Topic 2 - Nucleic Acids

Introduction to Nucleic Acids

Nucleic acids are essential biopolymers found in all living cells, responsible for the storage, transmission, and expression of genetic information. The two primary types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

• DNA: Stores genetic information and serves as the blueprint for protein synthesis.

• RNA: Functions in various roles, including acting as a messenger, structural component, and catalyst in protein synthesis.

Basic Building Blocks of Nucleic Acids

Nucleic acids are polymers composed of repeating units called nucleotides. Each nucleotide consists of three components:

• Phosphate group

• Pentose sugar (deoxyribose in DNA, ribose in RNA)

• Nitrogenous base (purines: adenine, guanine; pyrimidines: cytosine, thymine [DNA], uracil [RNA])

Nucleoside refers to a nucleotide without the phosphate group.

Phosphodiester bonds link nucleotides together, forming the backbone of DNA and RNA.

Structure of DNA: The Double Helix

DNA is characterized by its double helical structure, in which two strands are held together by hydrogen bonds between complementary nitrogenous bases.

• Base pairing: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).

• Antiparallel strands: The two DNA strands run in opposite directions (5' to 3' and 3' to 5').

• Helix of sugar-phosphates: The backbone consists of alternating sugar and phosphate groups.

RNA is typically single-stranded and contains uracil (U) instead of thymine.

Types of RNA and Their Functions

RNA molecules play diverse roles in the cell:

• 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.

• Catalytic RNA (ribozyme): RNA molecules with enzymatic activity.

• Small inhibitory RNA (siRNA): Involved in gene regulation and silencing.

The 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.

Expressed as:

Genetic Information and the Genome

The genome is the total amount of double-stranded DNA in an organism. Genes are segments of DNA that encode polypeptides or functional RNA molecules.

• Human genome: ~3,234.83 Mb (million base pairs), ~25,000 genes.

• Coding sequences: ~30% of the genome.

• Exons: Only ~1.5% of the genome are exons, which code for mature RNA after splicing.

Gene expression refers to the activation, transcription, and translation of genetic sequences, resulting in protein production. Spatial expression means different cell types express different sets of genes, leading to tissue-specific proteins.

Applications: Genetic Engineering

Understanding nucleic acids is fundamental for genetic engineering, which involves manipulating DNA and RNA to alter genetic traits, produce therapeutic proteins, or study gene function.

• Techniques include recombinant DNA technology, gene cloning, and CRISPR-Cas9 genome editing.

• Applications range from medicine (gene therapy) to agriculture (genetically modified crops).

Comparison of DNA and RNA

Self-Replication of Small Molecules

Self-replication is a fundamental property of nucleic acids, allowing genetic information to be faithfully transmitted during cell division. The process involves template-directed synthesis, where complementary base pairing guides the formation of new strands.

• Replication ensures genetic continuity.

• Errors in replication can lead to mutations, which may have biological consequences.

Key Terms and Definitions

• Nucleotide: Monomeric unit of nucleic acids, composed of a phosphate, sugar, and base.

• Nucleoside: Sugar and base without phosphate.

• Phosphodiester bond: Covalent bond linking nucleotides in a chain.

• Gene: DNA segment encoding a polypeptide or functional RNA.

• Genome: Complete set of genetic material in an organism.

• Exon: Coding region of a gene present in mature RNA.

• Introns: Non-coding regions removed during RNA splicing.

Summary Table: Nitrogenous Bases

Example: The Human Genome Project

The Human Genome Project mapped the entire human genome, revealing that only a small fraction codes for proteins. This knowledge has revolutionized medicine, biotechnology, and our understanding of genetic diseases.

Additional info: Some explanations and tables have been expanded for clarity and completeness based on standard biochemistry curriculum.