Transcript 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
Copyright © 2011 Pearson Education Inc.
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)
Copyright © 2011 Pearson Education Inc.
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
Copyright © 2011 Pearson Education Inc.
E. coli genome
1 single looped
chromosome
made of 4,300 genes
1.6 millimeters in length
(800 times the length of the
cell)
Copyright © 2011 Pearson Education Inc.
Figure 7.2
The structure of nucleic acids
Nucleic acid is made of repeating units of nucleotides
Nucleotides:
5 carbon sugar
phosphate
nitrogen base
Copyright © 2011 Pearson Education Inc.
Figure 7.1
The structure of nucleic acids
Copyright © 2011 Pearson Education Inc.
Figure 7.1
The Transfer of Genetic Information
Horizontal Gene Transfer
Copyright © 2011 Pearson Education Inc.
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.
Copyright © 2011 Pearson Education Inc.
Figure 7.4
DNA Replication
Copyright © 2011 Pearson Education Inc.
DNA Replication
Copyright © 2011 Pearson Education Inc.
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
Copyright © 2011 Pearson Education Inc.
DNA Replication
Fig. 8.3
Copyright © 2011 Pearson Education Inc.
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
Copyright © 2011 Pearson Education Inc.
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
Copyright © 2011 Pearson Education Inc.
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
Copyright © 2011 Pearson Education Inc.
The Transfer of Genetic Information
Horizontal Gene Transfer
Copyright © 2011 Pearson Education Inc.
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
Copyright © 2011 Pearson Education Inc.
Copyright © 2011 Pearson Education Inc.
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
Copyright © 2011 Pearson Education Inc.
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)
Copyright © 2011 Pearson Education Inc.
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.
Copyright © 2011 Pearson Education Inc.
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 =
Copyright © 2011 Pearson Education Inc.
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
Copyright © 2011 Pearson Education Inc.
Figure 7.8a
Prokaryotic mRNA can code for several polypeptides
Copyright © 2011 Pearson Education Inc.
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
Copyright © 2011 Pearson Education Inc.
Genetic Code
Universal Code
64 mRNA Codons
code for amino acid
20 Amino Acids
(wobble effect)
1 Start Codon
3 Stop Codons
Copyright © 2011 Pearson Education Inc.
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
Copyright © 2011 Pearson Education Inc.
Figure 7.13a-b
Ribosomal structures
Copyright © 2011 Pearson Education Inc.
Figure 7.14
Assembled ribosome and its tRNA-binding sites
Copyright © 2011 Pearson Education Inc.
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)
Copyright © 2011 Pearson Education Inc.
The initiation of translation in prokaryotes
Copyright © 2011 Pearson Education Inc.
Figure 7.16
The elongation stages of translation
Copyright © 2011 Pearson Education Inc.
Figure 7.17
One prokaryotic mRNA, many ribosomes and polypeptides
Copyright © 2011 Pearson Education Inc.
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
Copyright © 2011 Pearson Education Inc.
Translation
• http://highered.mcgraw-hill.com/olc/dl/120077/micro06.swf
Copyright © 2011 Pearson Education Inc.
Copyright © 2011 Pearson Education Inc.
CLASS EXERCISE
Copyright © 2011 Pearson Education Inc.
5’ AATTATGGGACTTTAATGA (DNA)
Copyright © 2011 Pearson Education Inc.
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
Copyright © 2011 Pearson Education Inc.
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
Copyright © 2011 Pearson Education Inc.
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)
Copyright © 2011 Pearson Education Inc.
Figure 7.19a
Inducible Operon
The lac operon
Inducer -Allolactose which is
formed when cell takes in
lactose
http://youtu.be/h5p05aFzWdA
Copyright © 2011 Pearson Education Inc.
Figure 7.19b
The lac operon, an inducible operon
Copyright © 2011 Pearson Education Inc.
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
Copyright © 2011 Pearson Education Inc.
Figure 7.20a
Repressible Operon
The tryp operon
Activated Repressor binds to Tryptophan
http://youtu.be/1arC3WkQNVQ
Copyright © 2011 Pearson Education Inc.
Figure 7.20b