Question

The most likely explanation for the difference in the length of time spent in S phase by α and ß cells is:

ß requires more time to synthesize components of the mitotic spindle

α has an excess of G-cyclin

ß has more DNA in the genome

Answer 1

According to the given question-

Cell cycle is divided into three states namely quiescent , interphase and mitosis states as well as into five phases called G0, G1, S, G2 and M phase. The quiescent states is also called senescent phase comprises G0 phase while interphase consists of G1, S and G2 phase and M phasr is divided into prophase, metaphase, anaphase and telophase. These are controlled by several check point present at the end of G1 , end of G2 and during the M phase.

Progression from one phase to another phase is regulated by cyclins, cyclin dependent kinases and tumor suppressors.

During G1 phase the progression is controlled by tumor suppressor genes that code for transcription factor, protein that are responsible for detecting DNA damage , enzymes that allow to cell to S phase and several signalling molecules that are responsible for activating these enzymes.

Gene E2F regulate expression of transcription gene , cyclin dependent kinases, cyclin E, and cyclin A. If there is any DNA damage then p53 molecule stimulating production of p21 which binds with the cyclin - cdk complex and inhibited them and causes cell cycle arrest.

Tumor suppressor retinoblastoma protein responsible for inhibition of E2F expression. CyclinD- cdk4 and CyclinD- cdk6 complex responsible for phosphorylation of retinoblastoma protein and causes inactivation of retinoblastoma protein as well as same time causes expression of E2F. E2F expression causes transcription gene expression and formation of CyclinE- cdk2 and pushed the cell into S phase. CyclinE-cdk2 is required for the synthesis of DNA while CyclinA-cdk1 ans CyclinB-cdk1 is responsible for promoting the event that are happening in mitosis.

At the end of mitosis anaphase promoting complex ubiquitination and destruction of CyclinA-cdk1 and CyclinB-cdk1 and termination of M phase.

So based on this the correct answer for the given question- The most likely explanation for the difference in the length of time spent in S phase by α and ß cells is the α has an excess of G-cyclin.