Download Chapter 8 Microbial Genetics

Chapter 8
Microbial genetics
• DNA
• Genetic information needed for the
structure and function of the cell
• Nucleotides
• Deoxyribose, phosphate, nitrogen base
• Adenine, guanine, cytosine, thymine
• Double helix
• 2 chains of nucleotides
• Alternating units of sugar and phosphate
• Nitrogen base is attached to the sugar
molecule
Nitrogen base sequence
Adenine
pairs with thymine
Cytosine pairs with
Guanine
Complementary base
pairing
• Gene – segment of DNA that codes for a
functional product
• Most genes code for proteins
• tRNA, rRNA
• Genes are passed on from one cell to
another – one generation to another
• DNA has to be replicated
• DNA is a long molecule
• E.coli chromosome has 4 million base
pairs (nucleotides)
• DNA is replicated segment by segment
The thread-like structure is the bacterial chromosome (DNA)
Segment – unwinds and separates
Each strand functions as a template for
the synthesis of a new strand
Free DNA nucleotides are in the
Area
Complementary base pairing takes place
Between the NB on free nucleotides
And the NB on the template strand
DNA polymerase links them together
New strand spirals around the old strand
Replication fork - region of DNA where the
Replication is taking place
Semiconservative – an old strand and a new
strand
Genetically
identical
Region of DNA where replication begins
Two replication forks
Move in opposite
directions
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Genetic information flows within the cell
Gene is transcribed to make the mRNA
mRNA is translated to make a protein
Transcription genetic information from the
gene is copied onto mRNA
• Gene is a segment of DNA – codes for a
functional product – protein
• E.coli chromosome has thousands of
genes
• Each gene has a unique NB sequence
• Promoter – gene begins
• Terminator – gene ends
• Coding sequence – transcribed onto
mRNA
Each gene has a unique nitrogen base sequence
RNA polymerase
• RNA polymerase
• NB sequence of mRNA is complementary
to the template strand of the gene.
• mRNA has the genetic information in the
language of RNA
• Language of RNA is in the form of codons
• Triplet of NBs - codes for an amino acid
Translation – interaction
Between mRNA, tRNA
And ribosomes
More than one codon
Each AA
Degeneracy of the genetic
Code
Stop – signal the end of
translation
Transfer RNA
Anticodon – complmentary
To a codon on mRNA
Specific group of tRNA
Each of the 20 AA
Specificity is based on the
anticodon
Picks up
AA - cytosol
Specific group
tRNA – each AA
specificity
Based on
anticodon
Triplet
Of NB
mRNA – brings the message
Ribosome – holds mRNA
tRNA – reads the message
Enzyme – catalyzes
Peptide bond formation
methionine
glycine
phenylalanine
leucine
• Sequence of AA – based on the sequence
of mRNA – based on the NB sequence of
the gene from which it was transcribed
• Genetic information flows from the gene to
mRNA to protein.
• Change in the NB of the gene – change
the codon on mRNA – change AA
sequence of the protein – protein becomes
less active or inactive
• Change in the NB sequence - mutation
n
o
Single NB at a specific site on the gene is
Replaced by another NB
• Missense mutation – sickle cell anemia
• Hemoglobin – polypeptide chains – specific AA
sequence
• Mutation – gene that codes for the polypeptide –
hemoglobin
• Thymine takes the place of adenine at a specific
site on the gene.
• AA sequence of the polypeptide chain is
changed
• Polypeptide chain – hemoglobin – AA valine –
AA glutamic acid – shape changed
• RBC – sickle shaped
• Mutation can take place spontaneously.
DNA polymerase makes a mistake and
inserts a wrong NB during DNA replication.
• Mutation frequency is increased by certain
agents – mutagens
• Chemicals – nitrous acid changes shape
Of adenine – cytosine
• X-rays – pull e- out of molecules – breaks
in the chromosome
• UV light – thymine dimers in DNA
Enzyme – separates thymine
Dimers
Too many thymine dimers –
Not all are separated
Accumulation of thymine
Dimers - mtations
In skin cells – skin cancer
Excessive sun tanning
Genetic transfer and recombination
• Contributes to genetic diversity in a
bacterial population.
• New strains pop up – genetic
recombination is partly responsible
• Two DNA are in the same cell – come in
contact – pieces of DNA are exchanged
• Genetic transfer – 2 DNA in the same cell
• Piece of DNA is transferred from a donor
to a recipient .
• Transformation, conjugation, transduction
Donor - dead cell
Live cell
Avirulent cell to virulent cell
F – fertility
conjugation
F - fertility
Hfr – conjugate with many cells and make a lot of recombinant cells
Avirulent – virulent , recipient – antibiotic resistant gene
Transduction
Transduction
Bacteriophage –
Virus – infects bacteria
Regulation of gene expression
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Most genes are expressed constantly.
Constitutive genes
Genes that code for enzymes of gycolysis
Hexokinase gene
Some genes are expressed only when
their products are needed
• Inducible genes
• Beta galactosidase gene
• Beta galactosidase breaks down lactose to
Glucose and galactose.
• Needed only when lactose is in the
medium
• Expressed in the presence of lactose
• Gene is part of the lactose operon
• Located on E.coli chromosome
• Operon – many genes are controlled by
the same control region (promoter)
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Lactose operon – 3 structural genes
Z – beta galactosidase
Y – permease – transports lactose
A – transacetylase
Controlled by the same promoter and
operator
• Medium has both lactose and glucose
• Operon is inactive until glucose is used up
• Catabolite repression
• Cyclic AMP , cyclic AMP receptor protein
(catabolite activator protein)
• Medium has both glucose and lactose
• Operon is inactive until glucose is used up
• Operon is active – absence of glucose
presence of lactose
Both conditions have to be satisfied for the
activation of the lactose operon
• Inducible gene
• Beta galactosidase gene
• Helps the cell to save its energy and
chemical resources such as amino acids.
• Cell is not making something that it does
not need
Plasmids
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Small circular DNA
R plasmids – resistance plasmids
Genes – code - antibiotic resistance
Enzymes that breakdown antibiotics
Not killed by antibiotics
R100 plasmid
Transferred between E.coli, Klebsiella
And Salmonella
• Dissimilation plasmids – enzymes –
breakdown – petroleum
• Pseudomonas – bioremediation
• Bacteriocin plasmids – code for toxins toxic to certain species of bacteria
• Lactococcus lactis - bacteriocin – nisin
• Prevents the germination of Clostridium
endospores
• Preserve cheese
Transposons
• Small segment of DNA
• Transposed from one region of DNA to another
• Jumping genes
• Simple transposons – insertion sequences
• Gene codes for an enzyme – transposase
• Cutting and resealing of DNA - transposition
• Inverted repeat sequence on each side of the
gene