5Nitrogen Bases continued… Adenine and guanine are purines – double rings of carbon and nitrogenThymine and Cytosine are pyrimidines – single rings of carbon and nitrogen
6Chargaff’s RulesBase pairing rules show the arrangement of nucleotides within a DNA moleculeAdenine-thymine (A-T) and the reverseCytosine-guanine (C-G) and the reverseBase pairs of DNA form a double helix and are complementary to each otherChargaff was the scientist who discovered that A and T along with C and G always occurred in the same proportions within DNA
9The Double HelixWatson and Crick (1953) discovered the structure of DNA to be a double helix or “spiral staircase” of two strands of nucleotides twisting around a central axis.Names for DNA…double helix, twisted staircase
10How DNA is copied…DNA Replication Replication is the process of synthesizing a new strand of DNAWhen does this occur? …before cell divisionPrior to replication, double helix must be unwoundThis is done by an enzyme called helicaseHelicase breaks up the hydrogen bonds that link the bases
13How DNA is copied…DNA Replication The point at which the DNA separates is called the replication fork (y shape)At the replication fork, new enzymes known as DNA polymerases move along each new strand adding nucleotides to exposed bases- forming two new double helicesDNA polymerases “proofread” to prevent errorsTypically, there are many replication forks along each linear DNA molecule
14Semi-conservative Replication Half of original DNA strand is “conserved” or used in the new strand.
15Two enzymes to know:Helicase – breaks the hydrogen bonds between bases to unwind the DNADNA polymerases – bring new nucleotides into place, proofreads new strands as they form