Question

9. In human cells four different cyclins drive the cell cycle (see picture). Please, note that the cyclins overlap (the picture shows the concentration of each cyclin protein in the cytoplasm). Write a short essay or outline of an experiment to test the following things: How can you figure out how much cyclin protein is present in the cytoplasm at different phases of the cell cycle? B. How can you figure out if there is overlap between the cyclins? For example, note that cyclin D overlaps with the other ones. c How can you figure out which cyclin drives the cell cycle at any given cell stage? For example, cyclin E overlaps with cyclin D. What would you do to test whether cyclin E or D drives the cell into S-phase? In your answers, be as specific as you can. Use techniques that we learned in class and explain the outcomes of your proposed experiments. In other words how would the results of your experiments test each one of the questions above? Cyclin Expression Cycle Cyclin E Cyclin A Cyclin B Concentration Cyclin D G1 Phase Phase G2 Phase Mitosis

Answer 1

1. Cyclin proteins work as core cell cycle regulators.

• Each cyclin is associated with a phase or set of phases.
• Cyclin D is associated with G1 phase
• Cyclin E is associated with G1/S phase
• Cyclin A is associated with S phase
• Cyclin B is associated with M phase.
• Cyclins helps in driving the functions associated with that phase. For example Cyclin B is associated with chromatin condensation.
• A typical cyclin will only increase at the stage when it is needed . Hence we can figure out how much cyclin protein is present in the cytoplasm i.e, Cyclin B increases at the phase of transition from G2 to M phase.
• Single cell time lapse microscopy of CDK2 activity followed by immunofluoresence can help us to determine how much cyclin protein is present.

2. Cyclin D is intiated during G1 and drives expression of Cyclin E and cyclin A . presence of increased cyclin concentration level in set of phases indicates overlap between cyclins.

3. Which cyclin drives the cell cycle at any given stage can be indicated by increased cyclin concentration at that particular stage. for example in M phase concentration of cyclin B will be higher compared to others.

Cyclin E drives the cell into S phase because Cyclin E (G1/S) rise in late G1 and fall in late S. it binds to G1 phase Cdk2 and thus helps in intiating DNA duplication.