G2 checkpoint

Examples of G2 checkpoint in the following topics:

• Regulation of the Cell Cycle at Internal Checkpoints

  • These checkpoints occur near the end of G1, at the G2/M transition, and during metaphase .
  • The G2 checkpoint bars entry into the mitotic phase if certain conditions are not met.
  • However, the most important role of the G2 checkpoint is to ensure that all of the chromosomes have been accurately replicated without mistakes or damage.
  • The cell cycle is controlled at three checkpoints.
  • Proper chromosome duplication is assessed at the G2 checkpoint.

• Non-Thermal Emission

  • $\displaystyle \frac{n_2 g_1}{n_1 g_2} = \exp \left (-\frac{h\nu}{kT} \right ) < 1$

• Einstein Coefficients

  • The second level has an energy $E+h\nu_0$and a statistical weight of $g_2$.
  • $\displaystyle \frac{n_1}{n_2} = \frac{g_1 \exp(-E/kT)}{g_2 \exp (-(E+h\nu_0)/kT)} = \frac{g_1}{g_2} \exp (h\nu_0/kT).$
  • $\displaystyle \frac{2 h}{c^2} \frac{\nu^3}{\exp ( h \nu_0 / k T) - 1} = \frac{ A_{21}}{ (g_1/g_2) \exp(h\nu_0/kT) B_{12} - B_{21}} \\ \displaystyle = \frac{A_{21}}{B_{21}} \frac{1}{(g_1/g_2) \exp(h\nu_0/kT) B_{12}/B_{21} - 1} .$
  • $\displaystyle A_{21} = \frac{2h\nu^3}{c^2} B_{21},\\ \displaystyle g_1 B_{12} = g_2 B_{21}.$

• Interphase

  • The three stages of interphase are called G1, S, and G2 .
  • In the G2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation.
  • There may be additional cell growth during G2.

• Calculating the Emission and Absorption Coefficients

  • $\displaystyle \alpha_\nu = \frac{h\nu}{4\pi} n_1 B_{12} \left ( 1 - \frac{g_1 n_2}{g_2 n_1}\right ) \phi(\nu) \\ \displaystyle S_\nu = \frac{2h\nu^3}{c^2} \left ( \frac{g_2 n_1}{g_1 n_2} - 1 \right )^{-1}

• Regulator Molecules of the Cell Cycle

  • In addition to the internally controlled checkpoints, there are two groups of intracellular molecules that regulate the cell cycle.
  • Two groups of proteins, called cyclins and cyclin-dependent kinases (Cdks), are responsible for the progress of the cell through the various checkpoints.
  • The different cyclins and Cdks bind at specific points in the cell cycle and thus regulate different checkpoints.
  • For the cell to move past each of the checkpoints, all positive regulators must be "turned on" and all negative regulators must be "turned off."
  • There is a direct correlation between cyclin accumulation and the three major cell cycle checkpoints.

• Synchrotron Absorption

  • $\displaystyle S_\nu = \frac{2h\nu^3}{c^2} \left ( \frac{g_2 n_1}{g_1 n_2} - 1 \right )^{-1}.$
  • $\displaystyle \frac{g_2 n_1}{g_1 n_2} - 1 = \frac{(E+h\nu)^2 E^{-p}}{E^2 (E+h\nu)^{-p}} - 1 = \left ( 1 + \frac{h\nu}{E}\right )^{p+2} - 1 \approx \left ( p + 2 \right ) \frac{h \nu}{E}$

• LTE

• Proto-oncogenes

  • For example, a mutation that allows the Cdk gene to be activated without being partnered with cyclin could push the cell cycle past a checkpoint before all of the required conditions are met.
  • In addition to the cell cycle regulatory proteins, any protein that influences the cycle can be altered in such a way as to override cell cycle checkpoints.

• Tumor Suppressor Genes

  • This discovery is not surprising in light of the multiple roles that the p53 protein plays at the G1 checkpoint.
  • Essentially, without a fully functional p53, the G1 checkpoint is severely compromised and the cell proceeds directly from G1 to S regardless of internal and external conditions.