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PROTEIN SYNTHESIS

PROTEIN SYNTHESIS. Outline: One gene-one polypeptide hypothesis Overview of protein synthesis. From DNA to Protein. Section 5.1: The One Gene-One Polypeptide Hypothesis Page 234. Definitions.

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PROTEIN SYNTHESIS

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  1. PROTEIN SYNTHESIS Outline: One gene-one polypeptide hypothesis Overview of protein synthesis

  2. From DNA to Protein Section 5.1: The One Gene-One Polypeptide Hypothesis Page 234

  3. Definitions • Gene – A sequence of nucleotides in DNA that performs a specific function, such as coding for a particular protein • Proteins – Complex macromolecules composed of one or more polypeptide chains • Functions: Antibodies, hormones, enzymes, membrane structure

  4. Experiments • Archibald Garrod • George Beadle & Edward Tatum • Vernon Ingram

  5. Archibald Garrod, early 20th century • British physician • Studied patients with alkaptonuria (inherited) • Urine contains alkapton, which makes it appear black • Theory – Two components: • Affected individuals have a defective alkapton-metabolising enzyme. • The error in the enzyme is the result of an error in the hereditary material.

  6. From theoretical to experimental… George Beadle and Edward Tatum • Experimentally demonstrated a relationship between genes and enzymes • Experimental model: Neurosporacrassa (bread mould) YUM!

  7. Steps: • Bombarded spore-forming parts of Neurospora with radiation  mutations • Collected spores and germinated them • Screened the strains to find mutants • How to recognize? Mutants are unable to grow on a medium composed of minimal nutrients • Wild-type Neurospora can grow easily on a minimal medium

  8. Identified three classes of mutants that were defective in their ability to produce arginine

  9. The mutant strains • All three were unable to produce their own arginine • Growth obtained only when arginine was added to minimal medium

  10. Known: The mutations mapped to different locations, on separate chromosomes (i.e. three different mutations) • Randomly named the arg-1, arg-2, and arg-3 genes • Wanted to ID the specific way in which each mutation altered the ability to synthesize arginine

  11. Side note: Biosynthetic pathways • Molecules are synthesized by a series of steps. • Each step is catalyzed by a particular enzyme. enzyme precursor

  12. Knock out function of gene/enzyme B… The precursor compound (2) will accumulate. • What happens if Compound 1 is added? Compound 2? • What if Compound 3, or 4 is added?

  13. Back to Beadle and Tatum… Discovered that the three strains differed in their response to the addition of two chemical compounds: ornithine and citrulline

  14. Observations Growth of arg Mutants in Response to Supplements Note: A plus sign means growth; a minus sign means no growth.

  15. Applied knowledge of biosynthetic pathways to mutant analysis. • Can you deduce the correct pathway? • In which order do the three compounds appear in the biosynthetic pathway of arginine?

  16. Looking only at growth when supplemented... If a compound appears later in the pathway, its addition will allow the growth of more mutant strains.

  17. Looking now at the specific mutations... They were able to conclude that the arginine-defective strains were each the result of a mutation to one specific gene: arg-1, arg-2, or arg-3.

  18. Conclusion: Genes act by directing the production of only one enzyme. • The “one gene-one enzyme” hypothesis But… • Genes code for more than just enzymes • Rename it the “one gene-one polypeptide” hypothesis

  19. Vernon Ingram • Studied sickle-cell anemia • Change in the shape of red blood cells • Found that a single amino acid substitution (glutamic acid  valine) is responsible • Demonstrated support for the one gene-one polypeptide hypothesis

  20. Protein synthesis: Overview Section 5.2, page 237

  21. DNA is in the nucleus • Ribosomes (which synthesize protein) are in the cytoplasm How does the ribosome synthesize the protein, without having access to the DNA?

  22. The central dogma of molecular biology RNA is an intermediate messenger molecule

  23. The central dogma involves two steps: • Transcription • Translation

  24. Transcription: The information in DNA is transcribed into a complementary mRNA message. Translation: The ribosomes read the information in RNA and translate it, in order to synthesize a protein.

  25. RNA: A nucleic acid uracil (U) in RNA pairs with the adenine (A) in DNA

  26. Checkpoint: A coding portion of DNA is shown below. What is the sequence of complementary RNA that will be transcribed? DNA: 3’- T A C T T A C G A T C G -5’ RNA: 5’ - -3’

  27. Classes of RNA Three types: • messenger RNA (mRNA) • transfer RNA (tRNA) • ribosomal RNA (rRNA)

  28. Homework Page 236 #1, 4-6 Page 241 #1-5

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