Nucleic Acids: Structure, Function, and Key Concepts

Nucleic Acids Module Guide

Introduction

Nucleic acids are essential biomolecules that store and transmit genetic information in all living organisms. This module covers the structure and function of nucleic acids, including DNA and RNA, and explores the molecular basis of genetic inheritance and expression.

Learning Objectives

• Describe the structure of a nucleotide: A nucleotide consists of three components: a nitrogenous base, a five-carbon sugar (pentose), and one or more phosphate groups.

• Recognize and name the three components of a nucleotide: The three components are the nitrogenous base, pentose sugar, and phosphate group.

• Distinguish between a purine nitrogenous base and a pyrimidine nitrogenous base: Purines (adenine and guanine) have a double-ring structure, while pyrimidines (cytosine, thymine, and uracil) have a single-ring structure.

• Distinguish between a deoxyribose pentose and a ribose pentose: Deoxyribose (found in DNA) lacks an oxygen atom at the 2' carbon, while ribose (found in RNA) has an OH group at the 2' carbon.

• Identify the 3' carbon and the 5' carbon of a nucleotide: The 5' carbon is attached to the phosphate group, and the 3' carbon is involved in forming phosphodiester bonds.

• Recognize, name, draw, or label the structure of ATP: ATP (adenosine triphosphate) consists of adenine, ribose, and three phosphate groups.

• Name the base components of ATP: The base component is adenine.

• Describe the cellular function of ATP: ATP serves as the primary energy currency of the cell, providing energy for biochemical reactions.

• Recognize and explain the formation of a phosphodiester linkage: Phosphodiester bonds connect nucleotides in a polynucleotide chain, linking the 5' phosphate of one nucleotide to the 3' hydroxyl of another.

• Identify the free 3' carbon end and the free 5' carbon end of a dinucleotide or polynucleotide: Polynucleotides have directionality, with a free 5' phosphate group at one end and a free 3' hydroxyl group at the other.

• Recognize and describe the structure and function of DNA: DNA is a double-stranded helix composed of nucleotides, storing genetic information.

• Recognize and describe the structure and function of RNA: RNA is typically single-stranded and plays roles in gene expression and protein synthesis.

• Give the nitrogenous base complementary pairing in DNA: Adenine pairs with thymine, and guanine pairs with cytosine.

• Explain antiparallel orientation in DNA: The two strands of DNA run in opposite directions (5' to 3' and 3' to 5').

• Name the purine nitrogenous bases on DNA and RNA: Adenine and guanine.

• Name the pyrimidine nitrogenous bases on DNA and RNA: Cytosine, thymine (DNA only), and uracil (RNA only).

Key Terms and Definitions

• Gene: A segment of DNA that contains the instructions for synthesizing a specific protein or functional RNA molecule.

• Gene expression: The process by which information from a gene is used to synthesize a functional gene product, such as a protein or RNA.

• Nucleotide: The basic building block of nucleic acids, consisting of a nitrogenous base, a pentose sugar, and a phosphate group.

• Polynucleotide: A long chain of nucleotides linked by phosphodiester bonds, forming the backbone of DNA or RNA.

• Antiparallel orientation: The arrangement of the two strands of DNA in opposite directions, one running 5' to 3' and the other 3' to 5'.

Structure of Nucleic Acids

Nucleotide Structure

Nucleotides are the monomers of nucleic acids. Each nucleotide is composed of:

• Nitrogenous base: Purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil).

• Pentose sugar: Ribose in RNA, deoxyribose in DNA.

• Phosphate group: Attached to the 5' carbon of the sugar.

Example: ATP (adenosine triphosphate) is a nucleotide with three phosphate groups, used for energy transfer in cells.

Phosphodiester Linkage

Nucleotides are joined together by phosphodiester bonds, forming the backbone of DNA and RNA.

• The bond forms between the 3' hydroxyl group of one nucleotide and the 5' phosphate group of the next.

• This linkage gives nucleic acids directionality (5' to 3').

Equation:

$\text{Nucleotide}_1\ (3'\text{OH}) + \text{Nucleotide}_2\ (5'\text{P}) \rightarrow \text{Nucleotide}_1\text{-phosphodiester-}\text{Nucleotide}_2$

DNA Structure and Function

DNA is a double-stranded molecule with complementary base pairing and antiparallel orientation.

• Double helix: Two strands twisted around each other.

• Base pairing: Adenine pairs with thymine (A-T), guanine pairs with cytosine (G-C).

• Antiparallel: One strand runs 5' to 3', the other 3' to 5'.

• Function: Stores genetic information for cellular processes and inheritance.

RNA Structure and Function

RNA is usually single-stranded and contains ribose sugar and uracil instead of thymine.

• Types: mRNA (messenger), tRNA (transfer), rRNA (ribosomal).

• Function: Involved in gene expression and protein synthesis.

Classification of Nitrogenous Bases

Type	Bases	Found in
Purines	Adenine (A), Guanine (G)	DNA and RNA
Pyrimidines	Cytosine (C), Thymine (T), Uracil (U)	Thymine in DNA, Uracil in RNA

Complementary Base Pairing in DNA

Base	Complement
Adenine (A)	Thymine (T)
Guanine (G)	Cytosine (C)

ATP: Structure and Function

ATP Structure

• Components: Adenine (base), ribose (sugar), three phosphate groups.

• Function: Provides energy for cellular processes.

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Summary Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Strands	Double	Single
Bases	A, T, G, C	A, U, G, C
Function	Genetic information storage	Gene expression, protein synthesis

Additional info:

• Students are expected to be able to define and explain all listed terms and concepts for exams and assignments.

• Hand-written definitions and explanations are encouraged for deeper understanding.

• Required textbook reading: Campbell Biology in Focus, 4th edition, pages 66-68 (Concept 3.6).