0, via Wikimedia Commons

References

Betts, et al. (2013, Apr 25). Figure 3.33 Control of the cell cycle [digital image]. Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-5-cell-growth-and-division

Rye et al. (2016, October 21). Figure 10.7 During prometaphase … [digital image]. Biology. OpenStax. https://openstax.org/books/biology/pages/10-2-the-cell-cycle

Long Descriptions

• Figure 3.2.1 The stages of the cell cycle are similar to a pie chart representation. A large circle is drawn, with sectors of the circle identified to correspond to various stages of the cell cycle, such as the G1 (cell growth), S (synthesis) and G2 (cell growth) stages. The size of this sector of the circle corresponds to the relative length of time of each stage. Various checkpoints which occur during the cell cycle are also identified where appropriate, along with the cyclin molecules which control the progression through the stages. [Back to Figure 3.2.1]
• Figure 3.2.2 A replicated chromosome (sister chromatids) shows the centromeric regions and the mitotic spindle fiber microtubules coming from the kinetochore regions during mitosis. [Back to Figure 3.2.2]