Transcript DNA : MUTATIONS

NOTES – CH
17, part 2
DNA &
MUTATIONS
MUTATIONS
● permanent changes in DNA that can
involve large chromosomal regions or a
single nucleotide pair
● POINT MUTATION: mutation limited to
about one or two nucleotides in a
single gene
● 3 Types of Point Mutations:
1) Base-pair substitution
2) Base pair insertion
3) Base pair deletion
1) Base-pair substitution:
● replacement of 1 nucleotide base pair
with another resulting in little or no
change in the protein encoded by the
mutated gene
-redundancy in the genetic code is why some
substitutions have no effect; a base pair
change may simply transform one codon into
another that codes for the same amino acid.
-even if the substitution
alters an amino acid,
the new amino acid
may have similar
properties to the one it
replaces, or it may be
in a part of the protein
where the exact amino
acid sequence is not
essential to its activity.
-some base pair substitutions result in
readily detectable changes in proteins that
may significantly alter protein activity
(i.e. sickle cell anemia; 1 amino acid in
hemoglobin is altered)
2) Base pair insertion:
● the insertion of 1 or more nucleotide pairs
into a gene
3) Base pair deletion:
● the deletion of 1 or more nucleotide
pairs from a gene
-because mRNA is read as a series of triplets
during translation, insertion or deletion of
nucleotides may alter the “reading frame” of
the genetic message
-this type of
FRAMESHIFT
MUTATION will occur
whenever the number
of nucleotides inserted
or deleted is not 3 or a
multiple of 3.
-this type of mutation
usually results in a
nonfunctional protein
(unless the mutation is
very near the end of
the gene)
FRAMESHIFT MUTATION:
MUTAGENS:
● MUTAGENS = physical or chemical
agents that interact with DNA to cause
mutations
MUTAGENS:
● most mutagens are also carcinogens
● CARCINOGEN = an agent that causes
cancer
PHYSICAL MUTAGENS:
● x-rays
● UV radiation
● radioactivity
CHEMICAL MUTAGENS:
● base analogues (similar to normal DNA
bases, but pair incorrectly)
● insert into DNA double helix & interfere
with correct DNA replication
● cause chemical changes in the bases
that change their pairing properties
CHEMICAL MUTAGENS:
● benzene (industrial solvent)
● DDT (& other pesticides)
● nitrogen mustard (in mustard gas, a
chemical weapon)
CHEMICAL MUTAGENS:
● cigarette smoke!
(contains 4000
chemicals, at least 43 of
which are
carcinogens!!!!)
-nicotine, tar, carbon
monoxide, formaldehyde,
hydrogen cyanide,
ammonia, arsenic, DDT