Exam #4: DNA Mutation, Replication & Repair Flashcards
Name three common DNA lesions
1) Depurination
2) Deamination
3) Pyrimidine Dimers
Somatic Mutation
A mutation that affects only the cell where the mutation occurred, and its progeny
Germline Mutation
A mutation that is passed to the offspring
Gene Mutation & Frequency
- Variation in nucleotide sequence via exchange, insertion, or deletion
- Occurs 10^-10 per base pair per cell division
- LEAST frequent
Chromosome Mutation & Frequency
- Rearrangement of chromosome following ds-DNA break or faulty recombination
- Occurs 6 x10-4 times per cell division
- Intermediate frequency
Genome Mutation & Frequency
- 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
Point Mutation
Exchange of one nucleotide for another
Silent Mutation
Change in nucleotide sequence DOES NOT result in a change in the amino acid sequence
Missense Mutation
Change in nucleotide sequence DOES result in a change in amino acid sequence
Nonsense Mutation
Change in the nucleotide sequence results in the generation of a STOP codon
List three processes that can lead to small insertions or deletions
1) Incorrect Recombination
2) Strand Slippage During Replication
3) Intercalating Agents (chemical agent)
Frameshift Mutation
Insertion or deletion of nucleotides alters the codon reading frame and produces a new protein
Meiosis
- Process by which 4 haploid germ cells are created from 1 diploid cell
- Two successive cell divisions without DNA replication
Crossing-over
Exchange of homologous chromosome fragments during meiosis
Disjunction
The pulling apart of homologous chromosomes during anaphase or meiosis
Non-disjunction
Incomplete separation of chromosomes during anaphase of meiosis
Consequences of Non-disjunction
- Germ cells with surplus chromosomes
- Germ cells missing chromosomes
In which autosomes is trisomy observed?
- 13, Patau Syndrome
- 18, Edward Syndrome
- 21, Down Syndrome
Mosaicism
- 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
Proofreading Activity of DNA Polymerase
- 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
Strand-Directed Mismatch Repair
- 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
HNPCC (Lynch Syndrome)
- “Hereditary Nonpolyposis Colorectal Carcinoma”
- 80% chance of developing colorectal cancer
- Caused by defect in DNA mismatch repair system
Base Excision Repair
- 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
Nucleotide Excision Repair
- 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
Xeroderma Pigmentosum
- Disorder characterized by extreme sensitivity to sunlight, pigmentation changes, & skin cancer
- Caused by mutation in proteins needed for nucleotide excision repair
Why are ds-DNA break particularly bad?
No intact template strand remains
ds-DNA break repair mechanisms
- Nonhomologous end-joining
- Homologous end-joining
Cytarabine
- 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
Cyclophosphamide
- 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
Doxorubicin
- Chemotheraputic drug that inhibits DNA replication & repair
- Used in a variety of cancers
- Topoisomerase II inhibitor
- Prevents re-ligation of ds-DNA breaks
Depurination
Loss of purine (A & G) bases because of spontaneous hydrolysis of the bond between the base and deoxyribose
Deamination
Removal of an amine group commonly resulting in conversion of cytosine to uracil (C–>U)
Pyrimidine Dimers
UV light causes dimerization of adjacent thymine bases on same DNA strand
What is an RNA processing mutant?
Change in nucleotide sequence changes an RNA splice site, or destroys a splice site
Incorrect Recombination
- 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
Strand Slippage
- 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
How many chromosomes should be in a normal haploid germ cell?
23
Outline the steps of base excision repair
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
Outline the steps in nucleotide excision repair
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
Non-homologous end-joining
- Broken ends of DNA are brought together & ligated
- Results in loss of nucleotide/s at the break point
Homologous end-joining
- Recombination processes are used to repair broken DNA
- NO loss of sequence information b/c the homologous chromosome provides the template
