Nucleic Acids: Structure, Function, and the Molecular Basis of Genetic Information

Nucleic Acids and the Information of Life

Nucleic acids are essential biological macromolecules that store and transmit the genetic information necessary for life. This chapter explores the structure and function of nucleic acids, focusing on DNA and RNA, and their roles in heredity, catalysis, and the origin of life.

Main Topics Overview

• What is a nucleic acid?

• DNA structure and function (specialized for stability and storage)

• RNA structure and function (specialized for versatility and catalysis)

• Could life have evolved from an RNA world?

Biological Polymers and Macromolecules

Cells are composed of large molecules called macromolecules, which are polymers made from smaller subunits (monomers). The two major types relevant here are proteins and nucleic acids.

MOLECULE	MONOMER	FUNCTION
Proteins	Amino Acids	Enzymes, Receptors, Transporters, Structural roles
Nucleic Acids	Nucleotides	Storage and Use of Genetic Information

What Is a Nucleic Acid?

Nucleic acids are polymers made up of monomers called nucleotides. Each nucleotide consists of three components:

• A phosphate group

• A five-carbon sugar (either ribose in RNA or deoxyribose in DNA)

• A nitrogenous base

There are two main types of nucleic acids:

• Ribonucleic acid (RNA): contains ribose sugar

• Deoxyribonucleic acid (DNA): contains deoxyribose sugar

Nitrogenous Bases

• Pyrimidines: cytosine (C), uracil (U, in RNA only), thymine (T, in DNA only)

• Purines: adenine (A), guanine (G)

Uracil is found only in RNA, while thymine is found only in DNA.

Structure of Nucleic Acids

Primary Structure

The primary structure of a nucleic acid is the sequence of its nitrogenous bases. This sequence encodes genetic information.

• In DNA, the sequence is written from the 5' to 3' direction, reflecting the orientation of the sugar-phosphate backbone.

Polymerization and the Sugar-Phosphate Backbone

Nucleic acids form when nucleotides polymerize via condensation reactions, creating phosphodiester linkages between the 5' phosphate group of one nucleotide and the 3' hydroxyl group of another.

The backbone is directional (5' to 3'), and the sequence of bases constitutes the primary structure.

ATP: A Substrate to Make RNA

Adenosine triphosphate (ATP) is a nucleotide that serves as a substrate for RNA synthesis and as an energy carrier in cells. The addition of phosphate groups increases the potential energy of the molecule.

DNA Structure and Function

Secondary Structure

The secondary structure of DNA is a double helix formed by two antiparallel strands held together by complementary base pairing:

• Adenine (A) pairs with thymine (T) via two hydrogen bonds

• Guanine (G) pairs with cytosine (C) via three hydrogen bonds

The sugar-phosphate backbone is on the outside, and the nitrogenous bases are on the inside. The double helix has major and minor grooves, which are important for protein binding.

Chargaff's Rules

• The total number of purines equals the total number of pyrimidines.

• The number of A's equals T's, and the number of G's equals C's.

Base Pairing and Helix Structure

• Only purine-pyrimidine pairs fit within the helix (purine-purine is too large, pyrimidine-pyrimidine is too small).

Function: Storage and Replication of Genetic Information

DNA's structure allows it to store and replicate genetic information. The sequence of bases encodes the instructions for cell growth and reproduction.

DNA Replication

Complementary base pairing enables each strand to serve as a template for the synthesis of a new strand. Replication involves:

1. Separation of the double helix

2. Hydrogen bonding of free nucleotides to the template strand

3. Formation of phosphodiester bonds to create the new strand

Stability and Catalytic Inactivity

DNA is highly stable and resistant to chemical degradation, making it an excellent long-term store of genetic information. However, this stability means DNA is not a catalyst and does not participate in chemical reactions as enzymes do.

RNA Structure and Function

Primary and Secondary Structure

RNA, like DNA, has a sugar-phosphate backbone and a sequence of four nitrogenous bases. However, RNA contains uracil (U) instead of thymine and ribose instead of deoxyribose. The presence of the 2'-OH group in ribose makes RNA more reactive and less stable than DNA.

RNA's secondary structure arises from complementary base pairing within the same strand, forming structures such as hairpins and short double helices.

Tertiary and Quaternary Structure

RNA molecules can fold into complex three-dimensional shapes (tertiary structure) and can associate with other molecules (quaternary structure), enabling diverse functions.

Versatility and Catalysis

• RNA can store information and self-replicate.

• Some RNA molecules, called ribozymes, have catalytic activity, similar to protein enzymes.

This versatility suggests that RNA may have played a key role in the origin of life (the "RNA world" hypothesis).

Comparison of DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strandedness	Double-stranded (usually)	Single-stranded (usually)
Stability	High	Lower
Function	Information storage	Information transfer, catalysis

Key Terms and Definitions

• Nucleotide: The monomer unit of nucleic acids, consisting of a phosphate group, a five-carbon sugar, and a nitrogenous base.

• Phosphodiester bond: The covalent bond linking nucleotides in a nucleic acid chain.

• Antiparallel: Refers to the opposite orientation of the two strands in DNA.

• Complementary base pairing: The specific pairing of A with T (or U in RNA) and G with C.

• Ribozyme: An RNA molecule with catalytic activity.

Key Equations

• Phosphodiester bond formation (condensation reaction):

$\text{Nucleotide}_1\text{-OH} + \text{Nucleotide}_2\text{-phosphate} \rightarrow \text{Nucleotide}_1\text{-phosphodiester-Nucleotide}_2 + \text{H}_2\text{O}$

• Chargaff's Rule:

$\text{[A]} = \text{[T]}$ $\text{[G]} = \text{[C]}$

Summary

• Nucleic acids are polymers of nucleotides that store and transmit genetic information.

• DNA is specialized for stability and long-term information storage; RNA is more versatile and can act as a catalyst.

• The structure of nucleic acids underlies their function in heredity, catalysis, and possibly the origin of life.