1. The Structure of DNA
The building block of DNA (and RNA) is the nucleotide.
Each nucleotide has 3 parts:
A sugar (deoxyribose in DNA)
A phosphate group
A nitrogenous base
There are 4 possible nitrogenous bases in DNA.

2. The 4 nitrogenous bases in DNA
nucleotides are:
Adenine
Thymine
Cytosine
Guanine

3. The nitrogenous bases (A, G, C and T) are the parts of the DNA molecule that encode genetic information.

4. In DNA, the nucleotides are arranged in double helix: 2 strands connected together and twisted like a spiral staircase. Watson and Crick (with data from Rosalind Franklin) determined this structure of DNA in the 1950’s. This knowledge helped scientists understand the role of DNA in heredity.

5. In the double helix, the sugar and phosphate group of the nucleotide form the backbone. Pairs of nitrogenous bases (A, C, T or G) form the “rungs” of the double helix. Adenine always pairs with Thymine Cytosine always pairs with Guanine

6. DNA Structure Video Click once on image to start

7. Build a DNA Molecule
Click here to build a DNA double helix

8. Applying the Concepts T T G C G A T A
If one strand of a DNA molecule has a base sequence of AACGCTAT what with the other strand be? A A C G C T A T T T G C G A T A

9. Applying the Concepts: Chargaff’s Rule
In a sample of DNA, 20% of the bases are adenine. What percentage are T, C, and G? Thymine = Cytosine = Guanine =
Adenine and Thymine always pair,
so equal amounts.
20%
30%
If 40% is A and T, remaining 60% is C and G, divided in equal amounts since they pair.
30%