Phage Infections

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Phage Infections
Single Phage Infection uniform plaque morphology.
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Phage Particles Can Recombine
r small plaque
r- large plaque
h clear plaque
h- turbid plaque
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What is a Gene?

• Bead Theory (1950s),
• The gene was viewed as a fundamental unit of
  structure, indivisible by crossing over.
• The gene was viewed as the fundamental unit of
  change.
• The gene is the fundamental unit of function?

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Genetic Fine Structure

• Seymore Benzer
• Demonstrated that a gene can be defined as a unit
  of function, a gene can be subdivided into a
  linear array of sites that are mutable and that
  can be recombined.
• Paved the way for the understanding that the
  smallest units of mutation and recombination are
  single nucleotide pairs.

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rII

• A mutant T4 phage was known to produce larger,
  ragged plaques
• this mutation was mapped to two genetic loci on
  the phage DNA molecule, rI and rII,
• rII mutants have an altered host range than
  wild-type T4.

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rII Host Range
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Mutation and Effect

• Base substitutions (point mutations),
• a change in a codon produces missense mutations,
• different amino acids may change the function of
  the translated protein.
• Deletions and insertions can cause frameshifts,
  usually resulting in missense and premature stop
  codons.

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Intergenic Recombination
Frequency of recombination indicates map didtance.
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Why Stop There?

• Deletion Mapping partial deletions in genes can
  be mapped in just the same way as other
  mutations
• In fact, the site of the deletion can be
  determined by defining which previously mapped
  mutations fail to to recombine into a wild-type
  gene.

Please Study, and master A moment to Think, pp.
314
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Deletion Mapping
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What Did We Learn?

• Mutations can exist within genes,
• Genetic Exchange can take place within a gene,
• Mutations are not produced at all locations in a
  gene,
• and are found at higher frequencies at certain
  locations.
• mutation hotspots correspond to key codons,
  hence key amino acids?