1. KEY CONCEPT DNA structure is the same in all organisms.

2. DNA = Deoxyribonucleic Acid
DNA is made up of a long chain of nucleotides.
Each nucleotide has three parts.
5-carbon sugar called deoxyribose
Phosphate group
Nitrogen base
phosphate group
deoxyribose (sugar)
nitrogen-containing
base
One Nucleotide

3. There are only four Nitrogen bases:

4. Nucleotides always pair the same way.
Nitrogen bases always pair up in DNA like this:
A T
G C
A pyrimidine (single ring) pairs with a purine (double ring) so the helix has a uniform width. C G T A

5. The sides are connected by strong covalent bonds.
The N-bases are connected by weak hydrogen bonds.
Hydrogen bonds
Sugar to phosphate bonds

6. Watson and Crick determined the structure of DNA by building models.
They realized that DNA is a double helix.