Mammalian cells that have just crossed the Restriction Point of the cell cycle are likely to have:

Correct option is C

Cyclin B is a key regulator of the cell cycle, particularly involved in the transition from G2 to M phase. After the restriction point in the G1 phase, cells are in a phase where they are preparing for DNA replication and entry into the S phase. At this point, mitotic cyclin B levels are low, because it is required later for the progression into mitosis. During interphase (G1, S, and G2 phases), the cell accumulates the appropriate cyclins for the upcoming phases, and cyclin B is synthesized in preparation for mitosis. It is only after cells have completed DNA replication and moved into G2 phase that cyclin B levels begin to rise significantly, eventually triggering the transition to mitosis.

Information Booster:

1. Mitotic cyclin B is synthesized during interphase and accumulates to activate Cdk1, which is essential for mitotic entry.

2. Low levels of cyclin B are seen just after the restriction point because the cell is still in G1 and preparing for DNA replication in the S phase.

3. The restriction point signifies the commitment to cell division, and cyclin B remains low in G1, with accumulation occurring later in G2 to drive mitotic events.

4. Cyclin B levels increase progressively in the G2 phase to prepare for the activation of Cdk1 and the progression into mitosis.

5. Cyclin B accumulation is tightly regulated, ensuring that cells enter mitosis only after DNA replication is complete and other cell cycle checkpoints are passed.

6. The restriction point is not directly associated with the activation of mitotic cyclins but rather with cell cycle progression from G1 to S phase, where cyclin B levels remain low.

Additional Knowledge:

• High levels of unphosphorylated retinoblastoma protein (option 1) are typically found in G1, before the cell passes the restriction point. Retinoblastoma protein is involved in preventing the cell from entering S phase unless it is phosphorylated.

• An active anaphase promoting complex (option 2) is involved in later stages of the cell cycle, specifically in regulating the metaphase-anaphase transition, and is not active immediately after the restriction point.

• Condensed chromosomes (option 4) are observed during mitosis, but not immediately after crossing the restriction point. Chromosome condensation occurs only during prophase in mitosis.