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Foundations in
Microbiology
Seventh Edition
Talaro
Chapter 9
Microbial Genetics
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
9.1 Genetics and Genes
Genetics – the study of heredity
The science of genetics explores:
1. Transmission of biological traits from parent
to offspring
2. Expression and variation of those traits
3. Structure and function of genetic material
4. How this material changes
2
3
Levels of Structure and Function of
the Genome
• Genome – sum total of genetic material of a cell
(chromosomes + mitochondria/chloroplasts and/or
plasmids)
– Genome of cells – DNA
– Genome of viruses – DNA or RNA
• DNA complexed with protein constitutes the genetic
material as chromosomes
• Bacterial chromosomes are a single circular loop
• Eukaryotic chromosomes are multiple and linear
4
Figure 9.2
5
Chromosome is subdivided into genes, the
fundamental unit of heredity responsible for a
given trait
–
–
Site on the chromosome that provides information
for a certain cell function
Segment of DNA that contains the necessary code to
make a protein or RNA molecule
Three basic categories of genes:
1. Genes that code for proteins – structural genes
2. Genes that code for RNA
3. Genes that control gene expression – regulatory
genes
6
• All types of genes constitute the genetic
makeup – genotype
• The expression of the genotype creates
observable traits – phenotype
7
Genomes Vary in Size
• Smallest virus – 4-5 genes
• E. coli – single chromosome containing
4,288 genes; 1 mm; 1,000X longer than cell
• Human cell – 46 chromosomes containing
31,000 genes; 6 feet; 180,000X longer than
cell
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Figure 9.3 E. coli cell has spewed out its DNA
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DNA
• Two strands twisted into a double helix
• Basic unit of DNA structure is a nucleotide
• Each nucleotide consists of 3 parts:
– A 5 carbon sugar – deoxyribose
– A phosphate group
– A nitrogenous base – adenine, guanine, thymine,
cytosine
• Nucleotides covalently bond to form a sugarphosphate linkage – the backbone
– Each sugar attaches to two phosphates –
• 5′ carbon and 3′ carbon
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DNA
• Nitrogenous bases covalently bond to the 1′
carbon of each sugar and span the center of the
molecule to pair with an appropriate
complementary base on the other strand
– Adenine binds to thymine with 2 hydrogen bonds
– Guanine binds to cytosine with 3 hydrogen bonds
• Antiparallel strands 3′ to 5′ and 5′ to 3′
• Each strand provides a template for the exact
copying of a new strand
• Order of bases constitutes the DNA code
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Figure 9.4
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Significance of DNA Structure
1. Maintenance of code during reproduction
- Constancy of base pairing guarantees
that the code will be retained
2. Providing variety - order of bases
responsible for unique qualities of each
organism
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DNA Replication
• Making an exact duplicate of the DNA involves
30 different enzymes
• Begins at an origin of replication
• Helicase unwinds and unzips the DNA double
helix
• An RNA primer is synthesized at the origin of
replication
• DNA polymerase III adds nucleotides in a 5′ to 3′
direction
– Leading strand – synthesized continuously in 5′ to 3′
direction
– Lagging strand – synthesized 5′ to 3′ in short
segments; overall direction is 3′ to 5′
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• DNA polymerase I removes the RNA
primers and replaces them with DNA
• When replication forks meet, ligases link
the DNA fragments along the lagging strand
to complete the synthesis
• Separation of the daughter molecules is
complete
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Figure 9.5
16
DNA replication is semiconservative because
each chromosome ends up with one new
strand of DNA and one old strand.
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Figure 9.7 Completion of chromosome replication
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9.2 Applications of the DNA code
• Information stored on the DNA molecule is
conveyed to RNA molecules through the
process of transcription
• The information contained in the RNA
molecule is then used to produce proteins in
the process of translation
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Gene-Protein Connection
1. Each triplet of nucleotides on the RNA
specifies a particular amino acid
2. A protein’s primary structure determines its
shape and function
3. Proteins determine phenotype. Living things
are what their proteins make them.
