Download Microbial Genetics

Microbial Genetics
What is the genetic material?

DNA

Nucleotide base pairs


Chromosomes



A-T, C-G
Bacteria: circular
Chromatin
Genetics



Genes
Genome
Genetic code
What is the purpose of DNA?


Recipe for making _____________
Genotype vs phenotype
What’s special about bacterial DNA?


Circular
Attached to PM at various pts
How is DNA copied for replication?


Recall DNA
structure
Replication
Overview




DNA helicase
unwinds double
helix
DNA polymerase
III copies at
replication fork
 Replication is
5’ to 3’
Ligase “glues”
nucleotides
Animation
What is semiconservative replication?
What are the
specifics of DNA
replication?




DNA strands are
antiparallel
Bidirectional replication
animation
Rolling circle animation
Replication always starts at
new 5’ end


Leading strand
Lagging strand
 Okazaki fragments
(started with an RNA
primer)

Error rate: 1 in 1010
Protein Synthesis
How does DNA direct protein synthesis?

Via RNA


Three types of RNA
 tRNA, rRNA, mRNA
Overview:


mRNA is copy of DNA gene
 Created by transcription
Protein made during translation
 Ribosome “reads” triplet genetic code
 tRNA delivers appropriate amino acid
What happens in transcription?

mRNA created


Often translation
occurs while
transcription
happens
Base pairing




No T in RNA,
instead U
Begins with RNA
polymerase
attaching to
promoter region of
coding strand
Stops when
reaches terminator
region
transcription
process
What happens in translation?

Ribosome attaches to mRNA
 Reads codons
 Code is redundant
(degenerate)
 20 aas, but 61 codons
 3 stop codons (nonsense
codes)
 Start codon (______) in
bacteria codes for
formylmethione
 tRNA brings in appropriate aa
 Matches to tRNA anticodon
How does the ribosome “read” the mRNA?

Ribosome finds
start codon


30S attaches, then
50S
First tRNA to P site



Second to A site
First tRNA transfers
aa to aa on A site
tRNA
Ribosome shifts




Moves 5’  3’
New tRNA into now
open A site
Process repeats
translation
How many ribosomes can work at once?
Is it the same process in eukaryotes?


Pretty much
Exception:


DNA is inside
nucleus
Posttranscriptional
modifications
5’ cap
 3’ poly-A tail
 Introns
removed

Control of Genes, Mutations
and DNA Recombination
What controls gene
expression?

Majority of genes are constitutive


Repression



Inhibit gene expression
Repressors: proteins that repress
Induction



Protein produced at constant rate
Turning on transcription
Inducer: substance that induces
Lac operon model demonstrates these two processes
What is induction?

Induction



Turning on
transcription
Inducer: substance
that induces
Lac operon model
demonstrates these
two processes
What is repression?

Repression


Inhibit gene expression
Repressors: proteins that
repress
What is the lac
operon model?

Study of E. coli


Inducible system when
lactose is present
Three genes for lactose
consumption


next to each other on
chromosome
These are structural genes

DNA nearby is control
region





Includes promoter and
operator
Together these are the
operon
Lac operon = 3 lac genes +
operon region
General regulation
animation
Animation
Regulation of Gene Expression
Figure 8.13
What happens if the DNA code is wrong?

Called a mutation

Causes change to
mRNA sequence
which can affect
translation and thus
___________
Spontaneous
 Induced
 Excision repair

Mutation

Nonsense mutation

Results in a nonsense
codon
Figure 8.16a, c
Mutation

Frameshift mutation

Insertion or deletion of
one or more nucleotide
pairs
Figure 8.16a, d
What types of mutation are there?

Original:


Point mutation:



THE BIG FLY HAD ONE RED EYE.
THE BIT FLY HAD ONE RED EYE.
Frame shift mutation:
Addition:

Deletion of G in BIG:




THE BIF LYH ADO NER EDE YE.
Which is more dangerous?


THE BIT GFL YHA DON ERE DEY E.
mutations movie
Spontaneous mutations
Mutagens

Can affect pathogeneticity
What can be a mutagen?

Chemicals


Nitrous acid
Nucleoside analog
 Similar to normal
nitrogenous base
 Causes
mismatching of
base pairs


AZT
(azidothymidine)
does this
Radiation

Formation of thymine
dimers
 Light-repair
enzymes
(photolyases)
 animation
Mutagens vs Carcinogens: what the difference?



Mutagens
Carcinogens
Ames test


Identifies possible carcinogens by identifying mutagens
Looks to see how many mutate Salmonella cells revert to a
nonmutant form
How can bacteria pass DNA?

Genetic
recombination



Vertical gene transfer


Exchange of genes
between
chromosomes
Gives new
combinations
Parent to offspring
Horizontal transfer

3 types…
What is conjugation?

Horizontal gene transfer (1%
of population)

Donor bacterial cell gives
DNA to recipient cell

Recipient now has
recombinant DNA

Conjugation

Process in E. coli



Donor is F+, recipient FF= fertility factor
Hfr cell (high frequency of
recombination
When F factors integrate
into chromosome
Conjugation-plasmid
Conjugation-chromosome



What is transduction?



Virus transfer DNA
general vs. specialized
animation
What is
transformation?


Gene transferred to recipient bacterium
Griffith (1928)

Studied Streptococcus pneumoniae


Two strains: one virulent, one not
Transformation animation
How could this
happen???!!
What are
transposons?


Jumping
genes
Can be
transferred to
other cells