DNA - STRUCTUREThe Double Helix

DNA - Chemistry

Composed of 5 types of elements – Carbon, Hydrogen, Oxygen, Nitrogen and Phosphorus

Organized in three main components Phosphate group DeoxyRibose Nitrogenous base

DNA - Chemistry

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

DNA - Chemistry

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

DNA - Chemistry

4 different nucleotides

2 double ring bases called purines Adenine (A) Guanine (G)

2 single ring bases called pyrimidines Thymine (T) Cytosine (C)

DNA - Chemistry

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

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

X-Ray Diffraction - Franklin

DNA Base Pairing

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

Finding the Secret

DNA Structure

DNA Replication

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

Dispersive

Conservative

Conservative

Semi-Conservative

Meselson and Stahl

DNA - 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