Nucleic Acids: Structure, Function, and Polymerization (Sections 4.1–4.3)

Nucleic Acids

Introduction to Nucleic Acids

Nucleic acids are essential biomolecules that store and transmit genetic information in all living organisms. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• DNA contains the genetic blueprint for cellular functions and heredity.

• RNA plays roles in protein synthesis and gene regulation.

Nucleotides and Nucleosides

Structure of Nucleotides

Nucleotides are the monomers that make up nucleic acids. Each nucleotide consists of three components:

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

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

• Nitrogenous base (adenine, guanine, cytosine, thymine in DNA; uracil replaces thymine in RNA)

The difference between ribose and deoxyribose is the presence (ribose) or absence (deoxyribose) of a hydroxyl group at the 2' carbon.

Nitrogenous Bases: Purines and Pyrimidines

• Purines: Double-ring structures; adenine (A) and guanine (G)

• Pyrimidines: Single-ring structures; cytosine (C), thymine (T) (in DNA), and uracil (U) (in RNA)

DNA contains adenine, guanine, cytosine, and thymine. RNA contains adenine, guanine, cytosine, and uracil.

Nucleoside Nomenclature

Nucleosides are molecules consisting of a nitrogenous base and a sugar, without the phosphate group. When one or more phosphate groups are added, they become nucleotides.

Name	Structure	Function
Adenosine triphosphate (ATP)	Adenine + ribose + 3 phosphates	Main energy currency of the cell
Adenosine diphosphate (ADP)	Adenine + ribose + 2 phosphates	Intermediate in energy transfer
Adenosine monophosphate (AMP)	Adenine + ribose + 1 phosphate	Component of RNA and energy metabolism

Polymerization of Nucleic Acids

Condensation and Hydrolysis Reactions

Nucleic acids are formed by joining nucleotides through condensation reactions (also called dehydration synthesis), which release water as a byproduct. The reverse process, hydrolysis, breaks polymers into monomers by adding water.

• Condensation reaction: Monomer in, water out

• Hydrolysis: Water in, monomer out

Equation for condensation:

$\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + H_2O$

Equation for hydrolysis:

$\text{Polymer} + H_2O \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

Phosphodiester Linkages

Nucleotides are joined by phosphodiester bonds between the 3' hydroxyl group of one sugar and the 5' phosphate group of the next. This forms the sugar-phosphate backbone of nucleic acids.

• Directionality: Nucleic acid strands have a 5' end (phosphate) and a 3' end (hydroxyl).

• New nucleotides are added to the unlinked 3' hydroxyl group.

ATP: Energy Carrier

ATP Hydrolysis and Energy Release

Adenosine triphosphate (ATP) is a nucleotide that stores and releases energy for cellular processes. Hydrolysis of ATP removes a phosphate group, releasing energy:

$\text{ATP} + H_2O \rightarrow \text{AMP} + 2\text{P}_i + \text{Energy}$

• Energy released is used to drive many biochemical reactions.

Identifying DNA and RNA Components

Components Found in DNA and RNA

Component	Found in DNA?	Found in RNA?
Adenine	Yes	Yes
Cytosine	Yes	Yes
Guanine	Yes	Yes
Thymine	Yes	No
Uracil	No	Yes
Deoxyribose	Yes	No
Ribose	No	Yes

Summary Table: Purines vs. Pyrimidines

Type	Bases	Structure
Purines	Adenine, Guanine	Double-ring
Pyrimidines	Cytosine, Thymine, Uracil	Single-ring

Additional info:

• These notes cover the fundamental structure and function of nucleic acids, including the chemical basis for DNA and RNA, and the role of ATP in energy transfer.

• Condensation reactions are responsible for polymer formation, while hydrolysis breaks polymers down.

• Phosphodiester bonds are covalent and form the backbone of nucleic acid polymers.