BCEM 393 Lecture Notes - Lecture 15: Adenine, Ethidium Bromide, Dna Mismatch Repair
22 Jun 2018
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DNA Repair
CLASSES OF DNA DAMAGE
-DNA damage can be caused by chemical agents, UV, or DNA replication itself
-Example: DNA replication —> Taq DNA Polymerase
-Error rate — 2.28x10^-5 / base pair
-22.8 errors per 10,000 base pairs
-Human genome: approximately 3 billion base pairs
-68,400 errors
DNA DAMAGE GIVES RISE TO MUTATIONS
-Mutations are a change in the nucleotide sequence
-Types of Mutations
-Points mutations (ex. SNPs —> single nucleotide polymorphism —> one nucleotide re-
places another)
-Deletions
-Insertions
-Breaks (ss or ds)
TYPES OF POINT MUTATIONS
-Synonymous Mutation
-With 61 codons for 20 amino acids, many of the codons are “synonyms,” coding for the
same amino acid
-Example: DNA codons CAA, CAG, CAT, and CAC all code for Val
-Missense Mutation
-Causes a codon to code for a different amino acid
-Example: sickle cell anemia —> GAA to GUA (glutamate to valine) mutation changes he-
moglobin (a protein that carries oxygen in the blood) and causes Hb aggregation which
sickles (changes shape of) red blood cells, resulting in sickle cell anemia
-Nonsense Mutation
-A codon that stands for an amino acid mutates to one of these three stop codons
EXAMPLES OF INSERTIONS/DELETIONS
- Insertions/deletions are particularly prevalent when the template DNA contains short repeat-
ed sequences
-It is when DNA polymerase becomes more prone to errors, therefore slowing down and
falling off. Since it falls off, the strands separate again and DNA polymerase binds again,
but at the wrong position, thereby causing deletions or insertions
-Repeated sequences can induce replication slippage, in which the template strand and its
copy shift their relative positions so that part of the template is either copied twice or missed
out
-Replication slippage underlies trinucleotide repeat expansion diseases such as Huntington’s
Disease
-Human HD gene contains the sequence 5’-CAG-3’ repeated between 6 and 35 times in
tandem (encodes a series of glutamine
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-In Huntington’s disease, this repeat expands to a copy number of 36-121 —> prone to mis-
fold and aggregate
EXAMPLES OF DSDNA BREAKS
-dsDNA, if left unpaired, facilitate mutagens
-DNA repair mechanisms for dsDNA breaks are prone to repair
-(A) dsDNA
breaks caused by environmental exposure to irradiation, other chemical agents, or ultraviolet
light (UV) (B) Unrepaired nick (ssDNA break) in DNA replication fork stalls replication be-
cause it causes the replication fork to collapse, further causing a dsDNA break in one strand
MUTATIONS ARISE IN 2 WAYS
-Spontaneous errors in replication that evade the proofreading function
-Rare —> error rate for DNA synthesis in E. coli is 1 in 10^7
-If base is left unpaired, mutation will carry to all future generations
-A mutagen (UV, chemical agents) reacts with the parent DNA, causing a structural change
that affects the base-pairing capability of the altered nucleotide
3 TYPES OF MUTAGENS CAUSE MUTATIONS IN 3 WAYS
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