1. DNA Structure
RHSA

2. What is DNA?
In 1953 James Watson and Francis Crick developed a double helix model for DNA.
DNA consists of 2 strands of nucleic acids that twist around each other.

3. DNA is made of nucleotides
The basic unit of a nucleic acid is called a nucleotide.
Each nucleotide is made of three parts:
A sugar
A phosphate group
One of 4 nitrogen bases:
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)

4. Antiparallel strands
The two nucleic acid strands are antiparallel to each other. That means that one strand is “upside down” compared to the other.
Look at the sugar shape!

5. Antiparallel Strands
The 2 nucleic acid strands are held together by hydrogen bonds between the nitrogen bases.
When the nitrogen bases bond together they follow a specific pairing rule called complementary base pairing where:
Adenine - thymine
Cytosine - guanine

6. James Watson and Francis Crick
Watson and Crick built models of a double helix to match the X-rays and chemistry of DNA.
Rosalind Franklin had concluded that there were two antiparallel sugar-phosphate backbones, with the nitrogen bases paired in the DNA molecule’s interior.
At first, Watson and Crick thought the nitrogen bases paired like with like (A with A, and so on), but such pairings did not result in a uniform width
Instead, pairing a purine (two rings) with a pyrimidine (one ring) resulted in a uniform width of DNA consistent with the X-ray

7. Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: DNA width
consistent with X-ray data

8. Watson and Crick reasoned that the nitrogen base pairing was more specific.
They determined that adenine paired only with thymine, and guanine paired only with cytosine

9. Videos
(Basic DNA structure)
(Watson Crick discovery)