Download MICR 130 Chapter 8

ninth edition
TORTORA ⏐ FUNKE ⏐ CASE
MICROBIOLOGY
an introduction
Chapter 8
Microbial Genetics
PowerPoint® Lecture Slide Presentation prepared by Christine L. Case!
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Structure and Function of the Genetic Material
§  Genetics – science of heredity
§  Study of what genes are, how they determine the
characteristics of an organism, how they carry
information, how the information is copied, how
information is passed on to subsequent generations
and between organisms
§  Genome – all the genetic information in a cell
§  Includes chromosomes and plasmids
§  Genomics – sequencing, characterization of genomes
Structure and Function of the Genetic Material
§  Chromosomes – a structure that contains the DNA
§  Physically carries the hereditary information, genes
§  Bacteria typically have one circular chromosome
§ Attached to membrane at several points
DNA is twisted and supercoiled
to fit into cell
Chromosome is 1000x longer
than width of cell
Chromosome erupting from
one E. coli cell
Structure and Function of the Genetic Material
§  Genes – segments of DNA that carry information to
produce functional products, proteins
§  Genetic code – the set of rules that determines how a
nucleotide sequence of a gene is converted into the
amino acid sequence of a protein
§  Linear sequence of nucleotides, bases provides the
information for making proteins
§  Much of anabolism is making proteins from DNA
§  When a product is made, the gene is expressed
Structure and Function of the Genetic Material
§  Genotype – an organism’s genetic makeup
§  The information found in its DNA
§  Represents potential characteristics
§  Phenotype – an organism’s expressed properties
§  Eg, an ability to resist an antibiotic
§  Phenotype is the display of an organism’s genotype
§  Genotype is collection of DNA in cell
§  Phenotype is collection of proteins in cell
The Flow of Genetic Information
Genetic information can flow in a number of ways
“Horizontal gene
transfer”
“Vertical gene
transfer”
“DNA replication”
DNA Replication
§  One parental DNA molecule converted into 2 identical
daughter molecule
§  Parental DNA strand acts as template for new strand
§  Structure of DNA
§  Complementary base pairing
§  DNA has direction
§ 3’ (3 prime) and 5’ (5 prime)
§ DNA strands are counter-parallel to each other
DNA has direction
Nucleotides added to 3’ OH
3’ OH required for nucleotide addition
DNA Replication
Structure of DNA
5’ end attached
to PO4
DNA strands run
anti-parallel
5’ à 3’
3’ à 5’
3’ end attached
to OH
DNA Replication
Overview
1 – double stranded
DNA unwound by
helicase enzyme
2 – exposed bases matched up
with bases in cytoplasm
A to T
C to G
3 - DNA polymerase joins
new nucleotides to forming
DNA molecule
4 – each new DNA
molecule contains one
parental strand, one
daughter strand à
semi-conservative
replication
DNA Replication
The Flow of Genetic Information
Genetic information can flow in a number of ways
“Horizontal gene
transfer”
“Vertical gene
transfer”
“DNA replication”
RNA and Protein Synthesis
§  Genetic information from DNA follows the “Central
Dogma of Biology”
§  DNA is used to make RNA, which is used to make
protein
§  DNA à RNA à Protein
§  DNA à RNA = Transcription, RNA synthesis
§  RNA à Protein = Translation, Protein synthesis
RNA and Protein Synthesis
Transcription – synthesis of RNA from DNA
§  Recall, RNA is single stranded, uses U instead of T
§  Three kinds of RNA
§  Ribosomal RNA, rRNA –integral part of ribosomes
§  Transfer RNA, tRNA – involved in protein synthesis
§  Messenger RNA, mRNA – carries information for
making protein
§  mRNA is synthesized from a gene by enzyme called
RNA polymerase
RNA and Protein Synthesis
§  Transcription begins when RNA polymerase binds to
the promoter sequence
§  RNA polymerase joins nucleotides into new mRNA
strand using DNA as template
§  New mRNA complementary to DNA template
§  A à U
§  T à A
§  C à G
§  G à C
RNA and Protein Synthesis
Transcription
2 – mRNA synthesized
by RNA polymerase
3 – RNA
polymerase binds to
DNA at the
promoter
1 – double stranded
DNA unwound by
RNA polymerase
enzyme
RNA and Protein Synthesis
RNA polymerase synthesizes mRNA
using DNA as template
DNA template is read 3’ à 5’
mRNA is made 5’ à 3’
3’
5’
5’
3’
RNA and Protein Synthesis
§  RNA is complementary to DNA
§  If DNA template is CGATAAG
§  What is the RNA strand formed?
