Chapter 6Microbial Genetics

Structure and Function of Genetic Materials

DNA & RNADNA=deoxyribonucleic acidRNA=ribonucleic acidBasic building blocks:

NucleotidesPhosphate groupPentose sugarNitrogenous base

Structure of DNA

Double 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 Replication

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

4 million base pairs1 mm long (over 1000 times larger that

actual bacterial cell. How it can be put into a 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 templateLeading 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

1 解旋酶打开DNA双螺旋

Protein Synthesis

DNA------- mRNA------ protein transcription translation

Central Dogma( 中心法则 ) 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 ssRNA sugar is riboseBase pairing-A-U

The structure of a bacterial gene

ATG TAA

transcript

Coding sequence

Transcription start site

PromoterRibosome binding site/translational start (ATG)

hisG

I. Single gene transcript

Translational end

Transcriptional terminator

ATG TAA

transcript

Transcription start site

hisG

Translational end

hisH

Multigene bacterial operon

-One promoter, one transcriptional stop; multiple translational starts and stops

TAA

Transcription

Types of RNA

Three types:mRNA: messenger RNA

Contains 3 bases ( codon)rRNA: ribosomal RNA

Comprises the 70 S ribosometRNA: transfer RNA

Transfers amino acids to ribosomes for protein synthesis

Contains 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 Code

Codons: code for the production of a specific amino acid

20 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)

Translation

Translation

Translation

Translation

MutationsChanges in base sequence of

DNA/lethal and inheritableCan be:

HarmfulLethalHelpfulSilent

Normal DNA/Missense Mutation

Nonsense Mutation/Frameshift

Mutation

Genetic Transfer in Bacteria

Genetic transfer results in genetic variation

Genetic variation is needed for evolutionThree 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

Generalized Transduction

Release of phage

Phage replication and degradation of host DNA Assembly of phages particles

Infection of recipient Homologous recombination

Infection of Donor

Potentially any donor gene can be transferred

Transformation

RecombinationLegitimate, homologous or general recA, recB and recC genes

• Significance– Phase variation in Neiseseria– Recombinant DNA technology

StepsUptake of DNA

Gram +Gram -

Transformation

Conjugation in E. coli

Conjugation continued…

Conjugation continued…