Download Chapter 7 Microbial Genetics

MICROBIOLOGY
WITH DISEASES BY TAXONOMY, THIRD EDITION
Chapter 7
Microbial Genetics
Lecture prepared by Mindy Miller-Kittrell, University of Tennessee, Knoxville
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The Structure and Replication of Genomes
• Genetics
– Study of inheritance and inheritable traits as expressed
in an organism’s genetic material
• Genome
– The entire genetic complement of an organism
– Includes its genes and nucleotide sequences
 Gene - Segment of DNA:
 Gene codes for a functional product (usually a protein or
regulation site)
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The Location of Genes
Chromosome –Genes essential for survival
Plasmid – Extra chromosomal DNA that replicate independently. Not
essential for normal bacterial metabolism, growth, or reproduction. Can
confer survival advantages
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E. coli genome
1 single looped
chromosome
made of 4,300 genes
1.6 millimeters in length
(800 times the length of the
cell)
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Figure 7.2
The structure of nucleic acids
Nucleic acid is made of repeating units of nucleotides
Nucleotides:
5 carbon sugar
phosphate
nitrogen base
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Figure 7.1
The structure of nucleic acids
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Figure 7.1
The Transfer of Genetic Information
Horizontal Gene Transfer
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DNA Replication
Protein Synthesis
Figure 8.2
DNA Replication
• Bacteria must replicate their DNA as the first step in binary
fission
• Each strand of nucleotides serves as a template for a
complimentary new strand.
• The process is semiconservative because each new
double helix is composed of an old strand of nucleotides
from the parent molecule and one newly-formed strand.
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Figure 7.4
DNA Replication
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DNA Replication
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DNA Replication
• DNA replicates only in a 5′ to
3′ direction
• Since strands are antiparallel,
new strands are synthesized
differently
– Leading strand synthesized
continuously
– Lagging strand synthesized
discontinuously
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DNA Replication
Fig. 8.3
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Leading Strand Synthesis
1.
2.
3.
Helicase unwinds DNA and creates replication fork
Stabilizing Proteins bind to prevent reannealing
RNA polymerase (Primase) synthesizes short RNA
sequences called primers, which serve as starting
points for DNA synthesis
4.
DNA polymerase III binds and adds nucleotides to
hydroxyl group at 3′ end of nucleic acid
5.
DNA polymerase I replaces RNA primer with DNA
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Lagging Strand Synthesis
1.
RNA polymerase (Primase) synthesizes short RNA
sequences called primers, which serve as starting points for
DNA synthesis
2.
DNA polymerase III binds and adds nucleotides to
hydroxyl group at 3′ end of nucleic acid
3.
Okazaki Fragments short, newly synthesized DNA
fragments
4.
5.
DNA polymerase I replaces RNA primer with DNA
Ligase seals gap between Okazaki Fragments
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Lagging Strand Synthesis
http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/00724373
16/120076/micro04.swf::DNA%20Replication%20Fork
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The Transfer of Genetic Information
Horizontal Gene Transfer
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DNA Replication
Protein Synthesis
Figure 8.2
Gene Expression
• Gene expression occurs when gene activity leads to a protein
product in the cell (Protein Synthesis).
• A gene does not directly control protein synthesis; instead, it passes
its genetic information on to RNA, which is more directly involved in
protein synthesis.
• Difference between DNA and RNA
• RNA is single-stranded;
• is composed of the sugar ribose;
• substitutes uracil for thymine
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RNA Types
• Three types of RNA are involved in gene expression:
– messenger RNA (mRNA) carries genetic information to
the ribosomes
– ribosomal RNA (rRNA) is found in the ribosomes
– transfer RNA (tRNA) transfers amino acids to the
ribosomes, where the protein product is synthesized
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Protein Synthesis
• Central dogma of genetics
– explains of the flow of genetic information
Steps:
1. Transcription = DNA transcribed to RNA
2. Translation = RNA translated to form polypeptides (proteins)
Transcription
DNA triplets
(genotype)
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Translation
RNA codons
(phenotype)
Protein
Transcription
• Process of making mRNA from a DNA template by complementary base
pairing.
• Only the section of DNA that codes for a needed protein is copied.
• Coding strand: strand of DNA that codes for a protein; the strand that
is the same as mRNA except for the substitution of bases - uracil for
thymine.
• Template strand: complimentary to coding strand; this is the strand that
is transcribed to make mRNA.
• RNA polymerase attaches to a site called the promoter. This is what
determines which strand of DNA is copied.
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Transcription
Coding Strand
A–T–G–T–T–G–A–C
Template Strand T – A – C – A – A – C – T – G
mRNA
A – U – G –U – U – G – A – C
Coding strand =
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Transcription

