Study Guide
Microbiology Principles and Explorations Chapter 7
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University:
Kentucky State University -
Course:
BIO 204 | Fund of Microbiology Academic year:
2024
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Views:
434
Pages:
89
Author:
Lamghari6kx
Microbiology: Principles and
Explorations
Chapter 7:
Microbial Genetics Microbial Genetics
• Most prokaryotes have a single, circular
chromosome
• When stretched out, this single DNA
molecule is about 1mm long (~1000 times
longer than cell)
• This immense molecule fits compactly
into the cell nucleoid by twisting around
itself (supercoiling) The Structure of DNA Supercoiled DNA
• DNA can be supercoiled in either a
positive or negative direction
• Negative supercoiling occurs when DNA
is twisted about its axis in the opposite
direction from that of the right-handed
double helix
• It is in this form that supercoiled DNA is
predominantly found in nature Topoisomerases
• Supercoiling of DNA in prokaryotes is typically
brought about in a much different manner than in
eukaryotes
• In bacteria and most archaea, DNA gyrase
(topoisomerase II) introduces negative supercoils
• Reverse gyrase: Found in hyperthermophilic
species of Archaea; introduces positive supercoiling
(overwinding) Vibrio cholerae
• This bacterial cell has two circular
chromosomes, one large and one small
• Chromosome: The DNA molecule must
contain genetic information essential for
the continuous survival of the organism
• Plasmids contain only genetic
information that could be helpful to
organisms, but that they could survive
without Definition of a Gene
• The basic unit of heredity; a linear
sequence of nucleotides of DNA that form
a functional unit of a chromosome or a
plasmid
• Genes with different information at the
same locus are called alleles
• Because prokaryotes have a single
chromosome, they generally have only
one version or allele of each gene A Partial Chromosomal Map of Escherichia coli Method used to
determine
which genes lie
on a specific
chromosome Transfer of Information from DNA to Protein DNA Replication in a Prokaryote DNA Replication in Some Prokaryotes
• Origin of replication (ORP): A single location on the
chromosome where DNA synthesis is initiated
– Consists of a specific sequence of about 300 bases
that is recognized by specific initiation proteins
• As replication proceeds, the site of replication
(replication fork), take on antiparallel arrangement
known as leading and lagging strands
• In circular DNA, this bi-directional replication leads to
the formation of characteristic structures called theta
structures Escherichia coli
• Has five different DNA polymerases
• DNA polymerase III is the primary enzyme of
replication at the replicating forks
• Helicases: Specific proteins that unwind the
double helix
• Single-stranded binding proteins (SSBP):
stabilizes the single-stranded DNA by
preventing the formation of hydrogen bonds As the two replication forks meet, the two new chromosomes
separate—each containing one new and one old strand • DNA polymerase I: Functions to not only
synthesize DNA but has a 5’ to 3’ exonuclease
activity
• Removes the RNA primer on the lagging strand
• DNA polymerase I is also involved in DNA
repair
• DNA ligase: Seals any nicks made in DNA
synthesis and also assists in DNA repair Protein Synthesis—Prokaryotes vs. Eukaryotes
• In Prokaryotes, both transcription and
translation occur in the cytoplasm
• In Eukaryotes: transcription takes place in the
nucleus; translation occurs in the cytoplasm
• Eukaryotes and Achaeans have regions of genes
that code for proteins (exons) and noncoding
regions called introns Transcription of RNA from the DNA Template Transcription of RNA from the DNA Template
• -PPP represents a high energy triphosphate and
–PP represents a pyrophosphate
• In RNA, uracil (U) pairs with adenine (A)
rather than thymine (T)
• RNA transcribed in the 5’ to 3’ direction
• Complementary RNA strand used as a
template to transcribe new DNA strand Eukaryotic genes differ in complexity from
Prokaryotic genes Eukaryotic genes differ in complexity from
Prokaryotic genes
• Coding sequences of DNA called exons (E) and
non-coding sequences called introns (I)
• After being transcribed into RNA, introns
removed and exons spliced together by
spliceosomes
• The resulting mRNA is ready to direct protein
synthesis and to leave the nucleus RNA Types
• Three types of RNA participate in protein
synthesis:
– Ribosomal RNA (rRNA)
– Messenger RNA (mRNA)
– Transfer RNA (tRNA)
• Each is synthesized by transcription using
single strand of DNA as a template Prokaryotic Ribosomal Structure Prokaryotic Ribosomal Structure
• Does not contain a genetic message, instead it
serves as site for protein synthesis
