Download Replication of the DNA

Chapter 12. Mutations: things
that go bump in the night
Prepared by Woojoo Choi
Mutations alter the DNA
1) Mutation: alteration in the genetic information
2) Mutation is a change in the base sequence of the DNA.
3) There are many possible changes
– Wild-type
the cat sat on the mat
– Substitution
the rat sat on the mat
– Insertion(single)
the cat spat on the mat
– Insertion(multiple) the cattle sat on the mat
– Deletion(single)
the ct sat on the mat
– Deletion(multiple) the cat the mat
– Inversion(small)
the tac sat on the mat
– Inversion(large)
tam eht no tas tac eht
Mutations alter the DNA
1) Altering the DNA base sequence has a
variety of effects.
– In replication, any changes will be
duplicated and passed on to the
next generation.
– In transcription, the mutation in the
DNA sequence will be passed on to
the RNA molecule.
– In translation, an altered RNA
sequence may be translated into an
altered protein (or enzyme).
2) So, the final result of a change in the
DNA sequence may be a defect in the
operation of some vital reaction.
Silent mutation
1) A mutation with no observable effect on cell growth or survival
2) Silent mutation do not alter the phenotype.
3) The ways to get a silent mutation
① The base change occurs in the non-coding DNA between genes.
Silent mutation
② The base change occurs in introns except for few important bases at
the splice recognition sites.
Silent mutation
③ The third main type is within the coding region of a gene and does
get passed on to mRNA.
• There are 64 different codons, most of the 20 possible amino
acids have more than one codon.
• A base change that converts the orginal codon into another
codon that codes for the same amino acid will have no effect on
the final structure of the protein.
• Third base redundancy: since many amino acids have several
codons, the third codon base can often be changed without
changing the amino acid which is encoded.
• For example, since codons that code for the amino acid alanine
have GC as the first two bases, any codon of the form
GCX(X=any base) will give alanine.
Nonsense and missense mutations
1) Missense mutation
– When a sequence change in DNA results in the replacement of one
amino acid by another amino acid.
– Conservative substitution: replacement of an amino acid in a protein
with another that is similar in its chemical properties(alanine->glycine)
– Changes will be relatively mild and usually non-lethal.
Nonsense and missense mutations
– Radical replacement: replacement of an amino acid in a protein with
another that is very different in its properties(alanine->glutamic acid)
– Changes will seriously cripple or even totally incapacitate our protein.
Nonsense and missense mutations
2) Nonsense mutation
– When a sequence change in DNA results in the replacement of the
codon for an amino acid with a stop codon, thus producing a
shortened protein
– The cell detects and digests a shortened and unfolded protein.
– The result is the total absence of this particular protein.
– Nonsense mutations are often lethal.
Deletion and indertions
1) So far we have been really rather restrained and only swapped a single
base for another.
2) We can excise one or more bases of the DNA sequence.
3) Deletion: removal of one or many nucleotides from DNA
Deletion and indertions
4) The more bases we remove, the
worse the mutation? Not
necessarily!!
– with a single base deletion or
insertion, the reading frame
and protein sequence have
been completely changed.
• Frameshift mutation: a
mutation which changes
the reading frame of the
protein encoded by a
gene
• Deletion or insertion of
two bases would also
change the reading frame.
Deletion and indertions
– Lets suppose three bases deletion.
• In this case, although an amino acid has been deleted, the
correct reading frame has been preserved.
• Apart from the single amino acid we lost, the rest of the protein
is unchanged.
• A three base insertion would also have the same effect.
Deletion and indertions
• If the deleted(or inserted) amino acid is in a relatively less vital
region of the protein, we may actually get away with this and
make a functional protein.
Deletion and indertions
5) A whole number of codons must be added or subtracted in order to
avoid the horrible consequences of changing the reading frame.
Rearranging DNA: Inversion and translocations
1) Inversion
– when a segment of DNA is removed, flipped and reinserted, facing
the opposite direction
– Inversions are definitely bad news.
