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BIO 105

BIO 105. Structure of DNA. Some History. DNA was discovered in 1869 by Miescher….4 years after Mendel published his findings. Levene’s chemical analysis (50 years later!) showed DNA contained 3 components in roughly equal proportions: PO 4 5 carbon sugar

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BIO 105

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  1. BIO 105 Structure of DNA

  2. Some History • DNA was discovered in 1869 by Miescher….4 years after Mendel published his findings. • Levene’s chemical analysis (50 years later!) showed DNA contained 3 components in roughly equal proportions: • PO4 • 5 carbon sugar • Nitrogen-containing bases (purines and pyrimidines) • Concluded that DNA is a polymer of nucleotides

  3. Nucleotide Structure

  4. Nucleotide Structure

  5. Nucleotide Structure

  6. More History 3. In the late 1940’s, Chargaff examined the proportion of bases (ATCG) in DNA. A T C G E. coli 26.0 23.9 24.9 25.2 Yeast 31.3 32.9 18.7 17.1 Rat 28.6 28.4 21.4 21.5 Human 30.9 29.4 19.9 19.8 • Equal proportions of A and T….and C and G • Equal proportions of purines and pyrimidines

  7. More History • Rosalind Franklin did X-ray diffraction on crystals on semi-purified DNA. • Her findings led her to conclude that DNA was shaped as a helix or corkscrew.

  8. Rosalind Franklin

  9. 5. Watson & Crick THE DOUBLE HELIX

  10. 5. Watson & Crick • Using Chargaff’s chemical analysis (A=T and C=G) and Franklin’s helical data, they postulated the double helix model (1953). • Base pairs holding together 2 antiparralel strands of polymerized nucleotides.

  11. The Double Helix Strands are complimentary to each other.

  12. DNA Replication • Three hypotheses were proposed A. Conservative model – a new double helix was made by the parent double helix and the parent molecule remained intact B. Dispersive model – parts of the parent were incorporated in the duplicating DNA • Semiconservative model – each strand of parent DNA makes a new strand and the new double helices are one strand parental and one strand newly synthesized. 6. Meselson & Stahl (1958) proved semiconservative replication of DNA

  13. Semiconservative Replication Parent strand Parent strand

  14. DNA Replication(as we know it today) • Eukaryotic chromosomes have multiple origins of replication. • There may be 100 origins of replication. • Each region is called a replicon. • This makes duplication of the chromosome about 100 times faster. • Replicons may be 10,000 to a million base pairs long.

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