Download DNA Repair - College of Arts and Sciences at Lamar University

DNA Repair
-Errors (at a rate of 1x10-9) are introduced during
DNA replication
-DNA in cells is constantly being altered by cellular
constituents, including active oxygen species that are
by-products of metabolism
-Many environmental agents attack and modify DNA
Thus maintenance of the genetic information requires
constant repair of DNA damage
Types of DNA Damage:
-Spontaneous deamination of some C, A, and G bases in
DNA
-Spontaneous depurination due to cleavage of the glycosyl
bond connecting purines to the backbone
(the depurinated sites are called abasic or AP [apurinic]
sites)
-Bases are oxidized by oxygen species
-Occasionally, bases are methylated or alkylated
-Ultraviolet radiation may covalently link adjacent
pyrimidines along one strand of the DNA to form
pyrimidine dimers
Mutations
-Mutations are inherited changes in the DNA sequence.
They can result (i) from replication errors,(ii) from damage to the
DNA, or (iii) from errors during repair of damage. Point mutations
are the changes of a single base pair. Transitions are mutations
in which one purine is substituted for another, or one pyrimidine
is substituted for another. Transversions are point mutations in
which purine is substituted for pyrimidine, or vice versa.
Missense mutations are point mutations that change a single
base pair in a codon such that the codon now encodes a different
amino acid. Nonsense mutations are point mutations that change
a single base pair in a codon to a stop codon that terminates
translation. Triplet expansion is a great increase in the number of
specific triplet. Triplet expansion causes many diseases, including
Huntington’s disease and fragile X disease.