§  RNA synthesized is GCUAUUC
RNA and Protein Synthesis
§  Transcription continues until RNA polymerase reaches
site on DNA called the terminator
mRNA, RNA polymerase
“fall off” DNA
5’
mRNA strand
Transcription
RNA and Protein Synthesis
Translation – protein synthesis from mRNA
§  “Translates” the “language” of nucleic acids into
“language” of proteins
§  Codons – groups of three bases used to translate
nucleic acids into amino acids
§  Each codon “codes” for an amino acid
§  Sequence of codons on mRNA determines
sequence of amino acids
§  Codons to amino acids is the genetic code
Translation: The Genetic Code
The Genetic Code
§  Written as mRNA, 5’ to 3’
§  Two types of codons
§  Sense codons – code for amino acids
§  61 codons for 20 amino acids
§  Degeneracy of genetic code – amino
acids coded for by multiple codons
§  Nonsense codons – code for stops in
translation
§  Aka stop codons
RNA and Protein Synthesis
§  Translation starts with AUG start codon
§  Codes for amino acid methionine
§  In Bacteria, proteins start with formylmethionine
§  tRNA carries amino acids to ribosomes
tRNA carries amino acid
on one end …
… and has anticodon at
other end
Anticodon recognizes codon in mRNA
What’s the codon this tRNA recognizes?
RNA and Protein Synthesis
RNA and Protein Synthesis
3 – tRNA binds to
next codon
1 - ribosome binds
at start codon
3’
5’
2 - ribosome moves
along mRNA in 5’ to
3’ direction
RNA and Protein Synthesis
4 – ribosome forms
peptide bond
between amino acids
3’
5’
RNA and Protein Synthesis
5 – translation continues
until ribosome reaches
stop codon
RNA and Protein Synthesis
6 – ribosome breaks up,
mRNA, protein released
Translation
Which of the following is a product of transcription?
1.  RNA polymerase
2.  DNA polymerase
3.  Proteins
4.  mRNA
RN
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Copies RNA from DNA
1.  DNA polymerase
2.  RNA polymerase
3.  tRNA
4.  DNA ligase
5.  DNA helicase
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Is required for protein synthesis
1.  DNA polymerase
2.  RNA polymerase
3.  tRNA
4.  DNA ligase
5.  DNA helicase
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Where does RNA polymerase bind at?
1.  AUG start codon
2.  The promoter
3.  The initiator
4.  Transcriptase
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Which of the following is a product of transcription?
1.  RNA polymerase
2.  DNA polymerase
3.  Proteins
4.  mRNA
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The template strand is the strand that the RNA polymerase uses to produce the
mRNA product. What is the sequence of the start codon on the template strand?
1.  AUG
2.  ATG
3.  UAC
4.  TAC
AC
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U
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Transcription and Translation
Consider the following "Template" strand of DNA AGA GGA TTA TAC TAT TCG TGG CGC AGT ATG TAA ACT TCT 1.  Transcribe this DNA molecule
