Question

1. Which checkpoint and phase of the cell cycle is the expression of BRCA1 and BRCA2 the greatest? What is the function of BRCA1 and why would it make sense that its expression is high at the checkpoint/phase of the cell cycle?

2. Outline the two parts of the eukaryotic cell cycle and describe the three main checkpoints in cell cycle regulation.

*PLEASE ANSWER QUESTIONS AS SIMPLY AS POSSIBLE. I AM IN A BASIC GENETICS COURSE*

*The answers should be completed in a paragraph or less for each question. Please and thank you!*

Answer 1

1. BRCA1 has a bell-shaped pattern of expression during the cell cycle (Note that active BRCA1 is tagged with a phosphate group in a process known as phosphorylation). The expression of BRCA1 slowly starts increasing at S phase and peaks at the G2 phase and deceases at mitosis. As far as the checkpoint is considered the increase of BRCA1 expression starts from the DNA replication checkpoint that is G2 checkpoint. BRCA2 is highly expressed in the S phase of the cell cycle. Maintenance of genomic integrity is a major role of BRCA2 which happens at multiple checkpoints hence they play a role in G1 and G2/M checkpoint as well as spindle assembly checkpoints. The main reason behind this being that DNA damage and integrity have to be checked at all points. especially during replication as S phase. In response to DNA damage During the transition from G1 to S phase DNA polymerase stalls, BRCA1 gets activated. BRCA1 along with other proteins at the arrested regions initiate a BRCA1-mediated recombination repair mechanism. once DNA gets repaired the cell cycle continues.

2. Major parts of the cell cycle are the interphase where the cell grows, genetic material replicates and the cell becomes ready for division and mitosis where the nucleus divides. Note that the eukaryotic cell cycle is divided into 3 parts majorly and not 2. The third part is cytokinesis where the cell cytoplasm divides giving rise to 2 daughter cells from a parent cell thus completing the cell cycle. The three major checkpoints in the cell cycle are as follows

1. G1 checkpoint (At G1/S phase): Primary checkpoint, checks for DNA damage, growth factors and nutrients. it is a primary checkpoint because the main decision of should the cell divide or not is taken at this checkpoint. if the criteria are not met the cells move into a quiescent GO (G zero) Phase
2. G2 checkpoint (At G2/M phase): checks of DNA integrity and DNA replication. If there are errors found cell cycle gets stalled and repair takes place and as mentioned above BRCA1 and BRCA2 have a major role to play in this repair
3. Spindle checkpoint (Between Metaphase and Anaphase): checks if the spindle is attached to the chromosome for separation(Note: chromosome at the metaphase stage are aligned at the metaphase plate). if there are errors the cells cycle will pause so that the spindle attaches to the chromosome.