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Decoding DNA: The Genetic Code Journey

Explore how DNA controls cells through protein synthesis, from Archibald Garrod's enzyme discovery to current genetic hypotheses. Learn about the genetic code, codons, transcription process, and evolution of the code. Understand the significance of reading frames, code redundancy, and transcription steps.

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Decoding DNA: The Genetic Code Journey

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  1. Chapter 17From Gene to Protein

  2. Question? • How does DNA control a cell? • By controlling Protein Synthesis. • Proteins are the link between genotype and phenotype.

  3. For tests: • Name(s) of experimenters • Outline of the experiment • Result of the experiment and its importance

  4. 1909 - Archibald Garrod • Suggested genes control enzymes that catalyze chemical processes in cells. • Inherited Diseases - “inborn errors of metabolism” where a person can’t make an enzyme.

  5. Example • Alkaptonuria - where urine turns black after exposure to air. • Lacks - an enzyme to metabolize alkapton.

  6. George Beadle and Edward Tatum • Worked with Neurospora and proved the link between genes and enzymes. Neurospora Pink bread mold

  7. Experiment • Grew Neurospora on agar. • Varied the nutrients. • Looked for mutants that failed to grow on minimum agar.

  8. Results • Three classes of mutants for Arginine Synthesis. • Each mutant had a different block in the Arginine Synthesis pathway.

  9. Conclusion • Mutations were abnormal genes. • Each gene dictated the synthesis of one enzyme. • One Gene - One Enzyme Hypothesis.

  10. Current Hypothesis • One Gene - One Polypeptide Hypothesis (because of 4th degree structure).

  11. Central Dogma DNA Transcription RNA Translation Polypeptide

  12. Explanation • DNA - the Genetic code or genotype. • RNA - the message or instructions. • Polypeptide - the product for the phenotype.

  13. Genetic Code • Sequence of DNA bases that describe which Amino Acid to place in what order in a polypeptide. • The genetic code gives the primary protein structure.

  14. Code Basis If you use: • 1 base = 1 amino acid • 4 bases = 4 amino acids • 41 = 4 combinations, which are not enough for 20 AAs.

  15. If you use: • 2 bases = 1 amino acid • Ex – AT, TA, CA, GC • 42 = 16 amino acids • Still not enough combinations.

  16. If you use: • 3 bases = 1AA • Ex – CAT, AGC, TTT • 43 = 64 combinations • More than enough for 20 amino acids.

  17. Genetic Code • Is based on triplets of bases. • Has redundancy; some AA's have more than 1 code. • Proof - make artificial RNA and see what AAs are used in protein synthesis (early 1960’s).

  18. Codon • A 3-nucleotide “word” in the Genetic Code. • 64 possible codons known.

  19. DNA vs RNA DNARNA Sugar – deoxyribose ribose Bases – ATGC AUGC Backbones – 2 1 Size – very large small Use – genetic code varied

  20. Codon Dictionary • Start- AUG (Met) • Stop- UAA UAG UGA • 60 codons for the other 19 AAs.

  21. For Testing: • Be able to “read” a DNA or RNA message and give the AA sequence. • RNA Genetic Code Table will be provided.

  22. Code Redundancy • Third base in a codon shows "wobble”. • First two bases are the most important in reading the code and giving the correct AA. The third base often doesn’t matter.

  23. Code Evolution • The genetic code is nearly universal. • Ex: CCG = proline (all life) • Reason - The code must have evolved very early. Life on earth must share a common ancestor.

  24. Reading Frame and Frame Shift • The “reading” of the code is every three bases (Reading Frame) • Ex: the red cat ate the rat • Frame shift – improper groupings of the bases • Ex: thr edc ata tat her at • The “words” only make sense if “read” in this grouping of three.

  25. Transcription • Process of making RNA from a DNA template.

  26. Transcription Steps 1. RNA Polymerase Binding 2. Initiation 3. Elongation 4. Termination

  27. RNA Polymerase • Enzyme for building RNA from RNA nucleotides.

  28. Binding • Requires that the enzyme find the “proper” place on the DNA to attach and start transcription.

  29. Binding • Is a complicated process • Uses Promoter Regions on the DNA (upstream from the information for the protein) • Requires proteins called Transcription Factors.

  30. TATA Box • Short segment of T,A,T,A • Located 25 nucleotides upstream for the initiation site. • Recognition site for transcription factors to bind to the DNA.

  31. Transcription Factors • Proteins that bind to DNA before RNA Polymerase. • Recognizes TATA box, attaches, and “flags” the spot for RNA Polymerase.

  32. Transcription Initiation Complex • The complete assembly of transcription factors and RNA Polymerase bound to the promoter area of the DNA to be transcribed.

  33. Initiation • Actual unwinding of DNA to start RNA synthesis. • Requires Initiation Factors.

  34. Elongation • RNA Polymerase untwists DNA 1 turn at a time. • Exposes 10 DNA bases for pairing with RNA nucleotides.

  35. Elongation • Enzyme moves 5’ 3’. • Rate is about 60 nucleotides per second.

  36. Comment • Each gene can be read by sequential RNA Polymerases giving several copies of RNA. • Result - several copies of the protein can be made.

  37. Termination • DNA sequence that tells RNA Polymerase to stop. • Ex: AATAAA • RNA Polymerase detaches from DNA after closing the helix.

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