Download Microbial Genetics

Microbial Genetics
Chapter 8
Structure and Function of
Genetic Material

DNA & RNA

DNA
• deoxyribonucleic acid

RNA
• ribonucleic acid

Nucleotides
• Phosphate group
• Pentose sugar
• Nitrogenous base
Structure of DNA

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Double stranded (double helix)
polymers of nucleotides
5’ to 3’ (strands are anti-parallel)
Complimentary Base Pairing
• A-T
• G-C
DNA Replication


Bacteria have only 1 chromosome (ccDNA)
E. coli
• about 4 million base pairs
• 1 mm long (1000 times longer than the cell)
• DNA takes up only about 10 % of cells volume

E. coli
DNA
1u
1,360u
1ft.
1,360 ft.
DNA Replication occurs at the
Replication Fork (5’ to 3’)


DNA Helicase
1. Leading Strand ( 5’ to 3’ )
• DNA Polymerase

2. Lagging Strand ( 5’ to 3’)
• RNA Polymerase (RNA Primer)
• DNA Polymerase (extends primer and digests
RNA)
• DNA Ligase
• Okazaki Fragments
Bidirectional replication

Origin of replication

Termination site
Protein Synthesis

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
DNA
Transcription
mRNA
Protein
Translation
Central Dogma of Molecular Genetics
Transcription

One strand of DNA is used as a template to
form a complimentary strand of mRNA

mRNA ----> messenger RNA
Transcription

3 ways RNA differs from DNA

Promoter
RNA Polymerase
Termination Site
5’ to 3’

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3 Types of RNA

1. mRNA
• Contains the codons

2. rRNA
• Ribosomes (70S)

3. tRNA
• Transfer amino acids to the ribosomes for
protein synthesis
• Anti-codon
Genetic Code

DNA

Triplet code

mRNA

codons

tRNA

anticodons
Genetic Code

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Codons code for a specific amino acid
20 amino acids
3 base code - 4 bases ( A,U,G,C )
64 possible combinations ( 43)
Amino acids are coded for by more than
one codon
Genetic Code is Degenerative
Genetic Code is Universal
Translation

rRNA
• ribosomal RNA (forms the ribosomes) (70S)

tRNA
• transfer RNA (transfers amino acids to the
ribosomes for protein synthesis)
• anticodon
• 3 base sequence that is complimentary to the codon
on mRNA
Regulation of Gene Expression

All Genes are not always being expressed

Genes turned on all the time - Constitutive

Other genes can be regulated:
• Turned On
• Turned Off
lac operon

Lac - Lactose (dissacharide)
• Glucose and Galactose

operon - series of structural genes all under
the control of a Regulatory Gene

lac operon is normally turned off

lac operon is an Inducible operon
Tryptophan operon


Tryptophan - amino acid
operon - series of structural genes all under
the control of a Regulatory Gene

Tryptophan operon is normally turned on

Tryptophan operon is a repressible operon
Mutations - a change in the base
sequence of DNA
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Mutations can be:
1. Harmful
2. Lethal
3. Beneficial
4. Silent (neutral)
• UUU - Phe
• UUC - Phe
Types of Mutations
Normal DNA
Base Substitution Mutation
C
Missense Mutation
Base Substitution Mutation
T
Nonsense Mutation
Frame Shift Mutation

ATG CAT GCA TGC ATT TCC TGC TTA AAA

1. Addition Mutation

AAT GCA TGC ATG CAT TTT CCT GCT TAA

Reading Frame is Shifted

2. Deletion Mutation

TGC ATG CAT GCA TTT CCT GCT TAA

Reading Frame is Shifted
Genetic Transfer in Bacteria

Genetic Transfer - results in Genetic Variation

Genetic Variation - is needed for Evolution

3 Ways Genetic Transfer can occur in Bacteria
• 1. Transformation
• 2. Cunjugation
• 3. Transduction
Transformation

Genes are transferred from one bacterium to
another as “naked” DNA

Frederick Griffith (1928)
Griffith’s Experiment

2 Strains of Streptococcus pneumoniae
• 1. Virulent strain with a capsule - Pneumonia
• 2. Avirulent strain without a capsule - no disease
Griffith’s Experiment
Griffith’s Experiment
Griffith’s Experiment
Griffith’s Experiment
Conjugation
One bacterium passes some DNA (in a plasmid) to
another bacterium
Transduction

DNA is transferred from one bacterium to
another by a virus

Bacteriophage
• virus that only infects bacteria