CHAPTER
7
Microbial Genetics
Chapter Outline
The Structure and Replication of Genomes (pp. 193–202)
The Structure of Nucleic Acids
Gene Function (pp. 202–216)
The Relationship Between Genotype and Phenotype
The Transfer of Genetic Information
The Events in Transcription
Translation
Regulation of Genetic Expression
Interactive Microbiology: Gene Regulation: Operons
Mutations of Genes (pp. 216–223)
Types of Mutations
Effects of Point Mutations
Mutagens
Frequency of Mutation
DNA Repair
Identifying Mutants, Mutagens, and Carcinogens
Genetic Recombination and Transfer (pp. 223–230)
Chapter Summary
The Structure and Replication of Genomes (pp. 193–202)
Genetics is the study of inheritance and inheritable traits. A genome is the sum of all the genetic
material in a cell or virus. Genes are specific sequences of nucleotides that code for RNA or
polypeptides molecules. The genomes of all cells are composed of DNA; some viruses use
DNA, and other viruses use RNA.
Chapter 7 Microbial Genetics
The Structure of Nucleic Acids
Nucleic acids are polymers of nucleotides. Nucleotides are composed of a phosphate attached to
a nucleoside composed of a pentose sugar attached to one of five nitrogenous bases: guanine
(G), cytosine (C), thymine (T), adenine (A), or uracil (U). The double helix of DNA is two
polymer strands held together by hydrogen bonds between complementary bases of nucleic
acids called base pairs (bp). In DNA, adenine bonds with thymine, and guanine bonds with
cytosine. In RNA, adenine bonds with uracil. The critical function of DNA is to store
The Structure of Prokaryotic Genomes
Bacterial genomes typically consist of one chromosome, which is a circular molecule of DNA
associated with protein and RNA molecules, folded into loops and localized in a region of the
cytoplasm called the nucleoid. Prokaryotes are haploid with a single copy of the chromosome.
The Structure of Eukaryotic Genomes
Eukaryotic genomes are typically composed of multiple linear chromosomes which are
sequestered in the nucleus. The double membrane around the nucleus is the nuclear envelope.
Many eukaryotes are diploid, having two copies of each chromosome. In addition to DNA,
DNA Replication
The basics of DNA replication are common to all cells. The key is the complementarity of the
bases. The two original strands serve as templates for the synthesis of new complementary
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strands. The process is semiconservative because each daughter DNA molecule is composed of
DNA replication starts at a specific nucleotide sequence called an origin. DNA helicase
“unzips” the double helix, breaking hydrogen bonds between complementary base pairs, to form
a replication fork. DNA polymerases bind to the strands. DNA polymerases always polymerize
The lagging strand is discontinuously synthesized in a direction away from the replication
fork in series of Okazaki fragments and thus lags behind the process occurring in the leading
strand. Each Okazaki fragment is initiated and synthesized as described for the leading strand,
Eukaryotic DNA replication is similar to that in bacteria with a few exceptions. Eukaryotic
cells use four DNA polymerases to replicate DNA. Due to the large size of eukaryotic
chromosomes, there are thousands of origins of replication. Okazaki fragments of eukaryotes are
smaller than those of bacteria. Finally, plant and animal cells methylate cytosine bases
exclusively.
Gene Function (pp. 202–216)
To understand gene function, it is necessary to understand the relationship between an
organism’s genotype and phenotype.
Chapter 7 Microbial Genetics
The Relationship Between Genotype and Phenotype
The genotype of an organism is the actual set of genes in its genome, whereas the phenotype is
The Transfer of Genetic Information
DNA stores genetic information but is not directly coded into protein. Transcription produces a
complementary RNA copy of the gene, and translation is the synthesis of a polypeptide from the
RNA copy. The central dogma of genetics states that DNA is transcribed to RNA, which is
translated to form polypeptides. Some RNA viruses are the exception using an RNA template to
generate a DNA product.
The Events in Transcription
Cells transcribe six types of RNA from DNA:
2. Messenger RNA (mRNA) molecules, which carry genetic information from chromosomes to
ribosomes.
4. Transfer RNA (tRNA) molecules, which deliver amino acids to the ribosomes.
6. Ribozymes, RNA catalysts.
Transcription occurs in the nucleoid of bacteria. RNA polymerase binds to a specific region
of DNA called a promoter near the beginning of a gene sequence. A bacterial subunit of RNA
polymerase, a sigma factor, recognizes the promoter sequence. Bacteria have different sigma
factors and promoter sequences. Each sigma factor recognizes specific promoter sequences, thus
which may destabilize the bonding between the DNA and RNA. During Rho-dependent
termination Rho proteins may assist in termination by binding to an RNA sequence. Rho then
moves toward the RNA polymerase and causes the RNA polymerase to dissociate from the
DNA template.