4. DNA is mainly a blueprint that tells the cell
which kinds of proteins to make and how to
make them
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Figure 9.9 DNA-protein relationship
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RNAs
• Single-stranded molecule made of nucleotides
– 5 carbon sugar is ribose
– 4 nitrogen bases – adenine, uracil, guanine, cytosine
– Phosphate
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RNA
• 3 types of RNA:
– Messenger RNA (mRNA) – carries DNA message
through complementary copy; message is in triplets
called codons
– Transfer RNA (tRNA) – made from DNA;
secondary structure creates loops; bottom loop
exposes a triplet of nucleotides called anticodon
which designates specificity and complements
mRNA; carries specific amino acids to ribosomes
– Ribosomal RNA (rRNA) – component of ribosomes
where protein synthesis occurs
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Figure 9.10
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Transcription:
The First Stage of Gene Expression
1. RNA polymerase binds to promoter region upstream
of the gene
2. RNA polymerase adds nucleotides complementary
to the template strand of a segment of DNA in the 5′
to 3′ direction
3. Uracil is placed as adenine’s complement
4. At termination, RNA polymerase recognizes signals
and releases the transcript

100-1,200 bases long
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Figure 9.11
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Translation:
The Second Stage of Gene Expression
• All the elements needed to synthesize
protein are brought together on the
ribosomes
• The process occurs in five stages: initiation,
elongation, termination, and protein folding
and processing
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Figure 9.12
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The Master Genetic Code
• Represented by the mRNA codons and the
amino acids they specify
• Code is universal
• Code is redundant
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Figure 9.13
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Figure 9.14
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Translation
• Ribosomes assemble on the 5′ end of an
mRNA transcript
• Ribosome scans the mRNA until it reaches
the start codon, usually AUG
• A tRNA molecule with the complementary
anticodon and methionine amino acid enters
the P site of the ribosome and binds to the
mRNA
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36
Translation
• A second tRNA with the complementary
anticodon fills the A site
• A peptide bond is formed
• The first tRNA is released and the ribosome
slides down to the next codon
• Another tRNA fills the A site and a peptide
bond is formed
• This process continues until a stop codon is
encountered
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Translation Termination
• Termination codons – UAA, UAG, and
UGA – are codons for which there is no
corresponding tRNA
• When this codon is reached, the ribosome
falls off and the last tRNA is removed from
the polypeptide
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Figure 9.15
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Polyribosomal complex allows for the synthesis of
many protein molecules simultaneously from the
same mRNA molecule.
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Eukaryotic Transcription and
Translation
1. Do not occur simultaneously – transcription
occurs in the nucleus and translation occurs in
the cytoplasm
2. Eukaryotic start codon is AUG, but it does not
use formyl-methionine
3. Eukaryotic mRNA encodes a single protein,
unlike bacterial mRNA which encodes many
4. Eukaryotic DNA contains introns – intervening
sequences of noncoding DNA – which have to be
spliced out of the final mRNA transcript
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Figure 9.17
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Genetics of Animal Viruses
• Viral genome - one or more pieces of DNA
or RNA; contains only genes needed for
production of new viruses
• Requires access to host cell’s genetics and
metabolic machinery to instruct the host cell
to synthesize new viral particles
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44
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9.3 Regulation of Protein Synthesis
and Metabolism
• Genes are regulated to be active only when
their products are required
• In prokaryotes this regulation is coordinated
by operons, a set of genes, all of which are
regulated as a single unit
46
Operons
• 2 types of operons:
– Inducible – operon is turned ON by substrate:
catabolic operons - enzymes needed to metabolize
a nutrient are produced when needed
– Repressible – genes in a series are turned OFF by
the product synthesized; anabolic operon –
enzymes used to synthesize an amino acid stop
being produced when they are not needed
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Lactose Operon: Inducible Operon
Made of 3 segments:
1. Regulator – gene that codes for repressor
2. Control locus – composed of promoter and
operator
3. Structural locus – made of 3 genes each coding
for an enzyme needed to catabolize lactose –
b-galactosidase – hydrolyzes lactose
permease – brings lactose across cell membrane
b-galactosidase transacetylase – uncertain function
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Lac Operon
• Normally off
– In the absence of lactose, the repressor binds
with the operator locus and blocks transcription
of downstream structural genes
• Lactose turns the operon on
– Binding of lactose to the repressor protein
changes its shape and causes it to fall off the
operator. RNA polymerase can bind to the
promoter. Structural genes are transcribed.
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Figure 9.18
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Arginine Operon: Repressible
• Normally on and will be turned off when
the product of the pathway is no longer
required
• When excess arginine is present, it binds to
the repressor and changes it. Then the
repressor binds to the operator and blocks
arginine synthesis.
