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Chapter 8
Microbial Genetics
Part 1
Structure and Function of the
Genetic Material
• Much of cellular metabolism is involved in
translating the genetic message of genes
into specific proteins
• Genetics: science of heredity and gene
function; study of what genes are, how they
carry information, how information is
expressed, and how genes are replicated
Structure and Function of the
Genetic Material
• Genome: one complete copy of genetic
information in a cell; All of the genetic
material in a cell
– Genome is organized into chromosomes
• Chromosomes: structures containing DNA
that physically carry hereditary information
Structure and Function of the
Genetic Material
• Gene: Segment of DNA that encodes a
functional product, usually a protein
– Usually codes for mRNA, but can also code for
rRNA or tRNA
• Genetic code: set of rules that determines
how a nucleotide sequence is converted into
the amino acid sequence of a protein; the
mRNA codons and the amino acids they
encode
Structure and Function of the
Genetic Material
• Genotype: genetic makeup of an organism
• Phenotype: external manifestations of an
organism’s genotype, or genetic makeup
• Genomics: study of genes and their
function; molecular study of genomes;
sequencing and molecular characterization
of genomes
DNA and Chromosomes
• DNA
– DNA exists as long strands of nucleotides
twisted together in pairs to form double helix
(two strands held together by H-bonds)
– Specific base pairing
two strands of DNA
are complementary
– Explain two primary features of biological
information storage
• linear sequence of bases (actual information)
• complementary structure allows precise duplication
of DNA during cell division
DNA and Chromosomes
• Bacterial DNA
– Typically a single circular chromosome consist
of a single circular molecule of DNA with
associated proteins
– Chromosome is looped and folded and attached
at one or several points to the plasma membrane
– Twisted or supercoiled by topoisomerase II (or
DNA gyrase)
– bacterial chromosome map (location of genes) is
marked in minutes (Fig. 8.1b)
Chromosome of E. coli
Figure 8.1a
The Flow of Genetic Information
• DNA replicates before cell division
• Each daughter cell receives an identical
chromosome from the parent
DNA
transcription (cytoplasm; nucleus)
mRNA
translation (cytoplasm)
protein
Flow of Genetic Information
Figure 8.2
DNA Replication
• One “parental” double-stranded DNA
converted to two identical “daughter” DNA
• Parental strand acts as a template for
replication (complementary base pairing)
• Initiation of DNA replication starts at the
origin of replication (form replication fork)
• Replication fork: point at which replication
occurs
DNA Replication
Figure 8.3
DNA Replication
• Polymer of nucleotides:
adenine, thymine,
cytosine, guanine
• Double helix associated
with proteins
• "Backbone" is
deoxyribose-phosphate
• Strands held together by
hydrogen bonds between
AT and CG
• Strands are antiparallel
Figure 8.4
DNA Replication
Figure 8.5
DNA Replication
• DNA is copied by DNA polymerase
– In the 5  3 direction
– Initiated by an RNA primer (synthesized by RNA
polymerase)
– Leading strand synthesized continuously
– Lagging strand synthesized discontinuously in small
fragments (Okazaki fragments)
– RNA primers are removed and Okazaki fragments
joined by a DNA polymerase and DNA ligase
DNA
Figure 8.6
DNA
• DNA replication is semiconservative
Figure 8.7
DNA Replication
• Any bases that are improperly base-paired
are removed and replaced by replication
enzymes (proof-reading capability of DNA
polymerase)
– Light-repair enzymes
– Nucleotide excision repair mechanism
Transcription
• DNA is transcribed to make RNA (mRNA,
tRNA, and rRNA)
– only one strand of two DNA strands serves as the
template for transcription
• Transcription begins when RNA polymerase
binds to the promoter sequence
• Transcription proceeds in the 5  3 direction
• Transcription stops when it reaches the
terminator sequence
Transcription
Figure 8.8
Transcription
RNA processing in Eukaryotes
• Transcription occurs in nucleus
• RNA needs processing
– exon: region of DNA expressed; contains genes
that encode proteins
– intron: intervening regions of DNA that does
not encode protein; noncoding DNA
• mRNA must be completely synthesized
before leaving nucleus to be translated in
cytoplasm
RNA processing in Eukaryotes
Figure 8.12
Translation
• Translation: process in which the
information in the nucleotide base sequence
of mRNA is used to dictate the amino acid
sequence of a protein
– proceeds also in the 5  3 direction
• Codon: a sequence of 3 nucleotides in
mRNA that specifies the insertion of an
amino acid into a polypeptide
• Anticodon: a sequence of 3 bases (on
tRNA) that is complementary to a codon
Translation
• mRNA is translated in
codons (3 nucleotides)
• Translation of mRNA
begins at the start
codon: AUG
– AUG also codes for
methionine
• Translation ends at a
STOP codon: UAA,
UAG, UGA
Figure 8.2
Translation
• Codes are degenerate
– 64 possible codons
– 61 sense codons
– 3 nonsense (stop)
codons
Figure 8.9
Translation
Figure 8.10.1
Translation
Figure 8.10.2
Translation
Figure 8.10.3
Translation
Figure 8.10.4
Translation
Figure 8.10.5
Translation
Figure 8.10.7
Translation
Figure 8.10.8
Translation
• Translation can
begin before
transcription is
complete in
bacteria
Figure 8.11
Regulation of Bacterial Gene
Expression
• Constitutive enzymes are expressed at a
fixed rate
– always present in a cell (e.g. most of the
enzymes in glycolysis)
• Other enzymes are expressed only as
needed
– Repressible enzymes (repressor)
– Inducible enzymes (inducer)
Repression and Induction
• Regulate the transcription of mRNA
• Regulation: regulatory mechanism that
inhibits gene expression and decrease
synthesis of enzymes
– Response to the overabundance of an endproduct of a metabolic pathway
• Induction: process that turns on the
transcription of a gene or genes
Repression
• Promoter
• Operator
• Repressor (coded by I
gene)
• Structural gene: a gene
that determines the
amino acid sequence of a
protein
Figure 8.13
Operon Model of Gene Expression
• Jacob & Monod (1961); lactose operon
• Operon: the operator and promoter sites and
structural genes they control
– Inducible operon and repressible operon
Figure 8.14.1
Regulation of Gene Expression
Inducible Operon
e.g. lac operon
* Lactose absent
Figure 8.14.2
Regulation of Gene Expression
*Lactose present
Figure 8.14.4
Regulation of Gene Expression
Repressible Operon
e.g.
Tryptophane operon
Figure 8.14.3
Regulation of Gene Expression
* Excess tryptophane
act as corepressor.
Figure 8.14.5
Operon Model of Gene Expression
• Transcription of the lactose operon requires
both the presence of lactose and the absence
of glucose in the medium
– cyclic AMP (cAMP) + cAMP repressor protein
(CRP) binds to the lac promoter
RNA
polymerase binds to the lac promoter
– alarmone: a chemical alarm signal the cell uses
to respond to environmental or nutritional stress
Regulation of Gene Expression
• Catabolite repression occurs when glucose and
lactose are present in the medium
– cAMP level is low when glucose is available
Figure 8.15