DNA Structure

SBI3U @ RHSA

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.

DNA is made of nucleotides

The basic unit of a nucleic acid is called a nucleotide.

Each nucleotide is made of three parts:

1. A sugar2. A phosphate group3. One of 4 nitrogen bases:

Adenine (A) Guanine (G)Cytosine (C)Thymine (T)

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!

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

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

James Watson and Francis Crick

Purine + purine: too wide

Pyrimidine + pyrimidine: too narrow

Purine + pyrimidine: DNA widthconsistent with X-ray data

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

Videos

http://www.youtube.com/watch?v=qy8dk5iS1f0 (Basic DNA structure)

http://www.youtube.com/watch?v=sf0YXnAFBs8 (Watson Crick discovery)