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Molecular Genetics

Molecular Genetics. Biology Chapter 12. Mystery of the Hereditary Material. Experiments in the 1950s showed that DNA is the hereditary material Scientists raced to determine the structure of DNA 1953 - Watson and Crick proposed that DNA is a double helix. Griffith Discovers Transformation.

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Molecular Genetics

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  1. Molecular Genetics Biology Chapter 12

  2. Mystery of the Hereditary Material • Experiments in the 1950s showed that DNA is the hereditary material • Scientists raced to determine the structure of DNA • 1953 - Watson and Crick proposed that DNA is a double helix

  3. Griffith Discovers Transformation • 1928 • Attempting to develop a vaccine • Isolated two strains of Streptococcus pneumoniae • Rough strain was harmless • Smooth strain was pathogenic

  4. Griffith’s “Transforming Factor”

  5. Hershey & Chase’s Experiments • Created labeled bacteriophages • Radioactive sulfur • Radioactive phosphorus • Allowed labeled viruses to infect bacteria • Asked: Where are the radioactive labels after infection?

  6. virus particle labeled with 35S virus particle labeled with 32P Hershey and Chase Results bacterial cell (cutaway view) label outside cell label inside cell

  7. DNA • Deoxyribonucleic acid • Nucleic acids are polymers that are the blueprints for proteins. • Nucleotides: The monomers that make up nucleic acids which consist of: • 5 carbon sugar • Phosphate group • nitrogenous base

  8. NITROGENOUS BASES • DNA • Adenine (A) Cytosine (C) • Thymine (T) Guanine (G) • RNA • In RNA uracil replaces thymine.

  9. Composition of DNA • Chargaff showed: • Amount of adenine relative to guanine differs among species • Amount of adenine always equals amount of thymine and amount of guanine always equals amount of cytosine A=T and G=C

  10. Rosalind Franklin’s Work • Was an expert in X-ray crystallography • Used this technique to examine DNA fibers • Concluded that DNA was some sort of helix

  11. Watson-Crick Model • DNA consists of two nucleotide strands • Strands run in opposite directions • Strands are held together by hydrogen bonds between bases • A binds with T and C with G • Molecule is a double helix

  12. Watson-Crick Model

  13. DNA Structure Helps Explain How It Duplicates • DNA is two nucleotide strands held together by hydrogen bonds • Hydrogen bonds between two strands are easily broken • Each single strand then serves as template for new strand

  14. DNA Replication • Each parent strand remains intact • Every DNA molecule is half “old” and half “new”

  15. Enzymes in Replication • Enzymes unwind the two strands • DNA polymerase attaches complementary nucleotides • DNA ligase fills in gaps • Enzymes wind two strands together

  16. DNA – RNA – Protein • Transcription: RNA copies the code of the DNA in the nucleus and leaves the nucleus (mRNA) • Translation: Converts the mRNA message into proteins using tRNA. Occurs in the ribosomes • Codon: A 3 nucleotide code for a specific amino acid

  17. DNA mRNA tRNA Protein

  18. Editing the RNA • Introns: Noncoding region of RNA • Exons: Coded regions of RNA • RNA splicing: The removing of introns and rejoining exons

  19. Three Classes of RNA • Messenger RNA • Carries protein-building instruction • Ribosomal RNA • Major component of ribosomes • Transfer RNA • Delivers amino acids to ribosomes

  20. Genetic Code • Set of 64 base triplets • Codons • 61 specify amino acids • 3 stop translation

  21. Base Pairing during Transcription DNA G C A U G C A T RNA C G T A C G T A DNA DNA base pairing in DNA replication base pairing in transcription

  22. Gene Transcription DNA to be transcribed unwinds transcribed DNA winds up again mRNA transcript RNA polymerase

  23. Translation • Anticodon: A three nucleotide group on the tRNA that is complimentary to the codon of the mRNA • A specific amino acid is attached to the other end of the tRNA

  24. Transcription Overview rRNA tRNA mRNA Mature mRNA transcripts ribosomal subunits mature tRNA Translation

  25. Gene Mutations Missense Nonsense Base-Pair Substitutions Insertions Deletions

  26. Base-Pair Substitution a base substitution within the triplet (red) original base triplet in a DNA strand During replication, proofreading enzymes make a substitution possible outcomes: or original, unmutated sequence a gene mutation

  27. Frameshift Mutations • Insertion • Extra base added into gene region • Deletion • Base removed from gene region • Both shift the reading frame • Result in many wrong amino acids

  28. Frameshift Mutation mRNA parental DNA arginine glycine tyrosine tryptophan asparagine amino acids altered mRNA DNA with base insertion arginine glycine leucine leucine glutamate altered amino- acid sequence Figure 14.12Page 234

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