Recall that one approach to the study of cell cycle regulation has been to fuse cultured cells that are at different stages of t
he cell cycle and observe the effect of the fusion on the nuclei of the fused cells (heterokaryons). When cells in Gl are fused with cells in S, the nuclei from the Gl cells begin DNA replication earlier than they would have if they had not been fused. In fusions of cells in G2 and S, however, nuclei continue their previous activities, apparently uninfluenced by the fusion. Fusions between mitotic cells and interphase cells always lead to chromatin condensation in the nonmitotic nuclei. Based on these results, identify each of the following statements about cell cycle regulation as probably true (T), probably false (F), or not possible to conclude from the data (NP). Sort each statement into the appropriate bin. 1. The transition from mitosis to G1 appears to result from the disappearance or inactivation of a cytoplasmic factor present during M-phase.
2. The activation of DNA synthesis may result from the stimulatory of one or more cytoplasmic factors.
3. Gl is not an obligatory phase of all cell cycles
4. The transition from G2 to mitosis may result from the presence in the G2 cytoplasm of one or more factors that induce chromatin formation.
5. The transition from S to G2 may result from the presence of a cytoplasmic factor that inhibits DNA synthesis.
A. T
B. F
C. NP
In somatic cells, the cell cycle can be divided into 1-the interphase, 2-the mitotic phase or M phase, and 3-the G₀ phase. In turn, the interphase of the cell cycle can be divided into three phases: 1-the gap 1 (G1) phase or growth 1 phase, 2-the S phase (DNA replication or DNA synthesis), and 3-the gap 2 (G2) phase or growth 2 phase. During the G1 phase, the cell synthesizes key enzymes which are required during DNA replication (S phase) and cell division (M phase). Cytoplasmic factors are proteins and messenger RNAs (mRNAs) that control cell functions during the cell cycle. This cycle is modulated by proteins known as control factors, i.e., cyclins and cyclin-dependent kinases (Cdks) that work together to control the progression through different phases of the cell cycle. The transition from S to G2 occurs after DNA replication. This transition (S to G2) is controlled by a DNA damage checkpoint orchestrated by the ATM (Ataxia telangiectasia mutated) and ATR (Ataxia Telangiectasia and Rad3 related) kinases which are recruited to DNA damage sites. Moreover, the transition from G2 to M phase is mediated by the accumulation of mitotic inducers, which lead to an increase in the activity of mitotic kinase and finally trigger mitotic entry.
Sensory information passes both the auditory and the limbic systems by the means of medial geniculate nucleus (MGN), a small oval mass that protrudes slightly from the underside of the thalamus, a big double-lobed structure buried under the cerebral cortex. Before the signal can travel on, however, it passes through another nearby structure called the thalamic reticular nucleus (TRN), which evaluates whether or not it should be passed on.
