Exam #4: DNA Mutation, Replication & Repair

Question 1

Name three common DNA lesions

Answer 1

1) Depurination
2) Deamination
3) Pyrimidine Dimers

Question 2

Somatic Mutation

Answer 2

A mutation that affects only the cell where the mutation occurred, and its progeny

Question 3

Germline Mutation

Answer 3

A mutation that is passed to the offspring

Question 4

Gene Mutation & Frequency

Answer 4

• Variation in nucleotide sequence via exchange, insertion, or deletion
• Occurs 10^-10 per base pair per cell division
• LEAST frequent

Question 5

Chromosome Mutation & Frequency

Answer 5

• Rearrangement of chromosome following ds-DNA break or faulty recombination
• Occurs 6 x10-4 times per cell division
• Intermediate frequency

Question 6

Genome Mutation & Frequency

Answer 6

• Missegregation or chromosomes during mitosis or meiosis that results in a surplus or loss of chromosomes
• Occurs 10-2 times per cell division
• MOST frequency & MOST devastating

Question 7

Point Mutation

Answer 7

Exchange of one nucleotide for another

Question 8

Silent Mutation

Answer 8

Change in nucleotide sequence DOES NOT result in a change in the amino acid sequence

Question 9

Missense Mutation

Answer 9

Change in nucleotide sequence DOES result in a change in amino acid sequence

Question 10

Nonsense Mutation

Answer 10

Change in the nucleotide sequence results in the generation of a STOP codon

Question 11

List three processes that can lead to small insertions or deletions

Answer 11

1) Incorrect Recombination
2) Strand Slippage During Replication
3) Intercalating Agents (chemical agent)

Question 12

Frameshift Mutation

Answer 12

Insertion or deletion of nucleotides alters the codon reading frame and produces a new protein

Question 13

Meiosis

Answer 13

• Process by which 4 haploid germ cells are created from 1 diploid cell
• Two successive cell divisions without DNA replication

Question 14

Crossing-over

Answer 14

Exchange of homologous chromosome fragments during meiosis

Question 15

Disjunction

Answer 15

The pulling apart of homologous chromosomes during anaphase or meiosis

Question 16

Non-disjunction

Answer 16

Incomplete separation of chromosomes during anaphase of meiosis

Question 17

Consequences of Non-disjunction

Answer 17

• Germ cells with surplus chromosomes

- Germ cells missing chromosomes

Question 18

In which autosomes is trisomy observed?

Answer 18

• 13, Patau Syndrome
• 18, Edward Syndrome
• 21, Down Syndrome

Question 19

Mosaicism

Answer 19

• A somatic mutation in early embryogenesis causing the affected individual to harbor a significant amount of mutant cells
• Person has normal cells & other cells with mutation
• Frequently occurs in Trisomy 21

Question 20

Proofreading Activity of DNA Polymerase

Answer 20

• 3’–>5’ exonuclease activity of DNA polymerase gamme
• Edits the 3’ end of growing DNA strand by pausing and re-reading the base pairing
• Tries again if incorrect

Question 21

Strand-Directed Mismatch Repair

Answer 21

• System identifies the new strand by nicks/gaps
• Repair enzymes recognize distortion in the DNA double helix
• Chews back DNA & allows polymerase to resynthesize DNA

Question 22

HNPCC (Lynch Syndrome)

Answer 22

• “Hereditary Nonpolyposis Colorectal Carcinoma”
• 80% chance of developing colorectal cancer
• Caused by defect in DNA mismatch repair system

Question 23

Base Excision Repair

Answer 23

• DNA glycoslyases recognize altered base in DNA & catalyze their hydrolytic removal
• E.g. Cytosine deaminated to Uracil recognized by uracil DNA glycosylase and cut out

Question 24

Nucleotide Excision Repair

Answer 24

• Repairs DNA damage caused by agents that result in large changes in the structure of DNA e.g. pyrimidine dimers
• Enzyme complex scans DNA & recognizes distortions in the double helix

Question 25

Xeroderma Pigmentosum

Answer 25

- Disorder characterized by extreme sensitivity to sunlight, pigmentation changes, & skin cancer - Caused by mutation in proteins needed for nucleotide excision repair

Question 26

Why are ds-DNA break particularly bad?

Answer 26

No intact template strand remains

Question 27

ds-DNA break repair mechanisms

Answer 27

- Nonhomologous end-joining | - Homologous end-joining

Question 28

Cytarabine

Answer 28

- Chemotheraputic drug that inhibits DNA replication & repair - Converted to Cytarabine Triphosphate that competes with deoxyribonucleotides for binding to DNA polymerase - Used to treat acute leukemia

Question 29

Cyclophosphamide

Answer 29

- Chemotheraputic drug that inhibits DNA replication & repair - Used to treat Hodgkin's Lymphoma, lung, breast, and testicular cancers - Converted to phosphoramide mustard in liver - Bi-functional alkylating agent that causes extensive damage to DNA that can induce apoptosis

Question 30

Doxorubicin

Answer 30

- Chemotheraputic drug that inhibits DNA replication & repair - Used in a variety of cancers - Topoisomerase II inhibitor - Prevents re-ligation of ds-DNA breaks

Question 31

Depurination

Answer 31

Loss of purine (A & G) bases because of spontaneous hydrolysis of the bond between the base and deoxyribose

Question 32

Deamination

Answer 32

Removal of an amine group commonly resulting in conversion of cytosine to uracil (C-->U)

Question 33

Pyrimidine Dimers

Answer 33

UV light causes dimerization of adjacent thymine bases on same DNA strand

Question 34

What is an RNA processing mutant?

Answer 34

Change in nucleotide sequence changes an RNA splice site, or destroys a splice site

Question 35

Incorrect Recombination

Answer 35

- One of three mechanisms that leads to small insertions or deletions - Unequal crossover event during homologous recombination in meiosis - Most likely at loci that contain repetitive DNA

Question 36

Strand Slippage

Answer 36

- One of three mechanisms leading to small insertions or deletions - Newly synthesized DNA in a repetitive sequence loops out in the ss-form - Second round of synthesis results in extension or the new strand

Question 37

How many chromosomes should be in a normal haploid germ cell?

Answer 37

23

Question 38

Outline the steps of base excision repair

Answer 38

1) DNA glycoslyase recognizes altered base and catalyzes removal 2) AP endonuclease recognizes missing base & cuts sugar-phosphate backbone 3) DNA phosphodiesterase removes the deoxyribose 4) Gap is filled by DNA polymerase & DNA Ligase

Question 39

Outline the steps in nucleotide excision repair

Answer 39

1) Enzyme complex scans DNA and finds distortion of double helix 2) Helicase separates strands 3) Backbone is cut 4) Oligonucleotide generates is peeled off 5) DNA polymerase and DNA ligase fill in the gap

Question 40

Non-homologous end-joining

Answer 40

- Broken ends of DNA are brought together & ligated | - Results in loss of nucleotide/s at the break point

Question 41

Homologous end-joining

Answer 41

- Recombination processes are used to repair broken DNA | - NO loss of sequence information b/c the homologous chromosome provides the template