2.  Translate the mRNA to find the "hidden message" in the protein.
a.  Hint #1 - Start at the first codon, not the first AUG
b.  Hint #2 - Use the first letter of each amino acid to spell out the
message
AGA GGA TTA TAC TAT TCG TGG CGC AGT ATG TAA ACT TCT
AGA GGA TTA TAC TAT TCG TGG CGC AGT ATG TAA ACT TCT
1.  Change 13th base from
“T” to “G”
2.  Change 15th base from
“T” to “A”
3.  Change 26th base from
“G” to “T”
AGA GGA TTA TAC TAT TCG TGG CGC AGT ATG TAA ACT TCT
1.  Change 13th base from
“T” to “G”
2.  Change 15th base from
“T” to “A”
3.  Change 25th base from
“G” to “T”
Mutation: Change in the Genetic Material
§  Mutation – change in the nucleotide, or base, sequence
of DNA
§  Change in nucleotide sequence can cause change in
protein sequence
§  Change in protein sequence can cause change in
protein function
§ Can become less active, more active, bind
different substrate(s), etc…
§  The effect of a mutation depends on type of mutation
Mutation: Change in the Genetic Material
Type of mutations
§  Base substitution (point mutation) – one base is
replaced by a different base
§  May cause change in one amino acid, stop codon
§  Frameshift mutation – one or a few bases are deleted
of inserted (not in multiples of three)
§  Shifts “translational reading frame” of mRNA
§  Causes change in all amino acids after frameshift
§  Almost always results in nonfunctional protein
Mutation: Change in the Genetic Material
Results of mutations
§  Silent mutations - mutations that have no effect
§  Change in base, no change in amino acid
§  Due to degeneracy of genetic code
§  Missense mutations - mutations that result in an amino
acid substitution in protein
§  Nonsense mutations - mutation that introduces
premature stop codon
Mutation: Change in the Genetic Material
Missense mutation
§  Mutated DNA molecule, makes
§  Mutated mRNA with codons AUG,
AAG, UUU, AGC, UAA, makes
§  Mutated protein Met-Lys-Phe-Ser
§  “Normal” DNA molecule, makes
§  “Normal” mRNA with codons AUG,
AAG, UUU, GGC, UAA, makes
§  “Normal” protein Met-Lys-Phe-Gly
Mutation: Change in the Genetic Material
Nonsense mutation
§  Mutated DNA molecule, makes
§  Mutated mRNA with codons AUG,
UAG, UUU, GGC, UAA, makes
§  Mutated short protein Met
§  “Normal” DNA molecule, makes
§  “Normal” mRNA with codons AUG,
AAG, UUU, GGC, UAA, makes
§  “Normal” protein Met-Lys-Phe-Gly
Mutation: Change in the Genetic Material
Frameshift mutations
Result in dramatic changes to protein
§  Mutated DNA molecule, makes
§  Mutated mRNA with codons AUG,
AAG, UUG, GCU, AA…, makes
§  Mutated protein Met-Lys-Leu-Ser-…
§  “Normal” DNA molecule, makes
§  “Normal” mRNA with codons AUG,
AAG, UUU, GGC, UAA, makes
§  “Normal” protein Met-Lys-Phe-Gly
Mutations
Mutagens
§  Spontaneous mutations – occur due to occasional
errors in DNA replication
§  Mutagens – environmental agents that cause mutations
§  May be physical or chemical
§  Any agent that interacts with DNA is potential mutagen
Mutagens
§  Chemical mutagens – interact with DNA to cause
improper base pairing, deletions, insertions
§  Analogs of DNA look like bases
§  But don’t base pair properly
§  HNO2 alters A
§  “A” binds with C instead of T
Mutagens
Radiation
§  Ionizing radiation – X-rays, gamma rays
§  Ionize molecules
§  Cause structural damage to DNA, leads to errors in
DNA replication
Mutations
A mutation is ____.
a.
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1.  a change in the
sequence in DNA
2.  a change in the
nucleotide in mRNA
3.  a change in the
sequence in tRNA
4.  a change in the
amino acid in
protein
Which type of mutation is the most damaging?
1.  Silent mutation
2.  Missense mutation
3.  Frameshift
mutation
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Genetic Transfer and Recombination
§  Genetic recombination – exchange of genes between 2
DNA molecules
§  Contributes to genetic diversity of population
§  In eukaryotes, genetic recombination happens regularly
as part of sexual cycle
§  Chromosomes are close together in nucleus
§  Recombination within one organism
§  In prokaryotes, transfer of genes happens by:
§  Vertical gene transfer
§  Horizontal gene transfer (recombination between 2
organisms)
Genetic Transfer and Recombination
Horizontal gene transfer –
genes passed between cells of
same generation
Vertical gene transfer –
transfer of genes from
parent to offspring
(daughter cells)
Genetic Transfer and Recombination
Horizontal gene transfer
§  Donor transfers part of its genome to recipient cell
§  Recipient can incorporate (recombine) part of donor
DNA
§  Rest is degraded for use as building blocks
§  Recipient that incorporates DNA is called recombinant
§  Rare event, occurs in less than 1% of population
§  Three mechanisms of horizontal gene transfer
§  Transformation, conjugation, transduction
Genetic Transfer and Recombination
Transformation – transfer of “naked” DNA
“Naked” DNA in
environment
Transformation
