1 DNA

2 DNA Stands for “Deoxyribose Nucleic Acid” Holds the genetic information that determines an organisms traits by way of proteins Long molecule called a polymer Made up of repeating subunits called nucleotides

3 Nitrogen Bases 4 kinds of bases: Purines: Adenine (A) & Guanine (G) Pyrimidines: Thymine (T) & Cytosine (C) In DNA the bases pair A – T & C – G A purine ALWAYS bonds with a pyrimidine by a hydrogen bond

4 Nitrogenous Bases Double ring PURINESDouble ring PURINES Adenine (A) Guanine (G) Single ring PYRIMIDINESSingle ring PYRIMIDINES Thymine (T) Cytosine (C) T or C A or G

5 Structure of DNA Nucleotides have 3 parts: a simple sugar, a phosphate group, and a nitrogen base –Simple sugar: Deoxyribose sugar –Phosphate group: one atom of phosphorous surrounded by 4 oxygen atoms –Nitrogen base: carbon ring structure that contains 1 or more atoms of nitrogen

6 DNA Nucleotide Phosphate Nitrogenous base (A, G, C, or T) (A, G, C, or T) Sugar

7 DNA Structure Two coiled strands called a double helix Sides made of a sugar bonded to phosphate molecule—sugar phosphate backbone Center made of nitrogen bases bonded together by weak hydrogen bonds

8 DNA Structure

9 Where Did it Come From? Rosalind Franklin took diffraction x-ray photographs of DNA crystals In the 1950’s, Watson & Crick built the first model of DNA using Franklin’s x- rays They proposed that DNA is made up of 2 chains of nucleotides joined by the nitrogen bases

10 Where Did it Come From? They also thought that the DNA was twisted or coiled, forming a “helix” shape Knowing that DNA was composed of two strands they called it a “double helix”

11 So How Does It Work? Nucleotides join together to form the long chains The phosphate groups and sugar molecules form the backbone of the chain and the nitrogen bases stick out (like “teeth” in a zipper) The phosphate group of one nucleotide ALWAYS bonds to a deoxyribose sugar of another

12 DNA Double Helix Nitrogenous Base (A,T,G or C) “Rungs of ladder” “Legs of ladder” Phosphate & Sugar Backbone