Chapter 6.4

Question 1

What is meant by the term oncogenes?

Answer 1

• they are mutated genes that can cause cancer
• before they are mutated
• > they are referred to as proto-oncogenes

Question 2

What are antioncogenes

Answer 2

• tumor suppressor genes
• > eg; p53 and Rb
• > function to stop tumor progression
• > do this by inhibiting the cell cycle or participating in DNA repair
• in order to inactivate the tumor suppressor activity, p53 and Rb alleles need to be suppressed
• > they both need to be suppressed. as long as one is active then it is all g

Question 3

What is the difference in methylation levels between the parent strand the template strand

Answer 3

Template strand

• > more heavily methylated
• > as it has existed in the cell for a longer period of time

Question 4

Why does the lagging strand have considerably more mutations than the leading strand

Answer 4

• because DNA ligase joins the lagging strands together
• > unfortunately, it lacks the proofreading ability
• > so there are more mutations in the lagging strand

Question 5

What is mismatch repair? In which stage of the cell cycle does it occur?

Answer 5

• it happens in the G2 phase
• > detect and remove errors introduced in the S phase of the cell cycle
• the enzymes are encoded by genes MSH2/MLH1
• > these enzymes are homologous of MutS and MutL in prokaryotes

Question 6

What two processes are used to recognize and remove damage found in the G1 and G2 cell cycle phases?

Answer 6

-nucleotide excision repair and base excision repair

Question 7

What is the role of nucleotide excision repair? Describe the process of this mechanism.

Answer 7

-to get rid of thymine dimers that interfere with DNA replication and normal gene expression

• first, specific proteins scan the DNA molecule and recognize the lesion because of a bulge in the strand
• then excision endonuclease makes nicks in the phosphodiester backbone of the damaged strand on both sides of the thymine dimer
• > removes the defective oligonucleotide
• > DNA polymerase then fills this gap and the nicks in the strand is covered by DNA ligase

Question 8

What is the role of base excision repair? Describe the process of this mechanism.

Answer 8

• the removal of uracil from DNA
• > uracil ends up in the DNA due to cytosine deamination
• first the affected base is recognized and removed by a glycosylase enzyme
• > leaves behind an apurinic/apyridiminic site(abasic site)
• > the abasic site is recognized by AP endonuclease that removes the damaged sequence from the DNA
• > DNA polymerase/ DNA ligase then fill in the gap