Introduction

After learning about the scientists that successfully identified DNA as the molecule of heredity and its structure, let us put it all together.

Learning

Recall from earlier modules that nucleic acids, including DNA, are composed of subunits called Nucleotides.

Nucleotides consist of a 5-Carbon sugar ring with a phosphate group attached to a nitrogenous base. The nitrogenous base defines the type of nucleotide.

Biochemist Erwin Chargaff was the first to decipher the relationship between the 4 nucleotides present in the DNA molecule. He looked at the ratio or percentage of each nucleotide present in DNA extracts.

Chargaff was the one who determined that the amount of adenine equals the amount of thymine and the amount of cytosine equals the amount of guanine.

These became known as ‘Chargaff’s Rule’:

• % A = % T
• % G = % C

Therefore, the DNA structure has two strands of nucleotides held together with weak ‘hydrogen bonds’ between the nitrogenous bases (A bonded to T; G bonded to C).

In addition, the two strands wrap around each other to form a structure we call a ‘double helix’ – double due to the two strands present, and helix indicating a spiral structure.

Summary

DNA consists of 4 Nucleotides:

• Adenine
• Thymine
• Cytosine
• Guanine

Chargaff’s rule states that in a DNA molecule A will always pair with T and G will always pair with C.

X-ray crystallography was used to determine that the DNA molecule had the following properties:

• Helical structure
• 3.4 nanometers/turn of the helix.
• 10 units/turn of the helix.
• 2 nanometers wide

Therefore, DNA is organized as a double helix, with two nucleotide chains held together with weak hydrogen bonds between complementary nucleotides.

Sources:

“DNA Structure and Sequencing. “By Biology 2e. Retrieved from https://openstax.org/books/biology-2e/pages/14-2-dna-structure-and-sequencing/. Licensed under: CC-BY: Attribution