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

Adenine pairs with thymine

Cytosine pairs withGuanine

Complementary basepairing

Nitrogen base sequence

• 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 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

Region of DNA where replication begins

Two replication forksMove in oppositedirections

Genetically identical

• 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

Templa

te

• 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 – interactionBetween mRNA, tRNAAnd ribosomes

More than one codonEach AADegeneracy of the geneticCode

Stop – signal the end oftranslation

Transfer RNA

Anticodon – complmentaryTo a codon on mRNA

Specific group of tRNAEach of the 20 AASpecificity is based on theanticodon

TripletOf NB

Picks upAA - cytosol

Specific grouptRNA – each AA

specificityBased onanticodon

mRNA – brings the message

Ribosome – holds mRNA

tRNA – reads the messageProtein and

rRNA

Enzyme – catalyzesPeptide bond formation

methionine leucine

glycine phenylalanine

• 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

no

Single NB at a specific site on the gene isReplaced 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 thymineDimers

Too many thymine dimers –Not all are separated

Accumulation of thymineDimers - mtationsIn 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

Live cell

Avirulent cell to virulent cell

Donor - dead 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

• 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)

• 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

• 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, KlebsiellaAnd 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