• Tightly binds to specific proteins to produce
two ribosomal subunits (large and small),
which combine together to form a ribosome
• The region of protein synthesis is at the
juncture of the three components mRNA
• It carries genetic information from the gene
(DNA) out of the nucleus, into the cytoplasm of
the cell where it is translated to produce a
protein
• Contains base triplets called codons that
constitute the genetic code
• Attaches to one or more ribosomes The Genetic Code The Genetic Code
• Standard three letter abbreviation for amino
acids known as codons
• UAA, UAG and UGA do not code for any
amino acids, instead all three code for Stop
designating terminator codons
• AUG codes for Start and the amino acid,
methionine (met)
• Therefore, protein synthesis always begins with
met Transfer RNA Structure Transfer RNA
• Function is to transfer amino acids from the
cytoplasm to the ribosomes for placement into
a protein molecule
• Each tRNA molecule consist of 75 to 80
nucleotides folded back on itself to form several
loops that are stabilized by complimentary base
pairing Transfer RNA Transfer RNA Structure
• Each tRNA has a three base anti-codon region
complimentary to a particular mRNA codon
• Each tRNA also contains an amino acid binding site,
specified by the mRNA codon
• Amino acid attachment to specific tRNA molecules
dictated through action of amino acid activating
enzymes and ATP
• There is a specific tRNA for each of the 20 different
amino acids Transcription vs. Translation Transcription vs. Translation
• Transcription is the synthesis of mRNA from a DNA
template
• Translation is the process where ribosomes synthesize
proteins using the mature mRNA transcript produced
during transcription
• Several ribosomes can be attached at different points
along a mRNA molecule to form a polyribosome
(polysome) Polyribosome
• The presence of many ribosomes all “riding”
simultaneously along one piece of mRNA
• Allows for concurrent transcription and
translation in prokaryotes Steps in Protein Synthesis Steps in Protein Synthesis Steps in Protein Synthesis Steps in Protein Synthesis Steps in Protein Synthesis Protein Synthesis Significance of Regulatory Mechanisms
• The survival of bacteria depends on their ability to
grow even when conditions are less than ideal
• Bacteria have developed mechanisms to turn
reactions on/off in accordance with their needs
• Thus cells use energy to synthesize substances in the
amounts needed and shut off synthesis before
wasteful excess are produced Replisomes
•
The two new strands of DNA are being synthesized
by a duplex of DNA polymerases
•
This is made possible by a “looping” of the lagging
strand
•
1.
2.
3.
4.
The replisome is a complex containing:
Helicases
Primase
Two DNA polymerase III molecules
Associated proteins Categories of Regulatory Mechanisms
• Feedback or End Product Inhibition
• Enzyme Induction
• Enzyme Repression Feedback/End Product Inhibition
• Enzyme activity is regulated directly
• The control mechanism determines how
rapidly enzymes already present will
catalyze reactions
• The end product of a biosynthetic
pathway directly inhibits the first enzyme
in the pathway Feedback Inhibition • Feedback inhibition allows the cell
to conserve energy in two ways:
1. The cell expends energy to
synthesize the end product only
when it is needed
2. Regulation of enzyme activity
requires less energy than the more
complex processes that regulate
gene expression Induction vs. Represssion Mechanisms
• Inducible Enzymes: Made only when their
substrates are present in the environment
• Repressible Enzymes: Enzymes whose
synthesis is inhibited in the presence of a signal
molecule (repressor)
• Constitutive Enzymes: Unregulated enzymes
that are always produced at a constant rate
because they are always needed Prokaryotic Operon
•
Operon: A set of closely related genes
that are regulated by a single operator
and are transcribed together
• An operon includes:
1. Structural genes: carry information for
the synthesis of specific proteins
2. Regulatory sites: Control the
expression of the structural genes;
located some distance from the operon Regulation of Transcription
• Most regulatory mechanisms act on
transcription
• They change the activity of RNA polymerase
• This is accomplished by changing the
frequency at which a particular gene is
transcribed
• If transcripts are made less frequently, less
protein is made • Enzyme induction: Typically
regulates catabolism
• Enzyme repression: Typically
regulates anabolism Regulatory Proteins that Bind to DNA
• Transcription is often regulated by
proteins that bind to DNA
• These proteins bind to specific regions of
DNA, called operators
• Operators are close to a gene’s promoter,
where RNA polymerase normally binds The Lac Operon
