All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources
Example Question #1 : Cellular Division
During which phase of the cell cycle are cells considered quiescent?
Any portion of interphase
The cell cycle is divided into several phases, with checkpoints that control transitions between phases of the cell cycle. The G1 checkpoint (restriction checkpoint) is the first of these barriers, and requires adequate quantities of the cyclin protein in order for the cell to continue maturing in preparation for division. When a cell fails to express cyclin, the cell reverts to an inactive quiescent state and stops preparations for division. This state is known as the G0 phase. The G0 phase can be overcome if cyclin is reintroduced to the cell environment.
The G1 and G2 phases are involved in protein production and organelle replication. DNA replication occurs during the S phase, between G1 and G2. The cell enters the M phase, mitosis, after passing a checkpoint that follows the G2 phase. G0, G1, S, and G2 phases are all considered part of interphase.
Example Question #2 : Cellular Division
What is the primary purpose of the S phase of the cell cycle?
Interphase is composed of three subphases: G1, S, and G2. While the two G phases are dedicated to cellular growth and organelle replication, the S phase is used to replicate the genetic material of the cell.
Example Question #1 : Help With The Cell Cycle
In the cell cycle, what is primarily responsible for cell cycle progression from G2 to M phase and is also referred to as the maturation promoting factor when in complex with cyclin-dependent kinase 1 (Cdk1)?
None of these
The correct answer is cyclin B. Cyclin B concentration in the cell spikes at the transition from G2 phase to mitosis/meiosis. Cyclin E controls pre-replication complex assembly which makes chromatin replicable during G1 to S phase. Cyclin A then replaces cyclin E in the nucleus, promoting DNA replication. Cyclin D is also important in driving G1/S phase transition and is sustained in proliferating cells the longest of the cyclins.
Example Question #2 : Help With The Cell Cycle
Which cyclin remains consistently expressed starting in G1 phase through mitosis to regulate cell cycle progression?
The correct answer is Cyclin D. This cyclin is expressed throughout the cell cycle, however, it directly regulates the transition from G1 to S phase. Cyclin A is also involved in the G1 to S phase transition and is expressed into the G2 phase. Cyclin E is expressed during the transition from G1 to S phase. Cyclin B is expressed during the transition from G2 to mitosis and regulates this progression. There is no Cyclin F in the cell cycle.
Example Question #5 : Cellular Division
The expression of which of the following protein classes is normally associated with G1 phase of the cell cycle?
I. Cyclin-dependent kinases (CDKs)
III. Nucleic acid polymerases
II and III
I and II
The two main protein types that are crucial to cell cycle regulation are cyclins and CDKs (cyclin-dependent kinases). CDKs are expressed at relatively stable levels at all phases of the cell cycle. Their regulatory partners are cyclins which, as the name suggests, are expressed only when they are needed to help activate a specific CDK and progress the cell to a new phase of the cell cycle. All of the choices are CDKs, and should be expressed during G1 of the cell cycle. Caspases are proteins that are involved in apoptosis. Nucleic acid polymerases would be expected to have elevated activity during S phase of interphase during which DNA is duplicated.