– uptake of DNA
Recipient cell
Recombination –
integration of
DNA into
genome
Cell that recombines
foreign DNA is a
recombinant cell
Genetic Transfer and Recombination
§  Griffiths experiment (1928) demonstrated transformation
2 - Nonencapsulated bacteria
don’t cause disease, mice live
when injected
3 – Dead encapsulated
bacteria injected into
mouse, mouse lives
4 – Dead encapsulated bacteria
and live nonencapsulated
bacteria injected, mouse dies
1 - Encapsulated
bacteria cause
disease, kill mice
when injected
1 - Many encapsulated
colonies isolated from
dead mouse
2 – Few nonencapsulated
3 – No colonies
colonies isolated, most
isolated
bacteria killed in mouse
Only encapsulated bacteria
cause disease
4 – Many encapsulated
colonies isolated from mouse,
“capsule” DNA incorporated
by nonencapsulated bacteria
Genetic Transfer and Recombination
§  In nature, some bacteria release DNA into environment
§  After cell death, cell lysis
§  Some bacteria can take up this DNA via transformation
§  Occurs naturally in Bacillus, Haemophilus, Neisseria,
Acinetobacter
§  Competence – physiological state in which recipient cell
can take up DNA via transformation
§  ie, Haemophilus can take up DNA only when
competent
Transformation
Genetic Transfer and Recombination
Transduction – phage mediated DNA transfer
§  DNA transferred as a part of bacteriophage infection
§  Two types of transduction
§  Generalized transduction - phage mediated transfer
of random segments of DNA
§  Specialized transduction - phage mediated transfer
of specific genes
§ ie, some phages transfer only toxin genes
Genetic Transfer and Recombination
Generalized
transduction
1 - Phage attaches to
donor cell, injects DNA
3 – Phage particles
assembled. Some phages
mistakenly package
bacterial DNA
2 – Phage DNA and proteins
made, bacterial chromosome
broken up
4 – Improperly packaged
phages transfer bacterial
DNA, not phage DNA
5 – Transferred
DNA can recombine
into recipient cell
Transduction
Plasmids
§  Plasmids are self-replicating circular molecules of DNA
§  Small, about 1-5% of genome
§  Often carry genes that are not essential for survival
§  Conjugative plasmids - carry genes for plasmid transfer
§  Conjugation
Genetic Transfer and Recombination
Conjugation – plasmid dependent DNA transfer
§  Requires cell to cell contact
§  Conjugating cells must be opposite “mating types”
§  Donor cell must carry plasmid
§  Recipient cell must not carry plasmid
§  “Bacterial sex”
Conjugation
Pilus
Genetic Transfer and Recombination
The F factor (fertility factor)
§  Conjugation requires contact between donor (F+) cell
and recipient (F-) cell
§  Genes for conjugation on F plasmid
Transfer of plasmid is
through a “sex pilus”
F+
F-
F+
F factor
Plasmid replicates
during transfer
F+
Recipient cell now F+
Genetic Transfer and Recombination
§  Sometimes F factor integrates into chromosome
§  F+ cell becomes Hfr (high frequency of recombination)
F+
F+
Hfr
Genetic Transfer and Recombination
§  Conjugation between Hfr and F- transfers part of donor
chromosome
§  Recipient is recombinant, but still F§  Why?
Hfr
F-
Only a part of F plasmid is transferred
Part of donor
chromosome
Hfr
Recombinant
F-
Conjugation
Plasmids
§  Conjugative plasmids - carry genes for plasmid transfer
§  Dissimilation plasmids - carry genes crucial for survival
§  Catabolism of unusual compounds
§  Toxins, bacteriocins (toxins that kill bacteria)
§  Enhance pathogenicity
§  R factors - provide antibiotic resistance
Which method of DNA transfer involves a plasmid?
1.  Transformation
2.  Conjugation
3.  Transduction
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Which of the following is not true of a bacterium that is R+?
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1.  R+ refers to the
possession of a
plasmid.
2.  R+ can be
transferred to a
recipient cell.
3.  It is resistant to
certain drugs.
4.  It is also F+
Transduction is _____.
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1.  transfer of genes
from parent to
offspring
2.  transfer of genes
due to a
bacteriophage
infection
3.  uptake of "naked"
DNA from
environment
4.  carried out by DNA
polymerase
When conjugation occurs between a Hfr donor and a F- recipient, which of the
following describes the most likely outcome of the recipient cell?
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1.  Conversion to an F
+ cell.
2.  Cell remains F- with
some chromosomal
DNA from donor.
3.  Cell remains F- with
no change in
genotype.
4.  Cell remains F- with
gain of F plasmid.
In transduction, the recipient is _____.
1.  a dead cell
2.  a cell infected by a
virus
3.  a phage
4.  a cell without a
plasmid
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In transformation, what is the donor cell?
1.  A virus infected cell
2.  A dead cell
3.  A competent cell
4.  A cell that contains a
plasmid
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5.  A cell without a