chapter 8
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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 - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 8
Microbial genetics
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• 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
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Adenine pairs with thymine
Cytosine pairs withGuanine
Complementary basepairing
Nitrogen base sequence
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• 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
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The thread-like structure is the bacterial chromosome (DNA)
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Segment – unwinds and separates
Each strand functions as a template forthe synthesis of a new strand
Free DNA nucleotides are in theArea
Complementary base pairing takes placeBetween the NB on free nucleotidesAnd the NB on the template strand
DNA polymerase links them together
New strand spirals around the old strand Replication fork - region of DNA where theReplication is taking placeSemiconservative – an old strand and a newstrand
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Region of DNA where replication begins
Two replication forksMove in oppositedirections
Genetically identical
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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
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• 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
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Each gene has a unique nitrogen base sequence
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RNA polymerase
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• RNA polymerase
Templa
te
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• 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
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Translation – interactionBetween mRNA, tRNAAnd ribosomes
More than one codonEach AADegeneracy of the geneticCode
Stop – signal the end oftranslation
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Transfer RNA
Anticodon – complmentaryTo a codon on mRNA
Specific group of tRNAEach of the 20 AASpecificity is based on theanticodon
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TripletOf NB
Picks upAA - cytosol
Specific grouptRNA – each AA
specificityBased onanticodon
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mRNA – brings the message
Ribosome – holds mRNA
tRNA – reads the messageProtein and
rRNA
Enzyme – catalyzesPeptide bond formation
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methionine leucine
glycine phenylalanine
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• 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
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no
Single NB at a specific site on the gene isReplaced by another NB
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• 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
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• 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
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• UV light – thymine dimers in DNA
Enzyme – separates thymineDimers
Too many thymine dimers –Not all are separated
Accumulation of thymineDimers - mtationsIn skin cells – skin cancer
Excessive sun tanning
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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
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• Genetic transfer – 2 DNA in the same cell
• Piece of DNA is transferred from a donor to a recipient .
• Transformation, conjugation, transduction
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Live cell
Avirulent cell to virulent cell
Donor - dead cell
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F – fertility
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conjugation
F - fertility
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Hfr – conjugate with many cells and make a lot of recombinant cells
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Avirulent – virulent , recipient – antibiotic resistant gene
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Transduction
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Transduction
Bacteriophage –Virus – infects bacteria
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Regulation of gene expression
• 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
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• 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
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• 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)
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• Medium has both glucose and lactose
• Operon is inactive until glucose is used up
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• Operon is active – absence of glucose
presence of lactose
Both conditions have to be satisfied for the activation of the lactose operon
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• 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
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Plasmids
• Small circular DNA
• R plasmids – resistance plasmids
• Genes – code - antibiotic resistance
• Enzymes that breakdown antibiotics
• Not killed by antibiotics
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R100 plasmid
Transferred between E.coli, KlebsiellaAnd Salmonella
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• 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
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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
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