Cell cycle progression is regulated at key transition points, called checkpoints, to ensure that all cellular components are present and functioning properly before the cell proceeds to the next stage. Checkpoints are controlled by the level of cyclins. Cyclins are proteins that bind to specific kinases, called cyclin dependent kinases (Cdks).
Once activated by cyclins, Cdk proteins phosphorylate specific proteins important for cell cycle progress. Relative amounts of cyclin proteins vary through the cell cycle, controlling progression through each stage (G1,S,G2,M).
The significance of checkpoints can be demonstrated by considering what happens when they are impaired.
What would occur if there was a gain-of-function mutation in the promoter for the cyclin E gene such that cyclin E protein was always made at high levels even under conditions in which cyclin E would not normally be made?
What would occur if there was a gain-of-function mutation in the promoter for the cyclin E gene such that cyclin E protein was always made at high levels even under conditions in which cyclin E would not normally be made?
A.Cells will skip the S phase and go directly to G2 phase and not complete DNA replication.
B.Cells will pass the M checkpoint with chromosomes unattached to spindles.
C. Cells will pass through the G1/S checkpoint even if conditions are not ideal for cell division.
D. Cells will pass through the G2/M checkpoint with damaged DNA.
Which of the following might explain why the level of cyclin B in a cell is not increasing heading into mitosis?
Select all that apply.
A. The cell has not reached an adequate size.
B. The attachment of spindle fibers to the kinetochores is inadequate.
C. Spindle fibers are not properly formed.
D. The cell has not repaired damaged DNA.
Quick Reply
