DNA

• James Watson, Francis Crick


  1953
• T.H. Morgan showed genes are on chromosomes
• DNA in heredity was found by studying bacteria infected by viruses
• Bacteriophages (phages)


  Viruses that infect bacteria
• Griffith discovered genetic role of DNA
• Transformation


  A change in genotype and phenotype due to assimilation of foreign DNA
• O. Avery, M. McCarty, C. MacLeod said transforming thing is DNA


  1944
• DNA


  A polymer of nucleotides, each with nitrogenous base, sugar, and phosphate group
• Chargaff's rules


  • Base composition of DNA varies between species
  • In any species the number of A and T bases are equal
  • The number of G and C bases are equal
• Rosalind Franklin produced picture of DNA using x-ray crystallography
• Antiparallel


  Subunits run in opposite directions
• Watson and Crick's semiconservative model of replication


  When a double helix replicates, each daughter molecule will have one old strand conserved from parent molecule and one newly made strand
• Conservative model


  Two parent strands rejoin
• Dispersive model


  Each strand is a mix of old and new
• Origins of replication


  Particular sites where replication begins, where two DNA strands are separated (opens like a bubble)
• Eukaryotic chromosome may have hundreds or thousands of origins
• Replication proceeds in both directions from each origin until entire molecule is copied
• Replication fork


  At end of each replication bubble, Y shaped region where DNA strands are elongating
• Helicases


  Enzymes that untwist the double helix at replication forks
• Single-strand binding proteins


  Bind to and stabilize single-stranded DNA
• Topoisomerase


  Corrects "Overwinding" ahead of replication forks by breaking, swiveling, and rejoining DNA strands
• DNA polymerases


  Cannot initiate synthesis of a polynucleotide, can only add nucleotides to 3' end (serves as starting point for new DNA strand)
• DNA polymerase (DNAP)


  Type of enzyme that is responsible for forming new copies of DNA in the form of nucleic acid molecules, catalyze elongation of new DNA at replication fork (require a primer and DNA template strand)
• Rate of elongation is ~500 nucleotides per second in bacteria, 50 per second in human cells
• New DNA strand can only elongate in the 5' to 3' direction
• DNA Polymerase synthesizing leading strand

  Moves toward the replication fork, elongating continuously along one template strand
• DNA Polymerase synthesizing lagging strand

  Works in direction away from replication fork, synthesized as series of Okazaki fragments that are joined together by DNA ligase
• Recent studies support model where DNA polymerase "reel in" parental DNA and "extrude" new daughter DNA molecules
• RNA primer


  The initial nucleotide strand, 5-10 nucleotides long
• Primase


  Can start an RNA chain from scratch, adds RNA nucleotides one at a time using parental DNA as template
• Nucleotide triphosphate


  Each nucleotide that is added to a growing DNA strand
• Proteins in DNA replication forms "DNA replication machine", may be stationary during process
• Information content of DNA is in specific sequences of nucleotides
• DNA inherited by organisms dictates specific traits by directing synthesis of proteins
• Proteins are the links between genotype and phenotype
• Gene expression


  Process where DNA directs protein synthesis, in two stages: transcription and translation
• Archibald Garrod suggested genes dictate phenotypes through enzymes that catalyze specific chemical reactions


  1902
• Gene-one enzyme hypothesis


  By George Beadle and Edward Tatum, stating that each gene dictates production of a specific enzyme
• Gene-one protein hypothesis revised the gene-one enzyme hypothesis, as some proteins aren't enzymes and many proteins are composed of several polypeptides, each with its own gene
• RNA


  The bridge between genes and the proteins for which they code