Microbial Genetics:

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Lecture 7

• Microbial Genetics
• Genetic Mutations
• Gene Transfer

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Genetic Mutations
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Mutations Changes in DNA

• Why are mutations in DNA important to humans?
• 2 types of mutations
• Spontaneous Mutations
• occur in the natural environment without the
  addition of mutagens (agents that cause
  mutations)
• Occur randomly and spontaneously
• Induced Mutations
• Mutations that are created by the addition of
  mutagens

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Spontaneous Mutations

• Two types
• 1. Base substitutions
• 2. Frameshift mutations

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Spontaneous Mutations Base substitutions

• Most common type of substitution
• Mistake during DNA replication, incorrect base
  incorporated into DNA
• Three types
• 1. Silent mutation no effect on protein
• (remember- several codons code for the same
  amino acid)
• 2. Missense mutation codon has changed and
  different amino acid is incorporated
• 3. Nonsense mutation codon has changed to a stop
  codon

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Figure 8.15 - Overview
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Spontaneous Mutation Base-pair deletion or
insertion

• Insert or delete a nucleotide- very disastrous
• Shifts codons of DNA when transcribed into RNA
  (also called frameshift mutation)
• All nucleotides downstream of mutation will be
  grouped into improper codons, and wrong amino
  acids will be added
• Protein will be non-functional

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Figure 8.16a
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Figure 8.16b
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Figure 8.16c
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Figure 8.16d
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Induced Mutation

• Mutations are induced by either certain chemical
  mutagens or physical mutagens
• Sometimes scientists intentionally mutate DNA to
  study it

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Chemical Mutagens- Chemical Modification

• Example nitrous acid
• Converts adenine so it no longer pairs with
  thymine
• Instead pairs with cytosine

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Figure 8.17 - Overview (1 of 3)
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Chemical Mutagens- Nucleoside Analogs

• Compounds that resemble bases closely

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Physical Mutagens Radiation
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• Repair of thymine
• dimers

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Gene Transfer
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Gene Transfer

• Gene Transfer
• Vertical Gene Transfer When genes are passed
  from an organism to its offspring
• Horizontal Gene Transfer Occurs between bacteria
  

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Horizontal Gene Transfer

• Two types of cells
• Donor transfers DNA to recipient
• Recipient receives the DNA

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Gene Transfer

• Three methods of horizontal gene transfer
• Transformation
• Transduction
• Conjugation

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Transformation

• Donor cell wall can rupture, and DNA that is
  normally tightly packed can break up into pieces
  and explode through cell wall
• Naked DNA
• These pieces can then pass through cell walls and
  cytoplasmic membranes of recipient cells and get
  integrated into their chromosomes

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Figure 8.15
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Transduction

• DNA is transferred from one bacteria to another
  by virus that infects bacteria (bacteriophage)

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Conjugation

• Transfer of DNA by contact of two bacterial cells
• Can transfer plasmid or chromosome

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Conjugation

• Two types of cells
• Donor cell- contains fertility plasmid and is
  designated F
• Recipient cell- does not contain fertility
  plasmid and is designated F-
• DNA transferred from F to F-

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Conjugation

• In some cells carrying F factors, the F factor
  integrates into the host chromosome
• Now called Hfr cell
• Conjugation between Hfr and F-
• Chromosome replicates
• Transferred to F- cell
• Usually chromosome breaks off before completely
  transferred
• Generally remains F- because does not receive F
  factor

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R plasmid

• Resistance plasmid- confer antibiotic resistance
• Two parts
• Resistance genes (R genes)
• Resistance transfer factor (RTF)

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Figure 8.22