Medical and Molecular Genetics
Lecture 11 Mutation & Repair of DNA
1) Define the following mutations: point, silent, missense, nonsense, and frameshift.
Point mutation – occurs when a single base is substituted with another base.
Silent mutation – the change, (often the third base in a codon) codes for the same base
as the original and has no effect on the gene.
Missense mutation – a base substitution that results in the generation of a codon that
specifies a different amino acid.
Nonsense mutation – a base substitution that results in a stop codon, which halts
transcription and results in an incomplete, nonfunctional gene.
Frameshift mutation – when one or two bases are deleted or inserted, the translational
reading frame is altered and a garbled message is transcribed, resulting in a
nonfuntional gene. If three deletions (or insertions) occur, the gene may only be
missing one or two amino acids.
2) Define the terms transition and transversion.
Transition – when a purine is substituted for another purine, or a pyrimidine is
substituted for another pyrimidine.
Transversion – occurs when a purine is substituted for a pyrimidine, or when a
pyrimidine is substituted for a purine.
3) Describe what is meant by a conservative and a nonconservative mutation. A
mutation that causes a codon to specify a different, but very similar, amino acid has
little or no effect on the function of the protein and is said to be conservative.
However, if the codon specifies a different, dissimilar amino acid the gene product
would be altered deleteriously, this is said to be a nonconservative mutation.
4) List five major ways that DNA can be damaged and/or mutated.
Polymerase inserts wrong base, very rare.
Errors in DNA replication
Errors in DNA recombination Sometimes DNA is lost during recombination
Chemical mutagens: man-made or metabolites
Chemical damage
Gamma rays and X-rays generate free radicals
Radiation
Bases undergo alterations that result in self-damage
Spontaneous Damage
5) Outline the major steps of mismatch repair and nucleotide excision repair.
Mismatch repair: relies upon a secondary signal within the DNA to distinguish
between the parental strand and the daughter strand – methylation of the sequence
GATC appears only on the parent strand since it takes some time after replication to
methylate the daughter strand. A protein called MutS recognizes and binds the
mismatched base pair. Another protein called MutL binds to MutS. The
hemimethylated GATC sequence is recognized and bound by an endonuclease called
MutH. The MutL/MutS complex links to MutH, which then cuts the unmethylated
DNA strand at the GATC site. A DNA helicase called MutU then begins to unwind
the DNA in the direction of the mismatch and an exonuclease degrades the strand.
Finally DNA polymerase fills in the gap and ligase seals the nick. Nucleotide excision
repair: the system recognizes and repairs lesions such as thymine dimmers, smoking
induced benzopyrene-guanine adducts, guanine-cisplatin adducts caused by
chemotherapy, and several other types of chemically induced damaged. NER begins
with the formation of a complex of protein factors (XPA, XPF, ERCC1, and RP-A) at
a lesion on the DNA. More factors (TFIIH, XPB, XPD, and XPC) then bind to the
complex in an ATP dependant reaction. Two different endonucleases (XPG and XPF)
make incisions on the strand of DNA that is damaged. One incision is made 5 bases
distal to the 3’ side of the lesion and the other is made 24 bases distal to the 5’ side of
the lesion. The resulting 29-nucleotide long segment of damaged DNA is then
unwound by helicases (XPB and XPD). The proteins RFC and PCNA then load either
DNA polymerase δ or ε onto the 5’ end of the gap, which is filled by polymerases and
sealed by ligase.
6) State the function of: apurinic endonuclease, uracil N-glycosidase, DNA
photolyase, guanine alkyltransferase.
Function
Recognizes and excises the deoxyribophosphate that is left over
Apurinic
following a spontaneous depurination.
Endonuclease
Regognizes any uracils that are formed by the spontaneous
Uriacil
deamination of cytosines, cleaves the glycosidic bond and removes
N-glycosidase
the uridine base.
DNA photolyase Directly cleaves the bonds that form the thymine-thymine dimers and
requires light to work.
Removes methyl and ethyl groups from alkylated guanines. A
Guanine
alkyltransferase ‘suicide’ enzyme, only works once.