The Structure of DNA: Nucleic Acids, Nucleotides, and DNA Organization

The Structure of DNA

Introduction

This chapter covers the molecular structure of DNA, focusing on the building blocks of nucleic acids, the organization of DNA, and the chemical and physical properties that underlie its function in heredity and information storage.

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) and ribonucleic acid (RNA).

• DNA and RNA store and transmit hereditary information.

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

• Example: DNA encodes genetic instructions; RNA is involved in protein synthesis.

Nucleotides

• A nucleotide consists of three components:

  • Pentose sugar (a five-carbon sugar: ribose in RNA, deoxyribose in DNA)

  • Nitrogenous base (five types: adenine, guanine, cytosine, thymine, uracil)

  • Phosphate group

• Example: The nucleotide adenosine triphosphate (ATP) contains adenine, ribose, and three phosphate groups.

Pentose Sugar

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

• The carbon atoms in the sugar are numbered 1' to 5'.

• The 3' and 5' carbons are important for forming the backbone of nucleic acids.

• Example: The absence of an -OH group at the 2' position distinguishes DNA from RNA.

Nitrogenous Bases

• Nitrogenous bases are classified as purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil).

• DNA contains adenine (A), guanine (G), cytosine (C), and thymine (T).

• RNA contains adenine (A), guanine (G), cytosine (C), and uracil (U).

• Example: Thymine is unique to DNA, while uracil is unique to RNA.

Phosphate Group

• The phosphate group gives nucleic acids their acidic properties and negative charge at physiological pH.

• Phosphate groups form high-energy bonds and are responsible for the name "nucleic acid."

• DNA is a negatively charged molecule due to its phosphate backbone.

• Example: The energy stored in the phosphate bonds of ATP is used in cellular processes.

Nucleosides and Nucleotides

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

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

• Example: Adenosine is a nucleoside; adenosine monophosphate (AMP) is a nucleotide.

Nucleotide Nomenclature

Naming Conventions

• Nucleosides are named as follows:

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

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

• Nucleotides are named by adding the number and type of phosphate (e.g., adenosine triphosphate, deoxyguanosine monophosphate).

• Example: dATP stands for deoxyadenosine triphosphate.

Table: Nitrogenous Bases in DNA and RNA

Base	Type	Present in DNA?	Present in RNA?
Adenine (A)	Purine	Yes	Yes
Guanine (G)	Purine	Yes	Yes
Cytosine (C)	Pyrimidine	Yes	Yes
Thymine (T)	Pyrimidine	Yes	No
Uracil (U)	Pyrimidine	No	Yes

Additional info:

• Further sections (not shown in the images) likely cover DNA secondary structure, DNA forms, supercoiling, and topoisomerases, as outlined in the chapter summary.