•The three enzymes needed to metabolize
lactose are encoded by the lac operon:
1.Galactosidase permease (lacY gene)
2.-galactosidase (lacZ gene)
3.Galactosidase transacetylase (lacA gene)
•Promoter-operator region (regulatory sites) Lac Repressor
• Regulator I gene directs synthesis of the
lac repressor
• Lac repressor is a protein that binds to
the operator and prevents transcription
of the adjacent, Z, Y and A genes
• The I gene is an example of a constitutive
gene; always undergoing protein
synthesis Enzyme Induction When lactose is absent, the repressor binds to the
operator, preventing transcription from occurring When lactose is present, it binds to the repressor,
inactivating it Enzyme Repression
• Controls processes in which substances needed
for growth are synthesized
• Synthesis of the amino acid tryptophan
• Trp operon
• Like many operons, the trp operon has more
than one type of regulation Trp Operon
• Contains five structural genes for the tryptophan
biosynthetic pathway
• The first enzyme in the pathway, anthranilate
synthetase is subject to feedback inhibition by
tryptophan
• When tryptophan is available to a bacterial cell, it
binds to an inactive repressor and activates it
• The activated repressor binds to the promoter, which
represses the synthesis of enzymes required to
produce tryptophan • Both enzyme induction and enzyme
repression are regulatory mechanisms that
control enzyme production by altering gene
expression
• Although these two mechanisms exhibit
different effects, they actually represent two
examples of the operation of a single
mechanism for turning genes on and off
• Table 7.2 Catabolite Repression
• Regulates expression of many genes and operons in
response to the availability of carbon sources for
growth
• Prevents expression of genes encoding catabolism
of carbon sources when a better carbon source is
available
• Saves the cell from expressing the lac operon if a
more suitable carbon source is available
• If glucose and lactose is available to E. coli, it uses
glucose first Catabolite Repression--The Growth Curve for Bacteria
Containing Glucose and Lactose MUTATIONS
• Heritable changes in the sequence of
nucleotides in DNA
• Account for evolutionary changes in
microorganisms (and larger organisms)
• Produces different strains within species Point Mutation—The
Effects of a Base
Substitution Point Mutation—The Effects of a Base Substitution Point Mutation—The Effects of a Base Substitution Point Mutation—The Effects of a Base Substitution Frameshift
Mutations Base Analogs Base Analogs
• The similarity between the structure of the
analog 5-Bromouracil, and thymine allows for
the analog to be taken up in place of the normal
base
• The analog can cause an error in base pairing
and increases the occurrence of a mutation,
such as guanine becoming inserted instead of
adenine Acridine—A Chemical Mutagen Radiation as a Mutagen
• UV rays can only affect the skin of humans but
can easily penetrate microorganisms
• X-rays and gamma rays have more energy and
can penetrate deeper into cells
• When these high energy rays strike DNA, they
can cause adjacent pyrimidine bases to bind
with each other forming dimers Thymine dimers caused
by Radiation Thymine Dimer Repair
• Many bacteria and organisms have enzymes
capable of repairing certain kinds of damage to
DNA
• Light repair (photoreactivation)—in bacteria
previously exposed to UV light, occurs in the
presence of visible light; removes the dimers
• Dark repair—can take place in the presence or
absence of visible light, but requires several
enzyme mediated reactions; the defective
segment of DNA is removed and replaced Thymine Dimer Repairs Thymine Dimer Repairs Thymine Dimer Repairs Thymine Dimer Repairs Thymine Dimer Repairs Thymine Dimer Repairs Thymine Dimer Repairs •Sunbathers acquiring thymine dimers by UV radiation
•If not repaired, skin cancer results •UV rays can cause genetic alteration of DNA—
Xeroderm pigmentosum
•Enzymes normally activated to repair UV damaged
DNA, are defective resulting in multiple skin cancers Ozone Biosensors—how deep does UV penetrate into
the ocean and what are the effects on sea life? Luria and Delbruck’s fluctuation test proves mutations
conferring antibiotic resistance occur randomly Replica plating allows for detection of antibiotic
resistance organisms Enterobacter cloacae
from 11,000 year old
mastodon fossil The mastodon’s remains—digestive tract is darker
colored, cylindrical mass bent into shape of intestinal
loops Polymerase Chain
Reaction (PCR) Plates used in Ame’s test for determination of
mutagenicity Methodology of Ame’s test for mutagenic properties of
chemicals
Microbiology Principles and Explorations Chapter 7
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