Rearranging DNA: Inversion and translocations
2) Translocation
– when a segment of DNA is
removed and reinserted in a
different place, either on the
same chromosome or on a
completely different
chromosome
– If an intact gene is merely
moved from one place to
another, little damage may
result.
– If half of a gene is moved and
stuck somewhere in the
middle of another gene, the
results will be chaotic and
severely detrimental.
What causes mutations?
1) Induced mutations: mutations caused by mutagen
– Mutagen: agent that can cause mutation
– Two main types: toxic chemicals and radiation
2) Spontaneous mutation: mutations which happen without mutagens
because of mistakes during DNA replication
What causes mutations?
3) The most common types of toxic chemicals
– EMS (ethyl methane sulfonate): add a methyl group to bases in DNA
and changes their shape
– Nitrite: replaces amino groups with hydroxyl groups and converts the
base cytosine to uracil
What causes mutations?
4) When the time comes for DNA replication, the DNA polymerase is
confused by the altered bases and puts in wrong bases in the new strand
of DNA.
What causes mutations?
5) Another type of chemical mutagen mimics the base found in natural
DNA
– Base analog: chemical that resembles a base of a nucleic acid well
enough to fool a cell into using it instead.(eg, bromouracil)
What causes mutations?
6) A more subtle form of chemical mimicry consists of imitating the
structure of a base pair
– Intercalation: when a chemical agent inserts itself into DNA between
two base pairs(eg, acridine orange)
– The DNA polymerase thinks the intercalating agent is a base pair and
it puts in an extra base when making a new strand.
– Insertion of an extra base will change the reading frame and this will
completely destroy the function of protein.
What causes mutations?
7) Teratogen: agent that can causes abnormal development of embyos
leading to monstrosities (eg, thalidomide).
What causes mutations?
8) High frequency electromagnetic radiation(UV, X-rays and γ-rays) directly
damages DNA
– UV radiation makes two neighboring thymine bases react with each
other to give thymine dimers.
– X-rays tend to produce multiple mutations and often yield
rearrangements of the DNA such as deletions, inversions and
translocations.
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Mutations caused by insertion of transposons
1) Transposon: mobile segment of DNA that moves from place to place on
a cell’s chromosome(ch 20)
Genetically engineered gene disruption
1) Mutations that serve to completely inactivate a gene are useful in
genetic analysis.
2) Sometimes foreign DNA is inserted into genes to disrupt them and then
study the results.
Genetically engineered gene disruption
1) 그림 12.29 추가
Mutational hot spots
1) Hot spots: regions in the genetic material that are unusually susceptible
to mutation
2) Most hot spots are due to the presence of occasional methylcytosine
bases in the DNA. (spontaneously disintegrates to give methyluracil)
Reversion and suppression
1) Reversion
– a second mutation that restores original characteristics to a mutant
organism
– It is a phenotypic term.
2) True revertant
– a revertant in which the original DNA base sequence is exactly
restored
– It happens rare.
3) Second-site revertant
– a revertant in which a second change in DNA base sequence cancels
out the effects of the first
– It happens more often.
Detecting mutagenic chemicals by reversion
1) Chemical mutagens can be detected by the Ames test, used routinely by
industry and government agencies to screen chemicals for possible
mutagenic effects.
Detecting mutagenic chemicals by reversion
2) Many chemicals are checked by this test that examines chemical’s effect
on bacteria in culture.
– Mutants carrying mutations in the genes for histidine synthesis are
used.
– These mutants cannot grow unless given histidine.
– When some mutants grow in the medium lacking histidine, these are
revertants.
– The frequency of reversion is also increased by mutagenic agents.
DNA repair
1) There are several DNA repair systems
① Mismatch repair: DNA repair system which recognizes wrongly paired
bases
DNA repair
① Mismatch repair
• We need to know which strand came from the mother cell and
which was the recently synthesized (end error-carrying) daughter
strand.
• In bacteria such as E. coli, the DNA is tagged by methyl group to
indicate this. (eg, GATC bases)
• Different organisms use different tagging systems, but the
principle remains the same.
DNA repair
② Excision repair: cutting out
a stretch of damaged DNA
and replacing it with new
DNA (DNA polymerase I)