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Microbial Genetics Prokaryotic Genetics

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Microbial Genetics Prokaryotic Genetics Powered By Docstoc
					Prokaryotic Genetics
                        Mutations
Change in genetic material
Mutations may be neutral, beneficial, or harmful.
Often silent (Degeneracy of the code)
Types of Mutation
– Base substitution (point mutation)
     Change in one base
     Three types:
       – Missense = changes the codon and codes for different amino acid
       – Nonsense= changes the codon and codes for a stop codon
       – Silent= changes the codon, but codes for same amino acid
            Usually occurs if there is a change in the 3rd base
– Frameshift mutation
     Insertion or deletion of a few nucleotides causing a shift
     Often involves a long stretch of altered amino acids leaving a non
     functional protein
Example of Point Mutations
Example of Frameshift Mutations
                      Problem
Below is the original DNA and different mutations. Determine what
amino acids would be produced from each and which contain
missense, nonsense, silent and frameshift mutations.

Normal template DNA: 3’ TACAACCTG 5’

Mutation 1: 3’ TACATCCTG 5’

Mutation 2: 3’TACAACCTA 5’

Mutation 3: 3’TACCACCTG5’

Mutation 4: 3’TACAACGCTC5’
           Types of Mutagens
Mutagens
– Agents that causes mutations.
– Spontaneous mutations
     Occur in the absence of a mutagen


Chemical and Radiation
                  Mutagens
Chemical
– Nucleoside analogs

– Molecules similar to
  normal bases but cause
  altered base pairing
               Mutagens
Radiation
– Ionizing radiation
  (X rays and gamma
  rays)

– Nonionizing
  radiation (UV light)
    Causes thymine
    dimers, which
    leads to
    problems with
    replication and
    transcription
         Transferring DNA
Donor DNA is transferred to recipient cell in
3 possible ways

– Transformation: donor DNA is free in the
  environment
– Transduction: donor DNA transfer is mediated
  by a virus
– Conjugation: donor DNA is transferred by cell to
  cell contact
Transformation

            Recipient takes up donor
            DNA through cell
            membrane
            DNA must recombine to
            notice possible change in
            genotype.
             – Recombination necessary
               for expression

             – Transformation used in the
               laboratory when bacteria
               are used to produce large
               amounts of DNA
               Conjugation
Requires cell-to-cell contact
Transfer of the F plasmid (from F+ to F- cell)
 – Plasmid replicates separately from the bacterial
   chromosome
Sex pilus forms
Replication and transfer of F plasmid by rolling circle
replication
Conjugation
                     Transduction
Bacterial DNA is transferred from a donor to a recipient inside a virus that
infects bacteria (phage or bacteriophage)

Phage injects DNA into host

Chromosomal DNA of host is chopped up

Some pieces of bacterial DNA are mistakenly packaged inside phage
protein coats

Meanwhile, Phage forces host to make phage DNA and phage proteins

Phage protein coat (capsid) surround pieces of DNA to make new phage
offspring and offspring are released from host (burst).

Phage with bacterial DNA are released and infect new hosts and recombine
with the recipient bacterial DNA.
Transduction
              Plasmids
Carry non essential genes
(F plasmids): Carries genes for sex pili
and for the transfer of the plasmid to
another cell
Encode enzymes for catabolism of
unusual compounds
Plasmids that encode antibiotic resistance

				
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posted:3/14/2012
language:English
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