DNA Repair

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DNA Repair

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-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 DNAThus
maintenance of the genetic information requires
constant repair of DNA damage
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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
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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 Huntingtons disease and
fragile X disease.
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