Chapter 8
Microbial Genetics
8.1 Prokaryotic DNA
• Geologic time
• Fossil evidence
• Archaea domains
• Microbial evolution
FIGURE 1: Fossil microbes
Courtesy of Abigail Allwood, Geologist at NASA Jet Propulsion Laboratory
Time-Line of Life on Earth
FIGURE 2: The appearance of life on Earth
8.1 DNA and Chromosomes
History: The Tortoise and the Hare
• Deciphering the structure
of DNA
• Rosalind Franklin (tortoise)
• James Watson (the hare)
working with Francis Crick
FIGURE MF01: Photo of Rosalind Franklin
© Vittorio Luzzati/Photo Researchers, Inc.
• Bacterial and Archaeal DNA is organized within
the nucleoid.
• The DNA exists as a single, circular chromosome.
• DNA within a chromosome
is highly compacted.
• Many microbial cells also
contain plasmids.
• Plasmids carry nonessential
but often useful information.
FIGURE 4a: DNA packing
8.2 DNA Replication
• Highly regulated process:
• Initiation
• Elongation
• Termination
• Leading strand and lagging strand
Figure 05: Replication of
Circular Chromosome in
E. coli.
8.3 Protein Synthesis
• Central dogma
identifies the flow of
genetic material
(DNA RNA
protein).
• Transcription copies
genetic information into
RNA.
FIGURE 7: The central dogma
8.3 Protein Synthesis
• Types of RNA
• mRNA
• tRNA
• rRNA
Figure 11: The Transcription of the Three Types of
RNA.
A Comparison of DNA and RNA
Figure T02: A Comparison of DNA and RNA.
The Transcription Process
• Transcription - gene DNA serves as a template for new
mRNA molecules
Figure 08: The Transcription Process.
The Genetic Code is Degenerate
• The genetic code
consists of 3 letter
words.
• More than one
codon specifies a
specific amino acid.
Figure T03: The
Genetic Code Decoder.
Translation is the process of making the polypeptide
at the ribosome.
• Chain initiation
• Chain elongation
• Chain
termination/
release
Figure 12A: Protein
Synthesis in a
Bacterial Cell.
Concept Map for Protein Synthesis
Figure 16: A Concept Map for Protein Synthesis.
• Many antibiotics interfere with protein
synthesis.
• Proteins synthesis can be controlled in several
ways.
• Transcription and translation are
compartmentalized.
Protein synthesis can be controlled in several ways.
FIGURE 14: The operon and negative control
8.4 Mutations – as a result of heritable,
permanent changes in the DNA
• Mutations can be
spontaneous or
induced.
• Physical
mutations
• Chemical
mutagens
Figure 17: Ultraviolet
Light and DNA
The Effect of Chemical Mutagens
Figure 18A: The Effect of Chemical Mutagens.
Categories and Results of Point Mutations
• Point mutations
are a result of
spontaneous or
induced mutations.
• Base-pair
substitutions
• Base-pair deletion
or insertion
Figure 19: Categories and Results of Point Mutations.
• Repair mechanisms attempt
to correct mistakes or
damage in the DNA.
• Cells have the ability to
repair damaged DNA.
• Mismatch repair
• Excision repair
FIGURE 21: Excision repair
mechanism
FIGURE 20: Mismatch repair mechanisms
• Transposable genetic elements can cause mutations.
• Barbara McClintock discovered transposons in maize.
• Insertion sequences and transposons move from one
DNA location to another.
FIGURE 22: Transposon "jumping"
and structure
8.5 Identifying Mutants – an organism carrying
a mutation
• Plating techniques
select for specific
mutants or
characteristics.
• The Ames test can
identify potential
mutagens.
FIGURE 25: Using the
Ames test
Positive Selection of Mutants
Figure 24: Positive Selection of Mutants.