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Lecture 7
Microbial Genetics:
Genetic Mutations
Gene Transfer
Genetic Mutations
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
Spontaneous Mutations
• Two types:
1. Base substitutions
2. Frameshift mutations
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
Figure 8.15 - Overview
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
Figure 8.16a
Figure 8.16b
Figure 8.16c
Figure 8.16d
Induced Mutation
• Mutations are induced by either certain
chemical mutagens or physical mutagens
• Sometimes scientists intentionally mutate
DNA to study it
Chemical Mutagens- Chemical
Modification
• Example: nitrous acid
• Converts adenine so it no longer pairs with
thymine
• Instead pairs with cytosine
Figure 8.17 - Overview (1 of 3)
Chemical Mutagens- Nucleoside
Analogs
• Compounds that resemble bases closely
Physical Mutagens: Radiation
Repair of
thymine
dimers
Gene Transfer
Gene Transfer
•
Gene Transfer=
•
Vertical Gene Transfer= When genes are
passed from an organism to it’s offspring
•
Horizontal Gene Transfer= Occurs
between bacteria
Horizontal Gene Transfer
• Two types of cells:
1.Donor: transfers DNA to recipient
2.Recipient: receives the DNA
Gene Transfer
•
Three methods of horizontal gene
transfer:
1. Transformation
2. Transduction
3. Conjugation
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
Figure 8.15
Transduction
• DNA is transferred from one bacteria to
another by virus that infects bacteria
(bacteriophage)
Conjugation
• Transfer of DNA by contact of two
bacterial cells
• Can transfer plasmid or chromosome
Conjugation
• Two types of cells:
1. Donor cell- contains fertility plasmid and
is designated F+
2. Recipient cell- does not contain fertility
plasmid and is designated F• DNA transferred from F+ to F-
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
R plasmid
•
Resistance plasmid- confer antibiotic
resistance
• Two parts:
1. Resistance genes (R genes)
2. Resistance transfer factor (RTF)
Figure 8.22