Nucleic Acids: Structure, Function, and Applications

Topic 2 – 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 deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These macromolecules are polymers composed of repeating nucleotide units and play a central role in heredity, gene expression, and cellular function.

• DNA: The genetic material in most organisms, responsible for storing hereditary information.

• RNA: Functions in gene expression and regulation, with several types involved in protein synthesis and other cellular processes.

The Basic Building Blocks of Nucleic Acids

The fundamental monomeric unit of nucleic acids is the nucleotide. Each nucleotide consists of three components:

• Phosphate group

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

• Nitrogenous base (Adenine, Guanine, Cytosine, Thymine [DNA], or Uracil [RNA])

A nucleoside is a molecule consisting of only the pentose sugar and nitrogenous base, without the phosphate group.

Nitrogenous Bases

• Pyrimidines: Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA)

• Purines: Adenine (A), Guanine (G)

These bases pair specifically in nucleic acids: A with T (or U in RNA), and G with C.

Structure of DNA: The Double Helix

DNA is composed of two antiparallel strands forming a right-handed double helix. The backbone consists of alternating sugar and phosphate groups, with the nitrogenous bases projecting inward to form base pairs.

• Base pairing: Adenine pairs with Thymine via two hydrogen bonds; Guanine pairs with Cytosine via three hydrogen bonds.

• Stabilization: The double helix is stabilized by hydrogen bonding between complementary bases and hydrophobic interactions (base stacking).

• Phosphodiester bonds: Link the 3' carbon of one sugar to the 5' carbon of the next, forming the sugar-phosphate backbone.

Example: The classic Watson & Crick model (1953) described the double helix structure, which was supported by X-ray diffraction data.

Structure of RNA

RNA is typically single-stranded but can form complex secondary structures through intramolecular base pairing. The sugar in RNA is ribose, and uracil replaces thymine as a base.

• Types of RNA: Messenger RNA (mRNA), Transfer RNA (tRNA), Ribosomal RNA (rRNA), Catalytic RNA (cRNA), Small inhibitory RNA (siRNA)

• Function: RNA molecules are involved in protein synthesis, gene regulation, and catalysis.

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a biological system:

• Replication: DNA is copied to produce identical DNA molecules.

• Transcription: DNA is transcribed into RNA.

• Translation: RNA is translated into protein.

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Genes and the Genome

A gene is a segment of DNA that encodes the sequence of a polypeptide or a functional RNA molecule. The genome is the total amount of DNA in an organism.

• Human cells contain approximately 25,000 different genes.

• The Human Genome Project determined the human genome to be about 3,234.83 megabases (Mb) in size.

• Only about 30% of the genome consists of coding sequences, and only ~1.5% are exons (the portions that code for proteins).

Exon: A region of a gene that codes for a part of the final mature RNA product, after introns are removed by RNA splicing.

Types and Functions of RNA

• Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes for protein synthesis.

• Ribosomal RNA (rRNA): Structural and functional component of ribosomes.

• Transfer RNA (tRNA): Brings amino acids to the ribosome during translation.

• Other RNAs: Catalytic RNA (cRNA), Small inhibitory RNA (siRNA) involved in gene regulation.

Self-Replication and Information Storage

Nucleic acids are capable of self-replication, a property essential for heredity. The sequence of bases encodes genetic information, which is faithfully copied during cell division.

• Each strand of DNA serves as a template for the synthesis of a new complementary strand.

• Complementarity ensures high fidelity in replication.

Codons and the Genetic Code

The genetic code is read in triplets of bases called codons, each specifying an amino acid or a start/stop signal during translation.

Codon (RNA)	Amino Acid
UUU, UUC	Phenylalanine (Phe)
UUA, UUG, CUU, CUC, CUA, CUG	Leucine (Leu)
AUG	Methionine (Met, Start)
UAA, UAG, UGA	Stop codons
GCU, GCC, GCA, GCG	Alanine (Ala)

Applications: Genetic Engineering

Understanding nucleic acids enables a range of biotechnological applications, including:

• Production of medically important proteins (e.g., insulin, antibodies)

• Protein engineering to improve natural proteins

• Genetic manipulation of plants for improved traits (e.g., pest resistance, delayed ripening)

• Gene therapy for treating genetic diseases (e.g., cystic fibrosis)

Genetic engineering involves identifying, isolating, and modifying genes to produce desired proteins or traits. Recombinant DNA technology allows for the production of biopharmaceuticals and the development of genetically modified organisms (GMOs).

Summary Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strandedness	Double-stranded	Single-stranded (usually)
Function	Genetic information storage	Gene expression, catalysis, regulation
Location	Nucleus (eukaryotes)	Nucleus, cytoplasm

Key Points for Review

• Understand the chemical and functional differences between DNA and RNA.

• Know the structure and components of nucleic acids.

• Appreciate the organization of DNA and RNA in vivo and their roles in gene expression.

• Recognize the significance of the genome and the complexity of protein expression.

• Be aware of the major applications of nucleic acid biochemistry in biotechnology and medicine.