Nucleic Acids: Structure and Properties of DNA

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):

$[A] = [T]$ $[G] = [C]$

• DNA is a double-stranded molecule with the base composition: $[A] + [G] = [T] + [C]$

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.

Feature	Description
Helix Diameter	2 nm
Helical Repeat	3.4 nm (10 base pairs per turn)
Grooves	Major and minor grooves provide access for protein binding
Base Pairing	A-T (2 H-bonds), G-C (3 H-bonds)

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.