Chapter 8
Microbial Genetics
Biology 1009
Microbiology
Johnson-Summer 2003
Structure and Function of
Genetic Material
 DNA
& RNA
 DNA=deoxyribonucleic acid
 RNA=ribonucleic acid
 Basic building blocks:
 Nucleotides
 Phosphate group
 Pentose sugar
 Nitrogenous base
Structure of DNA
Double stranded (double helix)
Chains of nucleotides
5’ to 3’ (strands are anti-parallel)
Complimentary base pairing
A-T
G-C
DNA Structure
Phosphate-P
Sugar-blue
Bases-ATGC
DNA Replication
Bacteria have closed, circular DNA
Genome: genetic material in an
organism
E. coli
4 million base pairs
1 mm long (over 1000 times larger that
actual bacterial cell)
DNA takes up around 10% of cell volume
DNA Replication-occurs at the
replication fork
5’ to 3 ‘
DNA helicase-unzips + parental DNA strand
that is used as a template
Leading stand (5’ to 3’-continuous)
*DNA polymerase-joins growing DNA strand after
nucleotides are aligned (complimentary)
Lagging strand (5’ to 3’-not continuous)
*RNA polymerase (makes short RNA primer)
*DNA polymerase (extends RNA primer then digests RNA
primer and replaces it with DNA)
*DNA ligase (seals Okazaki fragments-the newly formed
DNA fragments)
Replication Fork
Protein Synthesis
DNA------- mRNA------ protein
transcription
translation
Central Dogma
of Molecular Genetics
Transcription
One strand of DNA used as a template to
make a complimentary strand of mRNA
Promoter/RNA polymerase/termination site/5’
to 3’
Ways in which RNA & DNA differ:
RNA is ss
RNA sugar is ribose
Base pairing-A-U
Transcription
Types of RNA
Three types:
mRNA: messenger RNA
Contains 3 bases ( codon)
rRNA: ribosomal RNA
Comprises the 70 S ribosome
tRNA: transfer RNA
Transfers amino acids to ribosomes for protein synthesis
Contains the anticodon (3 base sequence that is
complimentary to codon on mRNA)
Genetic Code
DNA: triplet code
mRNA: codon (complimentary to triplet
code of DNA)
tRNA: anticodon (complimentary to
codon)
Genetic Code
Codons: code for the production of a
specific amino acid
20 amino acids
3 base code
Degenerative: more than 1 codon
codes for an amino acid
Universal: in all living organisms
Genetic Code
Translation
Three parts:
Initiation-start codon (AUG)
Elongation-ribosome moves along mRNA
Termination: stop codon reached/polypeptide
released and new protein forms
rRNA=subunits that form the 70 S ribosomes
(protein synthesis occurs here)
tRNA=transfers amino acids to ribosomes for
protein synthesis)
Mutations
Changes in base sequence of
DNA/lethal and inheritable
Can be:
Harmful
Lethal
Helpful
Silent
Normal DNA/Missense
Mutation
Nonsense Mutation/Frameshift
Mutation
Genetic Transfer in Bacteria
Genetic transfer-results in genetic variation
Genetic variation-needed for evolution
Three ways:
Transformation: genes transferred from one
bacterium to another as “naked” DNA
Conjugation: plasmids transferred 1 bacteria to
another via a pilus
Transduction: DNA transferred from 1 bacteria to
another by a virus
Transduction by a
Bacteriophage
Transformation
Conjugation in E. coli
Conjugation continued…
Conjugation continued…