DNA Structure DNA was discovered in 1869 By the early 1900s people knew that DNA was a very long molecule found in the nucleus of cells In 1919 it was discovered that each molecule of DNA is made up of millions of tiny subunits called nucleotides.

Nucleotide Structure Each nucleotide consists of: 1. Phosphate group – PO Pentose sugar – 5 carbon 3. Nitrogenous base – contains nitrogen

Nucleotides Phosphate Pentose Sugar Nitrogenous Base

4 Nucleotides A Adenine T Thymine G Guanine C Cytosine

By the mid 1900s the race to figure out DNA’s structure was on! Scientists knew the following things about DNA: It’s the hereditary material It’s made of 4 types of nucleotides It’s a long chain of these nucleotides It’s linked to the production of proteins

What scientists didn’t know How the nucleotides are connected The importance of the 4 bases for carrying information So they played around with potential structures, like playing with Legos

A critical piece of evidence: patterns discovered in base pairing What does this indicate? Species are different in the pattern of bases The amount of A matches the amount of T and the amount of C matches the amount of G

Nitrogen bases have to pair a certain way! Each base will only bond with one other specific base. Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Form a base pair.

But how could the nucleotides fit together? This x-ray picture of DNA by Rosalind Franklin was the last piece in the DNA puzzle—it showed James Watson and Francis Crick that DNA must be a double helix.

Double helix

Why do bases pair like this? 1.To fit: A 2-ring base has to pair with a 1-ring base 2.To allow hydrogen bonding: A and T: 2 bonds C and G: 3 bonds

DNA Structure Because of this complementary base pairing, the order of the bases in one strand determines the order of the bases in the other strand.

G G A T T A A C T G C A T C