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Mutations and
Genetic Exchange
Scope of Mutations
Mutation Types
Mutation Causes
Genetic Exchange via Recombination
Transformation
Conjugation
Transduction
Transposable Elements
Scope of Mutation:
• A mutation is any change in the proper nucleic acid sequence
of a specific gene in a cell’s genome. It may result from a
single base pair mismatch during DNA replication.
• Mutation can create genetic diversity within a population; either
beneficial, neutral, bad, or lethal.
• Mutation could result in a new phenotype that is advantageous
to successful reproduction of the mutated individual; this
depends on particular environmental conditions, called
selective pressures.
• Such beneficial mutations stay within a population from
generation to generation, and drive the evolution of that
species.
• Bad or lethal mutations are often lost from a population over
subsequent generations.
A single base substitution will change a codon; a new
amino acid in the gene’s protein product may result.
Spontaneous Mutation:
E.g., Human Sickle Cell Anemia
Why does this stay in human populations? Africans with one good and one mutated
gene are resistant to African Sleeping Sickness, a lethal infections disease.
NORMAL:
Base Substitutions:
2) MISSENSE:
1) NUETRAL:
3) NONSENSE:
NORMAL:
Frameshift Mutation:
• Results from 1 or 2 base
deletion or addition to the
DNA.
• Causes the “reading
frame” of codons to change.
DELETION:
• All codons read down
from a frameshift may
change every amino acid
during translation.
• Think of a simple
sentence of 3-letter words”
e.g. “The cat eat the fat rat.”
Now, “The ate att hef atr at”
Chemical Mutagens:
Base Modification
Nitric Acid (nitrite ion) reacts with amine groups to form
nitrosamines in adenine. This base modification causes
adenine to act like guanine.
Chemical Mutagens:
Base Analogs
Base analogs mimic the critical chemical form of a normal base,
so it gets incorporated during DNA replication. However, its
proper base pairing function is altered so subsequent
generations will maintain the mutation.
UV Light Mutation
• Thymine dimers can be
repaired by a few different
mechanism in the cells.
• One important repair
system for thymine dimers
and chemical mutations is
“excision repair”.
• Excision repair involves
cutting out the bad
sequence on the mutated
strand and replacing it
with the proper bases.
• The goal is to make
repairs in DNA before it is
replicated for the next
generations, as that would
pass on mutations.
Excision Repair:
• However, high UV
doses may cause
accumulation of too
many thymine dimers for
their repair prior to DNA
replication. DNA
replication is interrupted.
• Rather than stop
replication all together –
certainly lethal - the DNA
Polymerase is forced to
randomly add bases.
• This “error prone”
repair represents a last
chance for survival.
Another name for this is
“SOS repair”.
Genetic Recombination:
• Two DNA molecules may recombine
segments of their molecule in a process
called crossing over.
• This is a relatively common event
between chromosome copies in
eukaryotes during meiosis. (Note the
example here.)
• Prokaryote chromosomes, viral DNA,
and smaller fragments of “foreign” DNA
may recombine, adding new genes (or
different alleles) to an individual cell.
• Bacteria can receive a foreign source of
DNA for recombination through one of
three different mechanisms of Genetic
Exchange:
- Transformation (external fragment)
- Conjugation (bacterial “sex”)
- Transduction (viral mediated)
Transformation:
1) Foreign DNA from a dead
donor cell is released into the
environment as fragments.
2) Fragments can be taken up
by a recipient cell.
3) A portion of foreign DNA
may there recombine with
recipient’s chromosome.
• Recipient cells have been
transformed into a new
genotype.
• Non-recombined foreign
DNA eventually gets degraded
by recipient cell nucleases.
• A process that is performed
naturally by some bacteria, or
may be forced artificially in the
laboratory.
Conjugation:
• Live donor contains an special fertility plasmid (F factor) that allows it to mate a
recipient cell without an F factor and transfer a copy of the F factor into the recipient.
• This is referred to as a “F+ x F- mating” and results in both cells being F+.
1) The F+ cell produces a sex pilus that will only attach to F- cell.
2) Once attached, the F factor’s DNA is replicated by rolling circle replication.
3) A linear copy of F factor is transferred via the pilus into the recipient.
4) F factor DNA circularizes in recipient and the pilus detaches.
+
F → Hfr
• A small percentage of donor cells with an F factor will have that DNA
recombine into the donor cell’s chromosome DNA at a specific site.
• F+ cells that have their F factor integrated into their chromosome are
called high frequency of recombination (Hfr) cells.
• Like F+ cells, Hfr cells can produce a pilus and mate with an F- cell.
Htr x F mating
1) Once the pilus has attached, again the DNA from the F factor begins rolling
circle replication to transfer DNA across the pilus into the recipient.
2) However, only the initial portion of F factor DNA gets copied and transferred, the
remaining majority of what is copied and transferred is chromosomal DNA from
the donor cell. Only a fragment of the donor chromosome transfers.
3) Recombination between the recipient’s chromosome and the transferred donor
chromosome fragment happens at a very high frequency. The recombinant F-
cell is called a F’ cell, which now has new genes and/or alleles.
Transduction:
• Viruses are involved in genetic exchange called transduction.
• Viruses are not cellular life; we simply refer to them as a particle, or virion.
• Specifically, viruses of bacterial cells are called bacteriophage. The
bacterial cell is the host.
• Bacteriophage have an outer protein coating called a capsid and inside
resides its small genome (phage DNA)
• Viral infection will eventually result in controlling the host cell’s “machinary”
to replicates hundreds of new viral particles.
• Sometimes, although rare, host (donor) DNA gets packaged into capsid
proteins, forming what is called a tranducing particle.
• Transducing particles can infect other bacteria (recipient), where the donor
DNA fragment can recombine with the recipient chromosome.
Transposable Elements:
“Jumping Genes”
• Transposable elements (insertion sequences and transposons) can tranfer copies
of themselves to other DNA molecules (chromosome, plasmid, or viral DNA).
• Antibiotic resistance genes rapidly spread within and between bacterial populations
by transposons carried on F factors called R plasmids.