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15 Cards in this Set
- Front
- Back
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What are the different types of DNA mutations and rank them in order of their severity?
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(1) Silent
(2) Missense (3) Nonsense (4) Frameshift (5) Thymine-Thymine Dimers Order of severity from least to greatest detrimental effect: Silent<< Missense<<Nonsense<<< Frameshift |
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(1) What is Transversion?
(2) What is Transition? |
(1) Transversion: when a purine is substituted for a pyridimidine (e.g., A-->C) or a pyrimidine is substituted for a purine (e.g., T-->G).
(2) Transition: when a purine is substituted for another purine (e.g., A-->G) or pyrimidine for a pyrimidine (e.g., T-->C) *Transversion and Transition mutations apply ONLY to the: (1) Silent, (2) Missense, and (3) Nonsense mutation-types |
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What is a silent mutation?
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A silent mutation is when a simple substitution occurs that results in the coding for the SAME AMINO ACID; usually occurs in the 3rd position (tRNA wobble); due to the "REDUNDANCY" of the "Genetic Code"
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What is a Missense mutation?
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A missense mutation occurs when a simple substitution results in the coding for a DIFFERENT AMINO ACID from the ORIGINAL amino acid (example: Sickle Cell Anemia, whereby Valine replaces the normally coded Glutamic Acid=non-conservative; hydrophilic A.A. is replaced with a hydrophobic A.A.)
A "Conservative Missense Mutation" is when the "DIFFERENT" amino acid is similar in structure and/or function chemically to the originally encoded amino acid. |
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What is a Nonsense mutation?
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A Nonsense mutation occurs when a simple nucleotide substitution results in the coding of a PREMATURE STOP CODON (UGA, UAG, UAA)
The severity depends of the mutation depends on when/where the mutation occurs. If it occurs early-on in the DNA (mRNA transcription) sequence, the protein is likely super-truncated and non-functional vs. if it is near the last amino acid sequence, then you may have a normally/almost normally functioning protein. |
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What is a Frameshift mutation?
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The worst of the worst when it comes to mutations. A Frameshift mutation occurs when an insertion or deletion of nucleotides (1-2) results in the ENTIRE amino acid SEQUENCE from that point on being MISREAD and coded with the WRONG primary amino acid sequence. (If the insertion or deletion is divisible by 3, then a functional/semi-functional protein is possible).
Usually results in a truncated, NON-FUNCTIONAL protein (e.g., Duchenne Muscular Dystrophy) |
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What are the mRNA Stop Codons?
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Three Stop Codons: "U Are Away"; "U Go Away; and "U Are Gone"=
(1) UAA (2) UGA (3) UAG The "Start Codon" |
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What is a Thymine-Thymine Dimer mutation and when does it occur?
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A Thymine-Thymine dimer forms in response to UV light exposure within the SAME STRAND of DNA and --> a LARGE loop-like structure.
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What is the "Start Codon"?
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AUG and encodes for Methionine (humans) and formyl-Methionine (bacteria)
**Note: The AUG codon is the ONLY Codon that encodes for Methionine. Hence, ALL polypeptides BEGIN WITH Met (humans), or f-Met (bacteria) {Tryptophan is the ONLY OTHER A.A. that only has ONE CODON; UGG}; all other amino acids are encoded for by >1 codon. |
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What are the features of the Genetic Code?
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(1) Unambiguous: Each codon only encodes one amino acid.
(2) Degenerate/Redundant: 18/20 amino acids are encoded by more than 1 codon (except Methionine AUG and Tryptophan UGG, respectively) (3) Non-overlapping/Commaless: The sequences are read from a fixed starting point (AUG) and are read as a continuous sequences of bases from then-on. (with the exception of some viruses) (4) Universal: Conserved throughout evolution in all species (with the exception of Mitochondria) |
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Describe Nucleotide Excision Repair.
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Specific ENDONUCLEASES release the Oligonucleotide-containing damaged bases
DNA pol III and Ligase Fill-in and repair the defects, respectively and repairs "Bulky, Helix-distorting" mutations. (e.g., T-T dimers and is defective in Xeoroderma Pigmentosum) |
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Describe Base Excision Repair.
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(1) A base-specific Glycosylase recognizes an altered base and creates a AP-site (apurinic/apyridimic-site) (2) AP-Endonuclease cleaves the 5'-end and LYASE cleaves the 3'-end.
(3) DNA Polymerase-Beta (Eukaryotes only) replaces the missing nucleotides ("gap"), using the complementary strand as a sequence guide. (4) Ligase joins the ends. **Important in spontaneous or toxic deamination of a base. |
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Describe the Mismatchrepair mechanism.
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(1) Mismatching of DNA nucleotides occurs
(2) Mismatched nucleotides are recognized on the NEWLY SYNTHESIZED STRAND (repair mechanisms know which strand is the "correct" sequence) and removed (endonucleases and/or lysases??) (3) Missing Nucleotides are replaced and joined by DNA pol-beta and Ligase, respectively ***The DNA mismatch repair mechanism is DEFECTIVE in HNPCC. |
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What are the single-stranded DNA repair mechanisms? Order them from least to most specific/precise.
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(1) Nucleotide Excision Repair
(2) Base Excision Repair (3) Mismatch Repair Least-->Most Precise: Nucleotide Excision Repair<Mismatch Repair<Base Excision Repair |
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What is Non-Homologous End-Joining?
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Non-Homologous End-Joining is a DNA repair mechanism for double-stranded DNA breaks.
The 2-Ends of the broken DNA Fragments are brought together; no homology required, but since there is no homologous strand to use as a template, you simply join the ends and hope it wasn't in a coding region :) *Ataxia Telangiectasia is a disorder where the Non-Homologous End-Joining dsDNA break repair mechanism is defective. |
