BIOLOGY 111 1st Edition Lecture 19 Outline of Last Lecture I Parental Type Offspring II Recombinant Type Offspring III Frequency of Recombination IV Linkage Map V Alteration of Chromosome VI Epigenetics Outline of Current Lecture I Molecular Basis of Inheritance II Chargaff s Rule III DNA IV Base Pairing V Replication of E coli VI Replication Problems VII DNA packaging Current Lecture Molecular Basis of Inheritance Frederick Griffith s bacterial transformation in 1920 s o experiment studied the bacteria of S smooth pathogenic and R rough control non pathogenic to test for the trait of pathogenicity Avery MacLeod and McCarthy 1944 used purification methods to reveal that DNA is genetic material Alfred Hershey and Martha Chase 1952 DNA is the genetic material o viruses infecting a bacterial cell phages bacteriophages called T2 attach to the host cell and inject their genetic material through the plasma membrane while the head and tail parts remain on the outer bacterial surface Purines Adenine and Guanine Pyrimidines Thymine and Cytosine Chargaffs Rule o the base composition varies between species o within the species the number of A and T bases are equal and the number of G and C bases are equal These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute James Watson and Francis Crick with Maurice Wilkins 1953 constructed the DNA model o DNA is double stranded helix shaped sugar phosphate backbone and paired bases on the inside o A pairs with T o G pairs with C o keeps a consistent width they have to pair purine to pyrimidine because if two purines pair the diameter would be too wide and if two pyrimidines pair it would be too narrow o 2 DNA strands are complementary DNA goes from 5 end to 3 end 5 GCGGATTT 3 3 CGCCTAAA 5 Base Pairing of DNA o the pairs of nitrogenous bases in DNA are held together by hydrogen bonds o A and T are held together by two Hydrogen bonds o G and C are held together by three Hydrogen bonds o in addition to hydrogen bonding there is also van Derr Walls Interaction that adds to the stability of DNA Replication of E coli o begins at origin at replication where a replication bubble is formed o at each end of the replication bubble is a replication fork creating a Y shape o Helicase untwists the double helix at the replication fork and separates the two strands o single strand binding proteins protect the unwound strands from repairing while being copied o Topoisomerase works ahead of the replication fork to swivel to DNA strands relieving strain so that helicase can unwind with no problems o the unwound sections of parental DNA strands are now used as templates for synthesis of complementary DNA strands o Nucleotides ALWAYS added to the 3 end of each strand o synthesization of a new DNA strand by DNA polymerase which adds nucleotides to a preexisting chain DNA polymerase 3 synthesizes the leading strand and also has proof reading capabilities back up and fixes its mistakes o nucleotide excision repair mismatched nucleotides escaped proof reading or if they are damaged after replication the damage is corrected at this point o Dimer damage DNA polymerase 1 removes RNA primers and and replaces with DNA o Antiparallel elongation leading strand made by DNA polymerase 3 continuous elongation in the 5 to 3 toward direction as the replication fork progresses lagging strand moves away from the replication fork Okazaki fragments segments of the lagging strand Replication of eukaryotes o requires several replication bubbles and serveral replication forks Replication problem o chromosomes would become even shorter except for telomerase adds short stretches of nucleotides telomeres to the ends of each chromosome o telomeres are repeats of 6 nucleotides 100 1000 times they do not contain genes they carry their own template so it can extend DNA without having to copy information from the other strand Packaging of DNA o Histone Octamers 8 histones that come together make nucleosomes o sizes in diameter as DNA is packaged 2nm DNA 10nm nucleosome 30nm fiber 300nm fiber composed of the 30nm fibers looped and folded up 700nm chromatid 1400nm replicated chromosome Euchromatin is dispersed and can be expressed Heterochromatin is highly condensed and cannot be expressed until it is converted to euchromatin by modifying its histones