Amino is –NH2 and imino is =NH on top of C or A. o This unusual base pair then changes ¼ offspring – transition mutation.
Chemical mutagens: o Those mutagenic to replicating and non-replicating alkylating agents – add methyl, ethyl, etc. groups deaminating agents (nitrous acid), remove amino groups to from keto group (hydrogen acceptor) – changes cytosine into uracil, which is transition, and changes G to Xanthine, which still pairs with Cytosine. In the immune system there’s an enzyme that makes C-U mutations for variability in antibodies) o Those mutagenic to only replicating DNA base analogs (5-bromouracil– difficult to distinguish from normal bases, substitute for base, pairs with adenine, then undergoes tautomeric shift to pair with guanine, causes T:A <-> C:G transition mutations. 2- aminopurine same but incorporated in place of A or G acridine dyes (proflavin, ethidium bromide) – intercalate between bases (very planar, lengthens/distorts helix, causes skip of addition of nucleotide – frameshift, intentionally used to cause mutation, also fluorescent so can see DNA). Radiation Induced Mutations o More mutations cause less mutation. X-rays and shorter cause ionization (mostly filtered out by ozone layer) Short wavelengths break DNA, proteins, ionize electrons which then hit other things, releasing more electrons. Can cause deletions, duplications, inversions, and translocations as cell shoves pieces back together in order to survive. UV causes excitation (more common) UV induced lesions form cross-linking of adjacent thymine dimers (pyrimidine dimers – C-C or C-T also possible but most common, can also occur across strands) Block DNA replication/polymerase, which can lead to strain breakage. Activate DNA repair mechanisms. Huge damage due to tanning beds. Transposable elements: o Segments of DNA that can move between regions on a chromosome or between chromosomes. o Make up more than 40% of eukaryotic genome, in many organisms (bacteria, fungi, protists, plants and animals). Can give advantage or be genetic parasites using our cells to propagate. Cause some diseases. o When excises leaves double strand break that can be fixed but can modify structure of endogenous genes. o Transposable elements/viruses can disrupt functioning genes by insertion – stick in the middle and truncate polypeptide. DNA REPAIR MECHANISMS
Light-dependent repair – photoreactivation (prokaryotes) o Sole purpose is to remove thymine dimers o Photolyase binds to thymine dimer, activated by absorption of blue light, use energy to break covalent bond o Humans do not have photolyase but have similar gene that is involved in regulating biological clock. o Can use as diagnostic tool to detect thymine dimers.
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- Fall '13
- DNA, rna polymerase