Question

List and describe the mechanisms used to pause the progression through the cell cycle?

Answer 1

Answer-

According to the given question-

We know that during Cell cycle the content in cell is duplicated followed by distribution of these contents into their two daughter cells. This is divided into many phases with distinct function such as

• In S phase Chromosome duplication occur
• In M phase segregation of replicated chromosomes into two daughter cell through Cytokinesis.

Both the S phase and M phase are separated by G1 and G2 gap phases. In cell cycle there are several checkpoints to arrest the cell cycle if there is detection of error in previous phase of cell cycle such as non completion of DNA replication , chromosome separation and attachment of  mitotic spindle. protein kinases also called Cdks or cyclin-dependent kinases initiate as well as regulate events of cell cycle such as replication of DNA , mitosis, as well as Cytokinesis.   Cdk activity are controlled by a proteins called cyclins. activity of Cdks depend on cyclins.

There are four types of cyclins, such as -

• G1/S-cyclins which bind to Cdks at end of G1 responsible for committing cell to go for DNA replication.
• S-cyclins which bind with Cdks during S phase for DNA replication initiation.
• M-cyclins helps in mitosis.
• G1-cyclins helps in late G1.

cyclin-Cdk complex Activation occur by CAK or Cdk-activating kinase which phosphorylates Cdk active site. cyclin-Cdk complex activity is inhibited by Phosphorylation of active site by Wee1 and dephosphorylation by phosphatase  Cdc25 responsible for increasing Cdk activity. Cell-cycle control depends on activity of cyclin-Cdk complexes which are  inactivated through proteolysis of cyclins during several stages of cell-cycle. cyclin destruction is depend upon activity of enzyme ubiquitin ligases. During the  G1 and S phase, enzyme complex known as SCF causes ubiquitylation as well as destruction of G1/S-cyclins and Cdk inhibitor proteins or CKI proteins which are responsible for controlling initiation of  S-phase. While during M phase APC or  anaphase-promoting complex causes  ubiquitylation and proteolysis of M-cyclins.

negative regulatory molecules such as Rb or retinoblastoma protein ,  p53, and p21 are tumor-suppressor proteins which also control cell cycle.

retinoblastoma protein, p53, and p21 generally act on checkpoint G1 . p53 acts when there is presence of damaged DNA inside the cells.  p53 stop the cell cycle and then recruits several enzymes for DNA repair. In DNA Repair was not processed then p53 causes apoptosis and prevent the passage of damaged chromosomes. Increase in p53 levels leads to production of p21. Rb during its active Dephosphorylated state attaches to transcription factors which is a protein called E2F. Binding of Rb to E2F, causes blockage of G1/S transition.  phosphorylation of Rb causes its inactivation and releases of E2F from Rb this leads to turn on of  gene for production of transition protein which removed the block and cell moves to next phase of cycle.