Download Microbial Genetics Part 2

Microbial Genetics
Part 2
Genetic Mutation
• A genetic mutation is a change in the original DNA
nucleotide sequence.
– It can consist of a change in one or more base pairs;
– A deletion of one or more base pairs;
– An addition of one or more base pairs.
• Some mutations harmful, some beneficial, some neutral.
• A point mutation is one in which a single base pair is
changed, added, or deleted.
– It can cause a change in the codon sequence, thereby changing
the codons to code for different amino acids. It could even
change the codon to a stop codon in middle of the protein
resulting in a shortened and probably non-funtional protein.
(This type of mutation is called a nonsense mutation.)
– A point mutation can cause a change in overall shape and function of
the final protein. An example of a point mutation is sickle cell anemia.
The point mutation changes the shape of the red blood cell so that it
cannot function correctly. Ironically, it is this change of shape that often
protects Africans from contracting Malaria which is so common on that
continent. It is thought that this particular mutation developed as a
result of the high exposure to Malaria as a natural defense against the
disease. (In other words, an example of evolution.)
• A frameshift mutation is one in which a one or more base pairs are
added or deleted.
– When 1 base pair is added into the DNA sequence the effects could be
huge! If the base pair were added into the 1st position of the codon then
the whole sequence of the following codons are changed. Potentially
every single amino acid would be different from what the original code
specified.
– It is called a frameshift mutation because it cause a literal shift in the
reading frame of each subsequent codon.
• Another type of mutation is a silent mutation. A silent mutation
changes a base pair but the change in the base pair does not alter
the amino acid coded for by the codon.
– Take a look at Fig. 9.14 on page 264. Look for the amino acid, Glycine.
You’ll notice that Glycine has 4 different codons that code for it. So, if a
mutation in the DNA template changed GGU to GGC no harm has been
done because the amino acid remains the same.
• Of course any mutation that occurs can always
mutate back to the original sequence. This type
of mutation is called a back-mutation.
• Mutagens are agents in the environment that
directly or indirectly cause mutation.
– UV light and benzpyrene (chemical in smoke and
soot) cause frameshift mutations.
• Cells do have proofreading and repair enzymes
to help take care of mutations. However, when
the damage is widespread, the cells enzymes
simply cannot take care of them all. If the
damage is sever enough the cell will die.
Genetic Transfer and
Recombination
• Recombination is the exchange of homologous genes on
a chromosome.
– For example, when gametes are being formed the homologous
chromosomes line up side by side. When they are in close
proximity similar sequences of DNA can changes chromosomes.
So if the chromosomes code for eye color but on one
chromosome the gene was for brown eye color and on the other
chromosome the gene was for green eye color, they could switch
chromosome.
– This DNA trading or switching is recombination. It is a very
useful tool that we use in the laboratory too.
• Often the question is asked, “How can
bacteria become antibiotic resistant?”
• One way bacteria gain antibiotic
resistance by acquiring the resistance
genes from other bacteria in the
environment.
• Bacteria acquire new genetic material in 4
ways that we know of currently.
– Transformation
– Conjugation
– Transduction
– Transposons
• Transformation is when extracellular DNA
is picked up by a bacterial cell.
– After cell death, some bacteria are lysed
(broken open) and release their cellular
contents into the surrounding environment.
– Some bacteria have the ability to take up that
DNA and incorporate it into their chromosome
or use it in some other way.
– In order to take up extracellular DNA the cell
has to be competent.
– Competence means that the recipient cell is
in a physiological state that will allow it to take
up DNA.
– Transformation occurs naturally among only a
few organisms. (This is another important
tool that is used in the laboratory.)
• Some plasmids encode for genes that
enhance the pathogenicity of a bacterium.
– For example, some strains of E.coli contain a
plasmid that codes for toxins.
– Other plasmids code for antibiotic resistance.
• Conjugation uses pili to attach to a neighboring
bacterial cell and transfer DNA through it.
• Conjugation requires cell to cell contact in order
for the process to begin. In addiiton, both cells
must be opposing mating types.
– I’m sure that seems confusing since we already know
that bacteria aren’t male or female. Not to worry, here
is what that means.
– One cell must have the plasmid (F factor plasmid) that
codes for pili formation. This cell is called the F+ cell.
(Remember that a plasmid is a small circular piece of
DNA in addition to a chromosome.)
– The other cell must not have the F factor plasmid in
order for conjugation to occur.
• See Fig.9.24 for a great diagram and
explanation of this process.
Steps of conjugation
• 1. The pilus of the F+ cell attaches to the F- cell.
• 2. The F factor plasmid begins to replicate.
• 3. The plasmid copy begins to move through the
pilus from the F+ cell to the F- cell.
• 4. The connection between the two cells is
broken.
– When the recipient cell receives a complete copy of
the F factor plasmid, it is then called a F+ cell.
– Sometimes the connection between the two cells is
broken early and only part of the F factor plasmid is
transferred. In those cases, the cell generally
remains an F- cell because it cannot be a donor cell
without the complete copy.
• Occasionally the F factor plasmid will recombine
into the recipient chromosome. When that
happens the cell is called an Hfr.
– Hfr= high frequency of recombination
• It can still produce pili and attach to other
recipient cells.
• The difference is that when the donor begins to
replicate the plasmid it goes on to replicate the
chromosome as well.
– Because the chromosome is so much larger than the
F factor plasmid by itself, the connection between the
two cells is usually broken before a complete copy of
the plasmid and chromosome can be transferred.
• The DNA that is transferred usually recombines
into the recipient chromosome and any unused
DNA is degraded.
Transduction
• Transduction is a process that uses
bacteriophages to transfer DNA.
– Bacteriophages are viruses that infect
bacteria.
• See Fig. 9.26 for a great explanation of
this process. (You can look at 9.27 for
your information but you won’t be
responsible for that.)
A brief overview of phage
replication
• 1. The phage attaches to the cell wall of the target/host
bacteria.
• 2. The phage injects or releases its DNA into the host
bacteria.
• 3. The phage takes over the host cell and uses it to
replicate its own DNA and make its own proteins.
• 4. While phage DNA is replicated, bacterial DNA is
broken down to free up more nucleotides for more phage
DNA.
• 5. Phage body parts are made and assembled.
• 6. The phage DNA is packaged into the assembled
phage.
• 7. The cell breaks open and releases the phages into
the environment.
Steps in transduction
• 1. Phage infects the host bacterial cell.
• 2. During replication and assembly, a phage
incorporates a portion of the host cell DNA into
itself, instead of phage DNA.
• 3. The cell breaks open and the phages are
released.
• 4. The phage that contains the bacterial DNA
infects another bacterial cell.
• 5. Instead of producing more phages, the
bacterial DNA is incorporated into the host cell
chromosome.
Transposons
• Transposons were discovered by Barbara McClintok
while studying color variations in Indian Corn.
• She believed that the color variations were caused by
“jumping genes” that would jump into or out of the middle
of the chromosome.
• Her theories were met with a great deal of criticism and
weren’t accepted until almost 30 years later.
• Transposons contain genes that enable the short
segment of DNA to insert and remove itself from the host
genome.
• Transposons have been found to be more common than
we originally believed. They are even found in the
human genome.
• Transposons are important because when they insert
themselves into the host chromosome we never know
where it will happen. They can turn genes on or turn
them off just by where they insert themselves.
• All of the ways in which DNA can be
transferred from cell to cell are used in
laboratories to manipulate bacteria.
• Make sure that you understand them so
that when we start the biotechnology
chapter you will know what is going on.
• The hw for microbial genetics is due
Friday, Oct. 6, 2006 by midnight.