Nucleic Acids: DNA
I. Introduction
Deoxyribonucleic acid (DNA) is the genetic material of most living organisms (except some viruses, which have RNA as their genetic material). It carries the hereditary information that determines the structure and function of all proteins, controlling the growth, development, and reproduction of cells. DNA provides the blueprint for life and ensures that genetic information is accurately passed from one generation to the next.
II. Structure of DNA
- DNA is a polymer of deoxyribonucleotides, each composed of three components — a deoxyribose sugar, a phosphate group, and a nitrogenous base.
- The bases are of two types: purines (adenine and guanine) and pyrimidines (cytosine and thymine).
- The DNA molecule is organized as a double helix, first described by James Watson and Francis Crick in 1953. It consists of two polynucleotide strands running in antiparallel orientation (one 5’→3’, the other 3’→5’), twisted around each other to form a right-handed helix.
- The sugar and phosphate groups form the backbone of the molecule, while the nitrogenous bases project inward, pairing with complementary bases from the opposite strand.
- Base pairing follows Chargaff’s rule, where adenine pairs with thymine by two hydrogen bonds, and guanine pairs with cytosine by three hydrogen bonds. This complementary pairing ensures accuracy during replication and stability of the molecule.
- The two strands are complementary and coiled around a common axis, with about 10 base pairs per turn in the B-form of DNA (the most common form in cells).
- The overall diameter of the double helix is uniform because each base pair consists of one purine and one pyrimidine, maintaining consistent width. The structure also features major and minor grooves, which serve as binding sites for proteins involved in replication and transcription.
III. Types of DNA
There are several structural forms of DNA, primarily A-DNA, B-DNA, and Z-DNA.
- B-DNA is the predominant form under physiological conditions, right-handed, and has 10 base pairs per turn.
- A-DNA is a more compact, right-handed form observed under dehydrated conditions.
- Z-DNA is a left-handed helix, rich in G-C pairs, and is believed to play a role in gene regulation.
Types of DNA
IV. DNA Synthesis (Replication)
The process by which DNA makes an exact copy of itself is called DNA replication. It occurs during the S phase of the cell cycle, ensuring that each daughter cell receives a complete copy of the genome.