Eukaryotic transcription differs from bacterial transcription in several ways. Eukaryotic cells
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
Translation
In translation, the sequence of genetic information carried by mRNA is used by ribosomes to
synthesize polypeptides with specific amino acid sequences. To understand how four DNA
nucleotides can specify the 21 different amino acids commonly found in proteins requires an
understanding of the genetic code. Scientists define the genetic code as the complete set of
amino acids, a cell does not need to have 62 anticodons on 62 different tRNAs due to “wobble”
of the anticodon’s third nucleotide. Wobble allows the third nucleotide to base pair with a
nucleotide other than its usual complement.
Prokaryotes have 70S ribosomes composed of 50s and 30s subunits, while eukaryotic 80S
ribosomes contain 60s and 40s subunits. Structural differences between eukaryotic and
1. The A site accommodates a tRNA delivering an amino acid.
3. Discharged tRNAs exit from the E site.
Prokaryotic translation proceeds in three stages: initiation, elongation, and termination. A
variety of protein factors are involved in each stage. In initiation, an initiation complex
composed of the ribosome subunits, mRNA, tRNAfmet, and several protein factors is formed.
During elongation, tRNAs sequentially deliver amino acids as directed by the codons of the
Regulation of Genetic Expression
Many genes in bacteria are expressed at all times; other genes are regulated so that the
polypeptides they encode are synthesized only when a cell needed them. For example, in
quorum sensing, cells detect quorum-sensing molecules and synthesize new proteins in
response to the signals. Cells may regulate synthesis by initiating or blocking transcription or by
stopping translation directly. Two types of transcription regulation are induction and repression
of operons.
Interactive Microbiology: Gene Regulation: Operons
The tryptophan (trp) operon is a repressible operon that codes for five enzymes required for
tryptophan synthesis. A regulatory gene codes for a repressor protein that is normally inactive.
When tryptophan is available, it binds to the repressor protein, activating it. The repressor then
Mutations of Genes (pp. 216–223)
A mutation is a heritable change in the nucleotide sequence of a genome. Mutations may be
large changes in an organism’s genome or affect a single base pair.
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
Types of Mutations
Point mutations are mutations in which one nucleotide base pair is affected. Point mutations
include the following:
Substitutions, in which a single nucleotide base pair is substituted for another.
Effects of Point Mutations
Some base-pair substitutions produce silent mutations: the substitution does not change the
amino acid sequence because of the redundancy of the genetic code. A change in a nucleotide
sequence resulting in a codon that specifies a different amino acid is called a missense
nonsense mutations.
Mutagens
Mutations can be spontaneous, created when errors occur in replication or repair, or result from
recombination, which involves the movement of DNA from one location to another. Physical or
chemical agents called mutagens, which include radiation and several types of DNA-altering
chemicals, induce mutations. Ionizing radiation in the form of X rays and gamma rays can
Chemical mutagens include nucleotide analogs, compounds that are structurally similar to
normal nucleotides but, when incorporated into DNA, may interfere with DNA polymerase
function or cause base pair mismatching. Some nucleotide-altering chemicals change the
Frequency of Mutation
About 1 of every 10 million genes contains an error. Mutagens typically increase the mutation
rate by a factor of 10–1000 times. Mutations are usually deleterious but on occasion may
Chapter 7 Microbial Genetics
DNA Repair
Cells have numerous methods of repairing damaged DNA. Direct repair mechanisms repair
errors in single nucleotides. In base-excision repair, an enzyme system removes the incorrect
base and fills in the gap. Light repair corrects the damage of pyrimidine dimers, and many cells
Identifying Mutants, Mutagens, and Carcinogens
If a cell does not repair a mutation, the cell and its descendants are called mutants. By contrast,
cells normally found in nature are called wild-type cells. Researchers have developed methods
to recognize mutants among their wild-type neighbors. These include:
Positive selection, which involves selecting a mutant by eliminating wild-type phenotypes.
The rate of mutation can be calculated by the following equation:
Genetic Recombination and Transfer (pp. 223–230)
Genetic recombination refers to the exchange of nucleotide sequences between two DNA
molecules often mediated by segments composed of identical or nearly identical nucleotide
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Horizontal Gene Transfer Among Prokaryotes
Vertical gene transfer is the transmission of genes from parents to offspring. In horizontal
(lateral) gene transfer, DNA from a donor cell is transmitted to a recipient cell. A
recombinant cell results from genetic recombination between donated and recipient DNA.
Transformation, transduction, and bacterial conjugation are types of horizontal gene transfer.
Transposons and Transposition
Transposons are DNA segments that code for the enzyme transposase and contain palindromic
sequences known as inverted repeats (IR) at each end. (A palindrome is a word, phrase, or