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Figure 9.19
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9.4 Mutations:
Changes in the Genetic Code
• A change in phenotype due to a change in
genotype (nitrogen base sequence of DNA) is
called a mutation
• A natural, nonmutated characteristic is known
as a wild type (wild strain)
• An organism that has a mutation is a mutant
strain, showing variance in morphology,
nutritional characteristics, genetic control
mechanisms, resistance to chemicals, etc.
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Figure 9.20
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Causes of Mutations
• Spontaneous mutations – random change
in the DNA due to errors in replication that
occur without known cause
• Induced mutations – result from exposure
to known mutagens, physical (primarily
radiation) or chemical agents that interact
with DNA in a disruptive manner
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Categories of Mutations
• Point mutation – addition, deletion, or
substitution of a few bases
• Missense mutation – causes change in a
single amino acid
• Nonsense mutation – changes a normal
codon into a stop codon
• Silent mutation – alters a base but does not
change the amino acid
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Categories of Mutations
• Back-mutation – when a mutated gene
reverses to its original base composition
• Frameshift mutation – when the reading
frame of the mRNA is altered
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Repair of Mutations
• Since mutations can be potentially fatal, the cell
has several enzymatic repair mechanisms in
place to find and repair damaged DNA
– DNA polymerase – proofreads nucleotides during
DNA replication
– Mismatch repair – locates and repairs mismatched
nitrogen bases that were not repaired by DNA
polymerase
– Light repair – for UV light damage
– Excision repair – locates and repairs incorrect
sequence by removing a segment of the DNA and
then adding the correct nucleotides
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Figure 9.21
61
The Ames Test
• Any chemical capable of mutating bacterial
DNA can similarly mutate mammalian DNA
• Agricultural, industrial, and medicinal
compounds are screened using the Ames test
• Indicator organism is a mutant strain of
Salmonella typhimurium that has lost the ability
to synthesize histidine
• This mutation is highly susceptible to backmutation
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Figure 9.22
63
Positive and Negative Effects of
Mutations
• Mutations leading to nonfunctional proteins are
harmful, possibly fatal
• Organisms with mutations that are beneficial in
their environment can readily adapt, survive, and
reproduce – these mutations are the basis of
change in populations
• Any change that confers an advantage during
selection pressure will be retained by the
population
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9.5 DNA Recombination Events
Genetic recombination – occurs when an
organism acquires and expresses genes
that originated in another organism
3 means for genetic recombination in bacteria:
1. Conjugation
2. Transformation
3. Transduction
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Conjugation
• Conjugation – transfer of a plasmid or
chromosomal fragment from a donor cell to
a recipient cell via a direct connection
– Gram-negative cell donor has a fertility
plasmid (F plasmid, F′ factor) that allows the
synthesis of a conjugative pilus
– Recipient cell is a related species or genus
without a fertility plasmid
– Donor transfers fertility plasmid to recipient
through pilus
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Figure 9.23 (1)
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Figure 9.23 (2)
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Conjugation
• High-frequency recombination – donor’s
fertility plasmid has been integrated into the
bacterial chromosome
• When conjugation occurs, a portion of the
chromosome and a portion of the fertility
plasmid are transferred to the recipient
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Figure 9.23 (3)
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Transformation
• Transformation – chromosome fragments
from a lysed cell are accepted by a recipient
cell; the genetic code of the DNA fragment is
acquired by the recipient
• Donor and recipient cells can be unrelated
• Useful tool in recombinant DNA technology
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Figure 9.24
Insert figure 9.23
transformation
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Transduction
• Transduction – bacteriophage serves as a
carrier of DNA from a donor cell to a recipient
cell
• Two types:
– Generalized transduction – random fragments of
disintegrating host DNA are picked up by the phage
during assembly; any gene can be transmitted this
way
– Specialized transduction – a highly specific part of
the host genome is regularly incorporated into the
virus
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Figure 9.25
Generalized
transduction
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Figure 9.26
Specialized
transduction
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Transposons
• Special DNA segments that have the
capability of moving from one location in
the genome to another – “jumping genes”
• Cause rearrangement of the genetic material
• Can move from one chromosome site to
another, from a chromosome to a plasmid,
or from a plasmid to a chromosome
• May be beneficial or harmful
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Figure 9.27
Transposons
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