Question

P.S. Please answer atleast 2 questions.

1. Explain the basic phases of the cell cycle and main checkpoints.

2. Understand how cycle time range varies in different cell types with specific examples.

3, Explain START in yeast and the restriction point in animal cells. Do these points of the cell cycle make sense?

4, Why are cyclins so important? Explain cyclins and cyclin-dependent protein kinases.

Answer 1

1. The cell cycle is divided into two main phases first one is interphase and second is M-phase. The period of actual cell division, corresponds to the M phase (mitosis phase). The interphase is the time in which the cell preparing for cell division by undergoing both cell growth and DNA replication. The interphase is further divided into the following-

G1 phase- In this phase the cell prepare for the DNA replication.

S phase- In this phase the cell undergo DNA replication.

G2 phase- During this period following DNA replication and preceding the initiation of the mitotic prophase

Cell cycle checkpoints insure the fidelity of the cell division which examines that the process at each phase of the cell cycle has been accurately completed before progression into the next phase. An important checkpoint referred as start in yeast and restriction point in mammalian cell, this is the point at which the cell becomes committed to DNA replication and completes the cell cycle. It is during the G1 phase that the cell integrates mitogenic and growth inhibitory signals, and makes the decision to proceed, pause or exit the cell cycle.

Spindle assembly checkpoint (mitotic checkpoint) guarantees that anaphase onset is initiated only when all chromosomes are properly attached to microtubules and aligned at the metaphase plate.

DNA damage checkpoint involves a signal transduction pathway induced by DNA damage that blocks the cell cycle progression until DNA is properly repaired. According to the cell cycle the DNA damage checkpoints are divided into 3 checkpoints which are as follows- G1/S checkpoint, intra- S phase checkpoint and G2/M checkpoint

2. Different cells take different lengths of time to complete the cell cycle. A dividing cell spends approximately 95% of the whole time in interphase. The time duration of the phases named as G1, S and G2 varies species to species also in different cells of the same organisms. The G1 phase takes more time than the other phases of the cell cycle however the length of the G1 phase is depends upon the growth conditions. The most important phase named as M phase takes only one hour. A rapidly proliferating human cell culture the total time of the cell cycle is about 24 hours to divide, the G1 phase takes 11 hours, the S phase takes 8 hours, the G2 phase takes 4 and the M phase takes 1 hour whereas the fast-cycling mammalian cells, such as the cells of the intestine takes 9-10 hours to complete their cell cycle, when these cells are grown in culture.

3. one of the most important checkpoints in G1 has been identifies in both yeast and mammals cells. In yeast it is referred to as START whereas in mammals it is referred as Restriction point, this point at which the cell become committed to DNA replication and completes a cell cycle.

4. The protein complexes which drive each step of the cell cycle are made up of two subunits- Cyclin and cyclin dependent protein kinases (CDKs). Cyclin are the regulatory and the CDKs are worked as the catalytic protein kinases. Cyclins are also named because they undergo a ccle of synthesis and degradation in each cell cycle. There are four classes of Cyclins- G1- Cyclins, G1/S- Cyclins, S- Cyclins and M- Cyclins.

The CDKs are the protein kinases (enzymes) which add the phosphate group to target substrate. CDKs are named because their activities are regulated by proteins called Cyclin.

CDKs works with Cyclin in each phase which are as follows- CDK4 and 6+Cyclin D in G1 phase, Cyclin E + CDK 2 in G1/ S transition, CDK2 +Cyclin E in S phase and CDK1+Cyclin A and B in G2/ M phase transition.