BIOL 2041 - Ch. 8 Microbial Genetics

central dogma

DNA → RNA → protein

DNA structure

double helix, backbone of deoxyribose phosphate, two strands of nucleotides, anti parallel

DNA gyrase

relaxes supercoiling

DNA ligase

makes covalent bonds to join DNA strands, okazaki fragments

DNA polymerase

adds nucleotides to growing strand in the 5’→3’ direction, initiated by RNA primer, lagging strand creates okazaki fragments, removed RNA primers

helicase

unwinds double strand

rRNA

ribosomal, integral part of ribosomes

tRNA

transfer, transports amino acids during protein synthesis

mRNA

messenger, carries coded info from DNA to ribosomes

transcription in prokaryotes

synthesis of complementary mRNA strand from DNA template

Transcription process

RNA polymerase binds to promoter sequence on DNA, proceeds in 5’→3’, only one of the two DNA strands is transcribed, stops when reaches terminator sequence

translation process

61 sense codons encode the 20 amino acids, starts with start codon AUG ends at nonsense codons, tRNA transports required amino acids, tRNA have anticodon that base pairs with the codon, amino acids joined by peptide bonds

translation

mRNA is decoded to build proteins, occurs in ribosomes

repression

inhibits one gene expression and decreases enzyme synthesis, mediated by repressors that block transcription, default on

induction

turns on gene expression, initiated by inducer, default off

promoter

segment of DNA where RNA polymerase initiates transcription of structural genes

operator

segment of DNA controlling transcription of structural genes

operon

set of operator and promoter sites and the structural genes they control

inducible/lac operon

structural genes not transcribed unless and inducer is present, in absence of lactose the repress or binds to the operator and prevents transcription, in presence of lactose allolactose (inducer) binds to the repress or and transcription occurs

inducer in lac operon

allolactose

trp operon

excess tryptophan is a corepressor that binds and activates repressor to bind to operator, stopping tryptophan synthesis

spontaneous mutations occur when

absence of a mutagen

mutagen

agents that cause mutations

base substitution mutation

change in one base

missense mutation

base substitution results in change in an amino acid

nonsense mutation

base substitution results in a nonsense/stop codon

frame shift mutation

insertion or deletion of one or more nucleotide pairs

chemical mutagens

mutated DNA by directly or indirectly altering structure or sequence

nitrous acid

chemical mutagen, causes adenine to bind to cytosine

nucleoside analog

chemical mutagen, incorporates into DNA in place of a normal base

photolyase

an enzyme in bacteria that separate thymine dimers and fix mutations

nucleotide excision repair

enzymes cut out incorrect bases and fill in correct ones

positive/direct selection

detects mutant cells bc they grow or appear different

negative/indirect selection

detects mutant cells that can’t grow or perform a certain function

auxotroph

mutant that has a nutritional requirement absent in the parent

genetic recombination

exchange of genes between two DNA molecules; creates genetic diversity

crossing over

two chromosomes break and rejoin, results in insertion of foreign DNA into chromosome

vertical gene transfer

organism to offspring

horizontal gene transfer

organism to organism of same generation

transformation in bacteria

DNA fragments from donor cell is taken p by recipient, donor DNA aligns with complementary bases, recombination occurs

plasmids

self replication circular pieces of DNA

conjugative/fertility plasmid

carried genes for sex pili

resistance plasmids (r factors)

encode antibiotic resistance

conjugation in bacteria

plasmids transferred from one bacterium to another via sex pili

transposons

segments of DNA that can move from one region of DNA to another, contain insertion sequences that code for transposase

transposase

cuts and reseals DNA

transduction in bacteria

DNA transferred from one bacterium to another by a virus