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Chapter 8

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

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Chapter 8

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  1. Chapter 8 Microbial genetics

  2. 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

  3. Nitrogen base sequence Adenine pairs with thymine Cytosine pairs with Guanine Complementary base pairing

  4. 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

  5. The thread-like structure is the bacterial chromosome (DNA)

  6. 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

  7. Genetically identical Region of DNA where replication begins Two replication forks Move in opposite directions

  8. 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

  9. 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

  10. Each gene has a unique nitrogen base sequence

  11. RNA polymerase

  12. RNA polymerase Template 

  13. 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

  14. Translation – interaction Between mRNA, tRNA And ribosomes More than one codon Each AA Degeneracy of the genetic Code Stop – signal the end of translation

  15. Transfer RNA Anticodon – complmentary To a codon on mRNA Specific group of tRNA Each of the 20 AA Specificity is based on the anticodon

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

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

  18. methionine leucine glycine phenylalanine

  19. 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

  20. no Single NB at a specific site on the gene is Replaced by another NB

  21. 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

  22. 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

  23. Enzyme – separates thymine Dimers Too many thymine dimers – Not all are separated Accumulation of thymine Dimers - mtations In skin cells – skin cancer • UV light – thymine dimers in DNA Excessive sun tanning

  24. 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

  25. Genetic transfer – 2 DNA in the same cell • Piece of DNA is transferred from a donor to a recipient . • Transformation, conjugation, transduction

  26. Donor - dead cell Live cell Avirulent cell to virulent cell

  27. F – fertility

  28. conjugation F - fertility

  29. Hfr – conjugate with many cells and make a lot of recombinant cells

  30. Avirulent – virulent , recipient – antibiotic resistant gene

  31. Transduction

  32. Transduction Bacteriophage – Virus – infects bacteria

  33. 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

  34. 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)

  35. Lactose operon – 3 structural genes • Z – beta galactosidase • Y – permease – transports lactose • A – transacetylase • Controlled by the same promoter and operator

  36. 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)

  37. Medium has both glucose and lactose • Operon is inactive until glucose is used up

  38. 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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