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.

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.

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.

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.

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.

The contractile ring is made up of actin and myosin-2 and is used to initiate cytokinesis. Polar and kinetochore microtubules have important functions during earlier stages of mitosis. Polar microtubules interact within the cytoplasm during metaphase and anaphase, while kinectochore microtubules directly attach to chromosome centromeres to facilitate separation. Dyenin is a motor protein that interacts with microtubules to aid in protein transport.

During mitosis, sister chromatids will line up in the center of the cell so that they may be pulled to opposite ends by the spindle fibers. This lining up takes place in the second phase of mitosis: metaphase.

Cyclin proteins fluctuate in level during the different stages of the cell cycle (except for Cyclin D). For example, Cyclin E regulates the entry into S phase. The expression of the cyclin E gene increases, which leads to higher Cyclin E protein levels in the cell. After the cell passes S phase, Cyclin E protein is actively destroyed by ubiquitination.

When Cyclins are bound to Cdks, they can regulate the activity of proteins that regulate processes like replication and microtubule formation; however, of the cellular processes listed, blocking ubiquitination would be the only process that would directly interfere with cyclins' role to regulate the cell cycle as cyclin proteins need to be destroyed at the correct time to allow the cell cycle to progress.

激酶的蛋白质,使磷酸化潜艇trates, often activating these substrates. In the context of cell cycle progression, cyclins interact with cyclin-dependent kinases (CDKs) and activate the CDK kinase domain to phosphorylate substrates and promote cell cycle progression. CDK inhibitors interact with CDKs to inhibit the CDK kinase domain from phosphorylating substrates.

The correct answer is prometaphase, in which the nuclear membrane dissolves and the microtubules attach to the centromeres. Interphase occurs before mitosis begins, and includes S phase, where the chromosomes are duplicated. In anaphase, the chromosome duplicates are separated by the microtubules. In telophase, cell division begins with the newly separated chromosome copies.