Structure of DNA

Nucleotides Know that DNA contained 4 different types of nucleotides Purines – 2 ring bases –Adenine –Guanine Pyrimidines – 1 ring base –Thymine –Cytosine

Chargaff’s rules Found nucleotide content of DNA not fixed Variability between species All members of a species have same base composition # of A’s = # of T’s, C=G

Rosalind Franklin X-ray diffraction of DNA –Showed DNA was helix –Showed a feature was repeated over and over, = hydrogen bonded bases.

Watson and Crick Using data from x-ray diffraction, constructed model of DNA in 1953, –received Nobel Prize in 1962 –Sugar phosphate backbone on outside, base pairs inside. Complementary base pairing – A with T, C with G –Pyrimidine always with purine.

DNA Replication Copying of a DNA molecule Old DNA strands used as a template for new one new strand Semiconservative replication –One old strand conserved in each new daughter molecule.

Replication Steps 1. unwinding (unzipped) DNA helicase, breaking of weak hydrogen bonds 2.Complementary base pairing – bases are always available in nucleus 3.Complementary nucleotides join to form new strands (DNA polymerase) 4.DNA polymerase can also be used as a proofreader.

Meselson and Stahl 1958 Replication experiment using cells grown in heavy and light nitrogen Results: DNA replication is semiconservative

Prokaryotes versus Eukaryotes Prokaryotic replication: –Replication can occur in 1 or 2 directions –Very fast, 40 minutes for entire chromosome –Can happen concurrently –Eukaryotic Begins at numerous origins along chromosome Spread bidirectional until they meet replication fork – V shape when ever DNA is being replicated Slower but do cover significant DNA because of many origins of replication

Replication Errors Genetic mutation – permanent change in sequence of bases 1 per 100,000 base pairs at most, after proofreading, 1 per billion Mutations can be result of DNA damage –Environmental factors –DNA repair enzymes exist and are somewhat effective. Remember: Mutations are raw material for evolutionary process!!