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DNA Structure, Genetic Information, and Gene Concept

This chapter explains the structure of DNA based on the Watson and Crick model, describes DNA as the carrier of genetic information, and explores the concept of one gene encoding one polypeptide.

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DNA Structure, Genetic Information, and Gene Concept

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  1. Chapter 15 EXPRESSION of BIOLOGICAL INFORMATION

  2. DNA & Genetic Information Objectives • to describe the structure of DNA based on the • Watson & Crick model • to explain DNA as the carrier of genetic • information • to explain the concept of one gene one • polypeptide

  3. DNA STRUCTURE • Watson and Crick Model • Monomer : NUCLEOTIDE • Component : Deoxyribose (pentose sugar) Phosphate group Nitrogenous base : Adenine (A) Thymine (T) Guanine (G) Cytosine (C) *Base pairing rule : A-T and C-G

  4. DNA NUCLEOTIDE

  5. WATSON AND CRICK MODEL • DNA : 2 polynucleotide chain • Arrange in double helix • Antiparallel (3’ to 5’) and (5’ to 3’) • Each polynucleotide chain attach to another • polynucleotide chain by hydrogen bond • Nucleotide link by alternating sugar-phosphate • backbone : phosphodiester bond

  6. DNA as carrier of genetic information • Experiments are conducted by Frederick Griffith • (1928) and Avery(1944) • Griffith: Identify transformation process • Avery et.al. : identify the transformation agent

  7. DNA: The Carrier of Genetic Information • Griffith (1928) • - studied Streptococcus pneumoniae; • bacterium that causes pneumonia to mammals • two strains (varieties) of S. pneumoniae: • R (rough) strain • mutant strain • non-capsulated • non-virulent (non-pathogenic) • S (smooth) strain • capsulated • virulent (pathogenic / causing disease)

  8. DNA: The Carrier of Genetic Information 2 1

  9. DNA: The Carrier of Genetic Information 3 4 Transformation occur

  10. DNA: The Carrier of Genetic Information • transformation occur; live R strain is transformed • into live S strain by hereditary material obtained • from the dead strain-S cells • the hereditary substance was unknown • the transformed bacteria reproduce forming • S strain daughter cells (heritable)

  11. DNA: The Carrier of Genetic Information Definition • a change in genotypes and phenotypes of an • individual • due to the assimilation of foreign DNA by a cell. S strain R strain

  12. DNA: The Carrier of Genetic Information Transformation agent • In 1944, Oswald Avery, Maclyn McCarty and • Colin McLeod proved that DNA was the • transforming substance. • Two models of the experiment were carried • out.

  13. DNA: The Carrier of Genetic Information Transformation agent • Experiment 1 • heat-killed the S strain bacteria culture • purify the culture; removeproteins, • carbohydrates, lipids, RNA and DNA • add proteins, carbohydrates, lipids, RNA and • DNA to different cultures of living R strain Observation : After a few days, colonies of S strain grew in the R strain culture that was added with DNA

  14. 15.1 DNA: The Carrier of Genetic Information Transformation agent • Experiment 1 The observation • After a few days, colonies of the S strain • grew in the R strain culture which had • DNA added.

  15. DNA: The Carrier of Genetic Information Transformation agent S colonies extracts are added to different cultures of R strain carbohydrate lipid protein RNA DNA no change no change no change no change S strain found Transformation

  16. DNA: The Carrier of Genetic Information Transformation agent • Experiment 2 Enzyme Treatment The Procedures • The purified DNA of the S strain bacterium • was mixed with the R strain colonies. 2. Different enzymes were added to the different mixture. 3. Bacteria colony was grown in different culture.

  17. DNA: The Carrier of Genetic Information Transformation agent • Experiment 2 Procedures The enzymes added: • protease - degrades proteins • ribonuclease - RNase - degrades RNA • deoxyribonuclease - DNase - degrades DNA

  18. Experiment 2 R Strain R colonies DNA from heat-killed S cells no colonies Serum that precipitates R cells from mixture DNA from heat-killed S cells + + Living R Strain S colonies (transformation)

  19. Experiment 2 S colonies 1 transformation Serum that precipitates R cells from mixture DNA from heat-killed S cells + + + protease R strain 2 transformation Serum that precipitates R cells from mixture DNA from heat-killed S cells + + RNase + R strain S colonies 3 Serum that precipitates R cells from mixture DNA from heat-killed S cells + + DNase + R strain NO colonies

  20. DNA: The Carrier of Genetic Information Transformation agent • Experiment 2 The Inferences • The protease and the RNasefailed to stop • the transformation of the R strain into the • S strain by the purified DNA. • The DNase destroyed the transforming • activity of the purified DNA preparation.

  21. DNA: The Carrier of Genetic Information Transformation agent • Experiment 2 The Conclusion • DNA is the carrier of genetic information • DNA from the capsulated bacteria (S strain) • carries the gene that encode the production of • capsule • During transformation, the DNA is assimilated by • DNA of R strain; enables R strain to synthesize • capsule

  22. Gene concept: One-Gene-One-Polypeptide • George Beadle and Edward Tatum (1941) • study the relationship between genes and • enzymes • by using Neurosporacrassa(bread mold) • fungi (Ascomycota / sac fungi) Fungi that produce spores in structures called sacs

  23. Gene concept: One-Gene-One-Polypeptide Beadle & Tatum experiment Medium needed: Minimal medium consists of agar, glucose, inorganic salts and vitamin biotin Complete medium consists of agar, glucose, inorganic salts, vitamin biotin and 20 amino acids.

  24. Gene concept: One-Gene-One-Polypeptide Procedures • some conidia (asexual spores) were exposed • to X-ray; to induce mutation

  25. Gene concept: One-Gene-One-Polypeptide Procedures • conidia (mutant) were transferred to complete • mediumand grown; mycelia formed • mycelia (from mutant conidia) were crossed with • mycelia (from wild type conidia). • wild type conidia are NOT exposed to X-ray

  26. Gene concept: One-Gene-One-Polypeptide • producing asci (sin. ascus) that consist of eight • ascospores(four ascospores from each parental • mycelia – mutant and wild type)

  27. Gene concept: One-Gene-One-Polypeptide Procedures • the ascospores were dissected out and • transferred to complete medium. • ALL grew and formed mycelia • mycelia were placed • on minimal medium • NO growth occur

  28. Gene concept: One-Gene-One-Polypeptide • mycelia which did no grow were unable to • synthesize certain amino acids • to determine which amino acids were • synthesized, mycelia were transferred to minimal • media each containing a different amino acid • amino acid test

  29. Gene concept: One-Gene-One-Polypeptide Observations • the medium in which growth occur, contain the • amino acid which the mutantNeurospora • unable to synthesize

  30. Gene concept: One-Gene-One-Polypeptide Conclusion • the mutantNeurospora was defective in • biochemical pathway to synthesize arginine • does not have the enzyme to synthesize arginine • due to lacks ofgene that encodes the enzyme • results in the mutant could ONLY grow with the • supply of arginine “ One GENE ; One ENZYME ”

  31. Restatement • NOT all proteins are enzymes • e.g keratin (structural protein) • insulin (peptide hormone) • proteins that AREN’T enzymes also encoded by • genes  “ One GENE ; One PROTEIN ” • proteins are composed of different polypeptide • chains (e.g hemoglobin) • each polypeptide chain is encoded by gene  “ One GENE ; One POLYPEPTIDE ”

  32. Gene concept: One-Gene-One-Polypeptide From further experiments Beadle and Tatum concluded that • the defect is in the biochemical pathways • that normally synthesize arginine

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