Synaptotagmin is a calcium sensor that is associated with the vesicle to be exocytosed. In a high calcium environment, synaptotagmin becomes activated and interacts with syntaxin, a SNARE protein docked in the membrane from which the vesicle will be exocytosed. This interaction permits selective exocytosis during processes such as neurotransmission when there is a large calcium influx, indicating a message must be relayed to the next cell. 

Bax and Bak dimerize to form a pore in the mitochondria outer membrane, which allows cytochrome c to escape into the cytosol. When cytochrome c is found in the cytosol, procaspase becomes activated and is cleaved into caspase. Once the caspase cascade begins the cell is destined for death.

Bax and Bak have nothing to do with the apoptosome and, while Bcl2 does block Bax and Bak from dimerizing, Bax and Bak do not prevent the action of Bcl2.

A GEF activates a small GTPase by exchanging a bound GDP (which confers an inactive state) for a GTP (which is higher energy, and activates the protein). A GAP performs the opposite; GAPs enhance the intrinsic GTPase activity of the small GTPase, which causes hydrolysis of the GTP on the active protein, thus converting it back to GDP and an inactive state.

Interactions between , , , and PKC do indeed occur downstream of activation of PLC to contribute to numerous downstream cascades primarily initiated by protein kinase C (PKC). However, it is important to understand that the second messengers are  and _, which are specifically formed by the cleavage of , and each of the other molecules is considered an effector of those second messengers in this context.

Proteins have different level of protein structure, termed primary, secondary, and tertiary (quarternary is also a type in certain proteins). The 3D shape of proteins is largely due to the tertiary structure of a protein. This level is dictated by the specific amino acid sequence of the protein.

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.