Microbial Genetics Part 2

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Microbial Genetics Part 2
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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.)

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• 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. Youll 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.

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• 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.

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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.

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• 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

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• 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.

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• 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.

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• 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.
• Im sure that seems confusing since we already
  know that bacteria arent 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.

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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.

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• 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.

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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 wont be responsible for
  that.)

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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.

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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.

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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 werent 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.

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• 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.