1. True or False (and explain): Since there are about 1013 cells in an adult hum
ID: 56993 • Letter: 1
Question
1. True or False (and explain): Since there are about 1013 cells in an adult human, and about 1010 cells die and are replaced each day, we become entirely new people every three years.
2. CDK2 (cyclin-dependent kinase 2) is pretty much always present in growing andquiescent cells (though not usually senescent or terminally differentiated ones). However, it is only active at specific times. List the various things that must occur for CDK2 to become active.
3. When cells are grown in the laboratory, they are an “asynchronous” population, meaning that they will all be at a different point in the cell cycle. However, in order to do experiments concerning the cell cycle, it is often helpful to establish synchronized populations of cells. There are several ways to do this, listed below. Please briefly explain how each one works and what phase they synchronized cells in.
A.) Serum starvation
B.) Double thymidine block
C.) Nocodazole
4. The retinoblastoma protein (pRb) was named because of its discovery in a case of retinal cancer. However, it functions in every cell of the body and is encoded by the RB1 gene. RB1 is a classic “tumor suppressor” because, when it is mutated or lost, cancer often results.
A.) What is the normal function of pRb during the cell cycle?
B.) How is pRb inactivated?
C.) What happens when pRb is inactivated?
Explanation / Answer
False. Though regularly worn out cells are replaced by new ones, the rate of degradation of cells is not uniform to all the cells. For example, epithelial cells on the skin surface are rapidly shed due to wear and tear, and an entirely new skin may be formed in a couple of years. However, cells like neurons and nephrons have very long life span and are not replaced too often. Thus, an entirely new human is not formed.
For the CDKs to become active, they must first bind to cyclins (cyclin A/B), small proteins called suk1 or Ck. Apart from cyclin, CDKs are also activated by proteins like p3, p39.
Serum starvation is applied to achieve synchrony of cells
Retinoblastoma protein: Rbp binds and inhibits transcription factors of E2F family. He E2F proteins are dimers associated with a dimerization partner protein (DP). The E2F-DP complexes makes the cell enter into S phase. If the E2F-DP complex is inactivated, the cells remain in G1 phase.
4b. Phosphorylation of pRB inactivates it, and thus allows the cell cycle progression
4c. Phosphorylated pRb is unable to bind to E2F, and thus is unable to restrict the cell cycle progression from G1 phase to the S phase.
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