 DNA (deoxyribonucleic acid) is a two stranded molecule called double helix  Each strand are made of smaller parts called nucleotides  The two strands are bonded with weak hydrogen bonds Structure of DNA

 Nucleotides have 3 parts: deoxyribose (sugar) phosphate group Nitrogenous base (A,C,G,T)

 Double Rings called PURINES Adenine (A) Guanine (G)  Single Ring called PYRIMIDINES Thymine (T) Cytosine (C) A or G T or C

 Adenine bonds with Thymine using two hydrogen bonds T A

 Guanine bonds with Cytosine using three hydrogen bonds CG 3 H-bonds

 Base-Pairing Rules (1950) Erwin Chargaff discovered DNA from any species had equal amounts C’s and G’s, and equal amounts A’s and T’s In human DNA  A = 30%  T = ________________  C = 20%  G = ________________

 Structure of DNA (1953) Rosalind Franklin takes diffraction x-ray photographs of DNA Watson and Crick build the first model of DNA using Franklin’s pictures

Discovery of DNA

 When a cell divides (mitosis and meiosis) chromosomes (DNA) needs to divide to create  DNA division happens during Interphase  Carried by a series of enzymes (proteins)

 First, DNA is “unzipped” along the hydrogen bonds by DNA helicase  Each strand is a template to add new nucleotides

 DNA polymerase adds new nucleotides  Nucleotides are added using Base Pairing Rules A – T C - G

 New DNA consists of one parent (original) strand and one new strand of DNA

DNA Replication

 What would be the complementary DNA strand for the following DNA sequence?  Use base pairing rules A - T C - G CGTATG

Original Strand:CGTATG New Strand:GCATAC