Chapter 8Microbial Genetics

Biology 1009Microbiology

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 DNADouble stranded (double helix)Chains of nucleotides5’ to 3’ (strands are anti-parallel)Complimentary base pairing

A-TG-C

DNA Structure

Phosphate-PSugar-blueBases-ATGC

DNA ReplicationBacteria have closed, circular DNAGenome: genetic material in an organismE. coli

4 million base pairs1 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------" proteintranscription translation

Central Dogmaof Molecular Genetics

TranscriptionOne strand of DNA used as a template to make a complimentary strand of mRNAPromoter/RNA polymerase/termination site/5’ to 3’Ways in which RNA & DNA differ:

RNA is ssRNA sugar is riboseBase pairing-A-U

Transcription

Types of RNAThree types:

mRNA: messenger RNAContains 3 bases ( codon)

rRNA: ribosomal RNAComprises the 70 S ribosome

tRNA: transfer RNATransfers amino acids to ribosomes for protein synthesisContains the anticodon (3 base sequence that is complimentary to codon on mRNA)

Genetic CodeDNA: triplet code

mRNA: codon (complimentary to triplet code of DNA)

tRNA: anticodon (complimentary to codon)

Genetic CodeCodons: code for the production of a specific amino acid20 amino acids3 base codeDegenerative: more than 1 codon codes for an amino acidUniversal: in all living organisms

Genetic Code

TranslationThree parts:

Initiation-start codon (AUG)Elongation-ribosome moves along mRNATermination: 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)

MutationsChanges in base sequence of DNA/lethal and inheritableCan be:

HarmfulLethalHelpfulSilent

Normal DNA/Missense Mutation

Nonsense Mutation/Frameshift Mutation

Genetic Transfer in BacteriaGenetic transfer-results in genetic variationGenetic variation-needed for evolutionThree ways:

Transformation: genes transferred from one bacterium to another as “naked” DNAConjugation: 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…