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 with Guanine Complementary base pairing 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 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

Region of DNA where replication begins Two replication forks Move in opposite directions 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

Template 

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

Triplet Of NB Picks up AA - cytosol Specific group tRNA – each AA specificity Based on anticodon

mRNA – brings the message Ribosome – holds mRNA tRNA – reads the message Protein and rRNA Enzyme – catalyzes Peptide bond formation

methionineleucine glycinephenylalanine

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

nono 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

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

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, 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