Genetics 4

Question 0

DNA strand is methylated at a ______ sequence, resulting in ________

Answer 0

GATC sequence, ds hemimethylated DNA

Question 1

Ames Test

Answer 1

• method of testing for mutagenesis using the concept that bacterial colonies require amino acids to grow.
• Observed growth= result of spontaneous back mutatios.

Question 2

Mut S and MutL

Answer 2

Proteins that form a complex and bind ds DNA in an ATP-dependent manner.

• Pull DNA in to create loop until encountering site of methylation.
• Recognizes MutH protein bound at hemimethylated GATC sequence.

Question 3

MutH

Answer 3

• protein bound at hemimethylated GATC sequence.

- Cleaves unmethylated strand, causing nick in DNA helix that serves as site of error repair.

Question 4

Depurination

Answer 4

• results from cleavage of glycosidic bond between a ribose and base moiety
• Results in removal of a purine (A or G)
• Daughter strand has point deletion

Question 5

Deamination

Answer 5

Common on cytosine groups

• results from hydrolysis of amino group on cytosine to yield uracil.
• Another commone one: 5-Methylcytosine–> thymine
• Causes C–> U, mutated daughter strand will display an A instead of G.

Question 6

DNA glycosylase

Answer 6

Recognizes a damaged base and cleaves between the base and deoxyribose in the backbone.

Question 7

Nucleotide Excision Repair

Answer 7

• common in formation of pyrimidine dimers
• Large cut made in DNA strand
• Exinucleases bind to bulks site, cleave damaged strand on either side
• Helicase removes either 13 (E coli) or 29 (human) nucleotide segment.
• Gap filled with DNA Pol epsilon (human) or DNA Pol I (E Coli)
• DNA ligase seals strand break

Question 8

Nonhomologous End Joining

Answer 8

Predominate form in humans.

• Results from accidents ds break
• Ku Heterodimers recognize end break and bind
• LOSS OF NUCLEOTIDES due to degradation
• Ku proteins recruit additional proteins that process DNA ends, evoke limited repair synthesis and eventual ligation.
• End result: repaired DNA with loss of nucleotides

Question 9

Homologous Recombination

Answer 9

• Only occurs when break takes place early in S phase or G2: sister chromatid present used as template.
• following break, exonuclease degrades 5’ ends of break site
• Sister chromatic is used as template by one of the break sites to synthesize new DNA
• New DNA pairs with remaining broken strand. Two undergo DNA synthesis
• Ligation seals break.

Question 10

BRCA1

Answer 10

• protein that binds ds DNA breaks, acting as recognition site for recruitment of proteins involved in homologous recombination/repair.
• If repair unable to correct break, cell will attempt apoptosis (activates p53)

Question 11

p53

Answer 11

tumor suppressor gene.

• When normally activated, induces apoptosis
• Inactivated –> clonal expansion of cells even in presence of damages (leads to tumors)

Question 12

Homozygous BRCA1 deficiency

Answer 12

Causes DNA damage to proceed and avoid repair. Inability to repair DNA
-Either end up with cell death or tumor growth.

Question 13

Retrogene

Answer 13

When a cell expresses an RNA and then that RNA is retrogradely transcribed back to DNA.
That DNA eventually yields gene in genome with no introns, but can replace the ancestral gene and express.