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: • PO4 • 5 carbon sugar • Nitrogen-containing bases (purines and pyrimidines) • Concluded that DNA is a polymer of nucleotides
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
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.
5. Watson & Crick THE DOUBLE HELIX
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.
The Double Helix Strands are complimentary to each other.
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
Semiconservative Replication Parent strand Parent strand
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.