Which statement is correct regarding cyclin?Group of answer choicesIt phosphorylates CDK on Thr-160 to activate the CDK.Cyclin contains a destruction box sequence which is targeted by ubiquitin ligase.The activity of cyclin E is inhibited when bound by phosphorylated pRB.It is present at similar concentrations throughout the cell cycle.

The correct statement regarding cyclin is: "Cyclin contains a destruction box sequence which is targeted by ubiquitin ligase."

Here's why:

1. Cyclin does not phosphorylate CDK on Thr-160 to activate the CDK. It's the other way around. CDKs (Cyclin-dependent kinases) are activated by binding to cyclins and this is a crucial step in cell cycle regulation.

2. The activity of cyclin E is not inhibited when bound by phosphorylated pRB. In fact, cyclin E forms a complex with CDK2 which is necessary for the transition from the G1 phase to the S phase of the cell cycle. The phosphorylation of pRB by this complex actually leads to the release and activation of E2F transcription factors that are necessary for the cell to enter the S phase.

3. Cyclin is not present at similar concentrations throughout the cell cycle. The concentration of cyclins varies throughout the cell cycle. They are synthesized and then degraded in a cyclic manner, which is why they are called cyclins. Their levels rise and fall during the cell cycle, which helps to control the progression of the cycle.

4. Cyclin contains a destruction box sequence which is targeted by ubiquitin ligase. This is correct. The destruction box is a sequence of amino acids in cyclins that is recognized by a protein complex called the anaphase-promoting complex (APC). The APC adds ubiquitin molecules to the cyclins, marking them for destruction by the proteasome. This destruction of the cyclins is a key step in moving the cell cycle forward.