Nucleic Acids and the RNA World: Structure, Function, and Replication

Chapter 4: Nucleic Acids and an RNA World

Overview

Nucleic acids are essential biological macromolecules that encode genetic information and enable its transmission across generations. This chapter explores the structure, function, and replication of nucleic acids, focusing on DNA and RNA.

Nucleic Acids: Structure and Types

What is a Nucleic Acid?

• Nucleic acids are polymers made of nucleotide monomers.

• Ribonucleotides are the monomers of RNA:

  • Contain ribose as their sugar.

  • Ribose has an –OH group bonded to the 2' carbon.

• Deoxyribonucleotides are the monomers of DNA:

  • Contain deoxyribose as their sugar ("deoxy" = lacking oxygen).

  • Deoxyribose has an H instead of an –OH at the 2' carbon.

• Both sugars have an –OH group bonded to the 3' carbon.

The General Structure of a Nucleotide

• Each nucleotide consists of three components:

  • Phosphate group (attached to the 5' carbon of the sugar)

  • 5-carbon sugar (ribose in RNA, deoxyribose in DNA)

  • Nitrogenous base (attached to the 1' carbon of the sugar)

The Different Sugars at Carbon 2'

Sugar	Found in	Group at 2' Carbon
Ribose	RNA	–OH
Deoxyribose	DNA	H

Polymerization of Nucleotides

Polymerization: Bonding of Monomers

• Nucleotides are joined together to form nucleic acid polymers through condensation reactions (also called dehydration synthesis).

• During polymerization, a phosphodiester bond forms between the phosphate group of one nucleotide and the 3' –OH group of another.

• Water is released as a byproduct of this reaction.

Biological Polymers: Condensation and Hydrolysis

Reaction Type	Description	Result
Condensation	Monomers join, water is released	Polymer formation
Hydrolysis	Water added, polymer breaks down	Monomer release

Nucleotides Polymerize Via Condensation Reactions

• Phosphodiester linkages connect nucleotides in a chain.

• Directionality: Nucleic acids have a 5' end (phosphate group) and a 3' end (–OH group).

Primary and Secondary Structure of Nucleic Acids

Primary Structure

• The primary structure of DNA or RNA is the sequence of nucleotides.

• Sequence is written from 5' to 3' direction.

Complementary Base Pairing

• Base pairing is based on hydrogen bonding between specific nitrogenous bases.

• Purines (adenine, guanine) pair with pyrimidines (thymine, cytosine, uracil).

• In DNA:

  • Adenine (A) pairs with Thymine (T): 2 hydrogen bonds

  • Guanine (G) pairs with Cytosine (C): 3 hydrogen bonds

• In RNA:

  • Adenine (A) pairs with Uracil (U)

Secondary Structure of DNA: The Double Helix

• DNA's secondary structure is a double helix formed by two antiparallel strands.

• Strands are held together by complementary base pairing and hydrogen bonds.

• The helical structure provides stability and allows for efficient storage of genetic information.

DNA as an Information-Containing Molecule

DNA Replication

• DNA replication is the process by which DNA makes a copy of itself during cell division.

• Three main steps:

  1. Strand separation: Hydrogen bonds between base pairs are broken, separating the two strands.

  2. Base pairing: Each strand serves as a template for the formation of a new complementary strand. Free nucleotides pair with exposed bases.

  3. Polymerization: New strands are synthesized by forming phosphodiester bonds, restoring the double helix structure.

Template and Complementary Strands

• The original DNA strand serves as a template strand for the synthesis of a complementary strand.

• Replication is semiconservative: each new DNA molecule contains one old strand and one new strand.

Summary Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Structure	Double helix	Single strand (can form secondary structures)
Function	Genetic information storage	Protein synthesis, gene regulation, catalysis

Key Equations and Concepts

• Phosphodiester bond formation:

$\text{Nucleotide}_1\text{–OH} + \text{Nucleotide}_2\text{–PO}_4 \rightarrow \text{Nucleotide}_1\text{–O–PO}_2\text{–Nucleotide}_2 + \text{H}_2\text{O}$

• Base pairing rules:

$\text{A} \equiv \text{T} \quad (2\ \text{hydrogen bonds})\\ \text{G} \equiv \text{C} \quad (3\ \text{hydrogen bonds})$

Example: DNA Replication

During DNA replication, the sequence 5'-ATGC-3' on the template strand will guide the synthesis of a complementary strand with the sequence 3'-TACG-5'.

Additional info: RNA can also have catalytic functions (ribozymes) and play roles in gene regulation (e.g., microRNAs).