Chapter 18, Problem 30Q

A.

Summary Introduction

To explain: The consequence of mutation in a cell where M cyclin cannot be degraded.

Concept introduction: Cyclins are part of the heterodimeric kinase that forms its regulatory subunit. The cell cycle has its specific cyclins for specific cell cycle phases. They are produced only during the prior gap phase. The cyclins regulate the cyclin-dependent kinase (Cdk) activity. The Cdks will be present in all four phases but are activated only in the presence of the respective cyclins.  Once the work of cyclin is done, they are immediately degraded.

B.

Summary Introduction

To explain: The consequence of mutation in a cell where p21 is always expressed in high levels.

Concept introduction: p21 is CKI (cyclin-dependent kinase inhibitor). p21 has the ability to interact with all Cdk and inhibit them. They are primarily involving in halting the cell cycle for the cell to carry out DNA repair activity. The p21 halts the cell cycle by inhibiting the Cdks and the PCNA, which is the clamp for the polymerases thus inhibiting the assembly of replisome complex at the end of G1 phase, thereby preventing DNA replication until DNA repair is completed.

C.

Summary Introduction

To explain: The consequence of mutation in a cell where Rb cannot be phosphorylated.

Concept introduction: Retinoblastoma (Rb) is a tumor suppressor gene whose homozygous allele or loss of function mutation results in several types of cancer primarily retinoblastoma. It plays a very important role in the cell cycle. Rb prevents excessive cell proliferation and growth.