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Molecular Biology of the Gene

Molecular Biology of the Gene. Chapter 10. Is DNA the Hereditary Material?. Biologists knew that genes were located on chromosomes 2 components of chromosomes: Protein & DNA Proteins were in the lead Appeared more structurally complex & functionally specific.

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Molecular Biology of the Gene

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  1. Molecular Biology of the Gene Chapter 10

  2. Is DNA the Hereditary Material? • Biologists knew that genes were located on chromosomes • 2 components of chromosomes: • Protein & DNA • Proteins were in the lead • Appeared more structurally complex & functionally specific

  3. Is DNA the Hereditary Material? • Griffith (1928) • 2 Strains of Bacteria • R strain - harmless • S strain – disease-causing • Procedure of Experiment • Heat-killed S strain • Mixed with R strain • Injected mice with mixture & some of the harmless strain transformed to pathogenic

  4. Is DNA the Hereditary Material?

  5. Is DNA the Hereditary Material? • What materials were used & what was the set-up before the trials of the Hershey-Chase Experiment? • Materials: chem. containing radioactive isotopes, radioactivity detector, blender, centrifuge • Procedure: radioactive isotopes to label DNA & protein in T2 • Grew T2 w/ E. coli in a solution containing radioactive sulfur (protein contains sulfur, but DNA does not) - yellow • New phages were made, radioactive sulfur atoms were incorporated only into their proteins • Grew separate batch in a solution containing radioactive phosphorus (phosphorus is only in DNA) - green

  6. DNA Structure

  7. DNA Structure

  8. RNA Structure

  9. Double Helix

  10. DNA Replication • “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” • Semiconservative model – each of the two daughter molecules will have one old strand and one new strand

  11. DNA Replication

  12. DNA Replication

  13. DNA Replication

  14. Replication Process • http://www.wiley.com/college/pratt/0471393878/student/animations/dna_replication/index.html

  15. Flow of Genetic Info

  16. Flow of Genetic Info • Discovery: Archibald Garrod (1909) • Genes dictate phenotypes through enzymes • Inherited disease reflects a person’s inability to make a particular enzyme • Remember: • Each step in a metabolic pathway is catalyzed by a specific enzyme

  17. Flow of Genetic Info • Tatum & Beadle (1940s) • One gene-one enzyme hypothesis: • Function of a gene is to dictate the production of a specific enzyme • Modifications: • All types of proteins (not just enzymes) • Ex: keratin & insulin • One gene-one protein • Many proteins are made from two or more polypeptide chains • Ex: hemoglobin – two kinds of polypeptides, encoded by 2 different genes • One gene-one polypeptide hypothesis

  18. Flow of Genetic Info • Cells governed by a molecular chain of command • DNA  RNA  Protein • Chemical language of DNA/RNA • Polymers of nucleotide monomers • A, T(U), C, G • Written as a linear sequence of bases • Specific sequence of bases = a gene • Consists of hundreds/thousands of bases in a sequence

  19. Flow of Genetic Info

  20. Flow of Genetic Info • DNA/RNA nucleic acid language is translated into Protein polypeptide language • Polypeptide language • Polymers made up of monomers of amino acids • Written in a linear sequence • Sequence of RNA dictates sequence of amino acids • RNA is the messenger carrying genetic info from DNA

  21. Flow of Genetic Info • DNA & RNA consist of 4 bases • In translation, these 4 must specify 20 amino acids • Triplett code: Genetic instructions for the amino acid sequence of a polypeptide chain are written in DNA & RNA as a series of 3-base words, called codons • 3-base codons in DNA are transcribed into 3-base codons in RNA, and then the RNA codons are translated into amino acids that form the polypeptide

  22. Genetic Code • Set of rules giving the correspondence between codons in RNA and amino acids in proteins • 61 of 64 codons for amino acids • AUG – Methionine or start of polypeptide sequence • 3 other codons do not designate amino acids, but they stop the translation • Redundancy but no ambiguity • Nearly universal, shared by organisms from bacteria to plants and animals

  23. Genetic Code

  24. Genetic Code

  25. Transcription • Transfer of genetic info from DNA to RNA • Occurs in nucleus (Eukaryotes) • RNA is transcribed from DNA template • RNA nucleotides follow same base-pairing rules that govern DNA replication • U pairs with A, instead of T with A • RNA polymerase – link RNA nucleotides • Promoter – nucleotide sequence that says “start transcribing” • Binding site for RNA polymerase

  26. mRNA • Conveys genetic info from DNA to translation machinery • Transcribed from DNA – Translated into polypeptides • Modified before leaving nucleus • Add nucleotides • Cap (G nucleotide) & tail (50-250 A’s) • RNA splicing • Remove introns and join exons • Produce mRNA molecule that is continuous

  27. mRNA

  28. tRNA • Translates codons into amino acids • Amino acids can not recognize mRNA codons • Up to the tRNA to match amino acids to the appropriate codons • 2 Functions must be met: • Picking up the appropriate amino acids • Recognizing appropriate mRNA codons

  29. tRNA • Structure • Single strand of RNA – 80 nucleotides • Twists & folds itself • Anticodon: single-stranded loop at 1 end that contains a special triplet of bases • Complementary to codon triplet • Opposite end of anticodon contains the amino acid • tRNA molecules a slightly different for each amino acid it specifies • Specific enzyme that joins amino acids to tRNA

  30. tRNA

  31. rRNA • Make up the large and small subunits of a ribosome • Ribosome is the site where translation occurs • tRNA & mRNA bind here • Prokaryotic & Eukaryotic Differences • Medically significant • Antibiotic drugs can inactivate prokaryotic ribosomes while leaving eukaryotic ribosomes unaffected

  32. rRNA

  33. Steps of Translation • Initiation, Elongation, & Termination • Initiation • mRNA, tRNA w/ 1st amino acid, 2 subunits of ribosome • Role: establish exactly where translation begins – get correct sequence of amino acids • 2 Steps: • mRNA molecule binds to small subunit, and tRNA (UAC) binds to start codon (AUG) – carries methionine • Large subunit binds to small; initiator tRNA fits into large subunit – P site – holds growing peptide; A site ready for next tRNA

  34. Steps of Translation • Elongation • Codon recognition: anticodon pairs w/ codon at site A • Peptide bond formation: polypeptide separates from tRNA on P site & attaches by a peptide bond to the amino acid carried by the tRNA on the A site • Translocation: P site tRNA leaves, the ribosome translocates the tRNA in the A site to the P site • Codon & anticodon remain bonded • Brings next codon & anticodon to the A site

  35. Steps of Translation • Termination • When stop codon reaches the A site (UAA, UAG, UGA) • Don’t code for an amino acid • Stop translation • Polypeptide is released from last tRNA & exits ribosome • Ribosome subunits separate

  36. Steps of Translation

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