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Chapter 6 Microbial Genetics. Structure and Function of Genetic Materials. 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 - PowerPoint PPT Presentation
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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…
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