The Structure of DNA

Introduction

The structure of DNA is fundamental to understanding molecular biology. DNA and RNA are nucleic acids that store and transmit genetic information in all living organisms. This chapter covers the chemical building blocks of DNA and RNA, their nomenclature, and the organization of these molecules into higher-order structures.

Nucleic Acid and Nucleotides

Nucleic Acids

• Nucleic acids are large biomolecules essential for all known forms of life.

• There are two main types:

  • Deoxyribonucleic acid (DNA): Stores genetic information.

  • Ribonucleic acid (RNA): Involved in protein synthesis and gene regulation.

• Nucleic acids are polymers made up of monomers called nucleotides.

Nucleotides: Structure and Components

• Each nucleotide consists of three components:

  1. Pentose sugar (five-carbon sugar): Either ribose (in RNA) or deoxyribose (in DNA).

  2. Nitrogenous base: Five types—adenine (A), guanine (G), cytosine (C), thymine (T, in DNA), and uracil (U, in RNA).

  3. Phosphate group: Provides the acidic property and negative charge to nucleic acids.

Pentose Sugar

• The sugar in DNA is 2'-deoxyribose; in RNA, it is ribose.

• The carbon atoms in the sugar are numbered 1' to 5'. The 5' and 3' carbons are important for the directionality of nucleic acids.

Nitrogenous Bases

• There are two categories:

  • Purines: Adenine (A) and Guanine (G) – double-ring structures.

  • Pyrimidines: Cytosine (C), Thymine (T, in DNA), and Uracil (U, in RNA) – single-ring structures.

Phosphate Group

• The phosphate group is attached to the 5' carbon of the sugar.

• At physiological pH, the phosphate group is negatively charged, making DNA and RNA acidic and negatively charged molecules.

• Phosphate groups contain high-energy bonds, especially in nucleoside triphosphates (e.g., ATP).

Nucleosides and Nucleotides

• A nucleoside consists of a nitrogenous base linked to a five-carbon sugar.

• A nucleotide is a nucleoside with one or more phosphate groups attached.

Nucleotide Nomenclature

Rules of Nomenclature

• Nucleoside names:

  • Base + sugar (for purines): osine suffix (e.g., adenosine, guanosine).

  • Base + sugar (for pyrimidines): idine suffix (e.g., cytidine, thymidine, uridine).

• Nucleotide names:

  • Add the name of the phosphate (e.g., adenosine monophosphate, AMP).

  • For deoxyribonucleotides, add 'deoxy' (e.g., deoxyadenosine monophosphate, dAMP).

Summary Table: Nitrogenous Bases in DNA and RNA

Key Points and Applications

• Nucleic acids are essential for genetic information storage and transfer.

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

• The phosphate group imparts a negative charge and is involved in the formation of the nucleic acid backbone.

• Nomenclature distinguishes between nucleosides (base + sugar) and nucleotides (nucleoside + phosphate).

• Understanding the structure and naming of nucleotides is foundational for studying DNA replication, transcription, and other molecular biology processes.

Example: ATP (Adenosine Triphosphate)

• ATP is a nucleotide consisting of the base adenine, the sugar ribose, and three phosphate groups.

• It serves as the primary energy currency in the cell.

Additional info: Later sections (not shown in these slides) would cover polymerization, secondary structure, DNA forms, supercoiling, and topoisomerases, which are also essential for a full understanding of DNA structure and function.