Promoter– specific nucleotide sequence at beginning of a
gene; tells the RNA polymerase where to start transcription



RNA Polymerase binds to promoter by sigma factor that
unwinds DNA
begins reading the DNA sequence on the template strand
from 3’ to 5’;
makes mRNA in 5’ to 3’ direction
http://www.youtube.com/watch?v=ztPkv7wc3yU
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Figure 7.8a
Prokaryotic mRNA can code for several polypeptides
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Figure 7.12
Translation
• Process of translating the DNA code from mRNA into an
enzyme or other protein.
• Occurs on the ribosomes.
• Ribosomes move along the mRNA, reading the genetic code
three bases at a time which is a codon
• Each mRNA codon codes for a specific amino acid
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Genetic Code
Universal Code
64 mRNA Codons
code for amino acid
20 Amino Acids
(wobble effect)
1 Start Codon
3 Stop Codons
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Figure 8.9
Transfer RNA
 Short (about 75 nucleotides) piece
of folded RNA containing 3 loops
 tRNA anticodon is found on the
bottom loop of the molecule and is
complementary to the mRNA codon
 tRNA has an acceptor stem specific
for an amino acid
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Figure 7.13a-b
Ribosomal structures
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Figure 7.14
Assembled ribosome and its tRNA-binding sites
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Figure 7.15
Gene Function
• Translation
– Three stages of translation
– Initiation
– Elongation
– Termination
– All stages require additional protein factors
– Initiation and elongation require energy (GTP)
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The initiation of translation in prokaryotes
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Figure 7.16
The elongation stages of translation
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Figure 7.17
One prokaryotic mRNA, many ribosomes and polypeptides
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Figure 7.18
Gene Function
• Translation
– Stages of translation
– Termination
– Release factors recognize stop codons
– Modify ribosome to activate ribozymes
– Ribosome dissociates into subunits
– Polypeptides released at termination may function alone
or together
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Translation
• http://highered.mcgraw-hill.com/olc/dl/120077/micro06.swf
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CLASS EXERCISE
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5’ AATTATGGGACTTTAATGA (DNA)
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Regulation of “Gene Expression”
• 75% of genes are expressed constantly
• 25% of genes are regulated by inhibiting translation or
transcription to conserve energy
• Operons – a group of genes that work together
– Inducible operons– gene usually not transcribed (off);
must be turned on by a substance
– Repressible operons– gene always transcribed (on);
must be turned off
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Operon Model
• Regulatory Gene
•
codes for a repressor protein that controls process of transcription
• Promoter
•
DNA segment where RNA polymerase attaches to begin transcription
• Operator
• DNA segment where the repressor protein binds to prevent the attachment of the
promoter gene
• Structural genes
• code for specific proteins
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Inducible Operon
The lac operon
Inducer - substances that initiates transcription, i.e. turns on gene
Inducible enzymes – enzymes synthesized in the presence of an
inducer (ex. B-galactosidase, permease, transacetylase)
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Figure 7.19a
Inducible Operon
The lac operon
Inducer -Allolactose which is
formed when cell takes in
lactose
http://youtu.be/h5p05aFzWdA
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Figure 7.19b
The lac operon, an inducible operon
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Figure 7.20
Repressible Operon
The tryp operon

Repressors - block the ability of RNA polymerase to initiate
transcription; usually inactive until activator is present.
 Used when enzymes are not needed – (ex.enzymes produced
to synthesize amino acid tryptophan)
 Tryptophan in environment represses gene expression of
enzymes
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Figure 7.20a
Repressible Operon
The tryp operon
Activated Repressor binds to Tryptophan
http://youtu.be/1arC3WkQNVQ
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Figure 7.20b