The Double Helix

 Composed of 5 types of elements – Carbon, Hydrogen, Oxygen, Nitrogen and Phosphorus  Organized in three main components  Phosphate group  DeoxyRibose  Nitrogenous base

 Orientation of the DNA molecule is based on the number of the carbon atoms in the sugar part of the nucleotide  Based on this; every molecule has a 3’ and 5’ side

 Phosphate is attached to the 5’ carbon  The nitrogenous base is attached to the 1’ carbon  Phosphate is joined to sugar by an ester bond  Nitrogenous base is joined by a glycosidic bond

 4 different nucleotides  2 double ring bases called purines  Adenine (A)  Guanine (G)  2 single ring bases called pyrimidines  Thymine (T)  Cytosine (C)

 DNA is a polymer created by the linking of bases using a phosphodiester bond  Link is between the phosphate of one base (5’) to the 3’ carbon of its neighbour  Dehydration synthesis

 DNA structure was determined by Watson and Crick using information discovered by Rosalind Franklin  DNA consists of two strands of linked bases  Each strand is a right handed helix

 Hydrogen bonding between specific bases in opposite strands keeps the pair of strands together  Since both are right handed strands, they have to pair up in opposite directions..\..\Bio Graphics\DNA and Genetics\25- basepairing.mp4

 With the discovery of base pairing explaining DNA replication becomes a matter of finding mechanisms for getting new nucleotides to form in the required pattern  3 possible scenarios;  Dispersive replication  Conservative replication  Semi-Conservative replication

 Involves several steps  Each step is mediated by a protein or an enzyme  Helicase – opens the DNA at the point of replication – Replication Fork  DNA-Polymerase – attach to the template molecule of DNA and add bases using pairing rules  DNA-Ligase – joins the new nucleotides together to create the new strand