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

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

DNA mRNA Protein

Transcription 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’

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

mRNA

tRNA

Triplet code

codons

anticodons

Genetic Code

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

Mutations can be: 1. Harmful 2. Lethal 3. Beneficial 4. Silent (neutral)

• UUU - Phe• UUC - Phe

Types of Mutations

Normal DNA

Base Substitution MutationC

Missense Mutation

T

Nonsense Mutation

Base Substitution 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