Nucleic Acids

Chargaff’s Rules and DNA Composition

Chargaff’s rules describe the base composition of DNA, which was foundational for the discovery of DNA’s double helical structure by Watson and Crick.

• Chargaff’s Rules: In double-stranded DNA, the amount of adenine (A) equals thymine (T), and the amount of guanine (G) equals cytosine (C):

• DNA is a double-stranded molecule with the base composition:

Discovery of DNA Structure

• Watson and Crick used X-ray crystallography data from Rosalind Franklin to determine the double helix structure of DNA.

• The X-ray diffraction images revealed a helical structure with a diameter of approximately 2 nm and a helical repeat every 3.4 nm.

Double Helix Structure

• The structure of DNA is called a double helix.

• It contains a major groove and a minor groove, which provide access to the nucleotide bases for protein binding and regulation.

• Proteins interact with DNA via these grooves, often recognizing specific base sequences.

Base Pairing

• DNA bases pair via hydrogen bonds: Adenine pairs with Thymine (A-T), and Guanine pairs with Cytosine (G-C).

• Base pairs are stabilized by hydrogen bonding and base stacking interactions.

Helical Parameters

• Each turn of the DNA helix contains about 10 base pairs and spans 3.4 nm.

• The diameter of the helix is about 2 nm.

Chemical Structure of DNA

• The backbone of DNA consists of alternating sugar (deoxyribose) and phosphate groups, connected by phosphodiester bonds.

• All bonds in the DNA backbone are single bonds, allowing for flexibility and helical twisting.

Nucleotides and Base Conformation

• Nucleotides are the monomeric units of DNA, each composed of a phosphate group, deoxyribose sugar, and a nitrogenous base.

• Nucleotides can have different conformations, but in B-form DNA (the most common form in cells), the bases are in the anti conformation.

Example: B-Form DNA

• B-form DNA is the predominant conformation in cells, characterized by a right-handed helix and anti conformation of bases.

Additional info: The anti conformation refers to the orientation of the nitrogenous base relative to the sugar in the nucleotide, which is important for proper base pairing and helix stability.