Nucleic Acids

Overview and Biological Roles

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

• DNA: Stores genetic information, found in chromosomes, and transmits genetic info to offspring.

• RNA: Functions in the transmission of genetic information and directs protein synthesis.

• Polymer: Both DNA and RNA are polymers made of nucleotide monomers.

Nucleotides: Building Blocks of Nucleic Acids

Nucleotides are the monomers of nucleic acids, each consisting of three components:

• Phosphate group

• Pentose sugar: Deoxyribose in DNA, ribose in RNA

• Nitrogenous base: Purines (Adenine, Guanine) and Pyrimidines (Cytosine, Thymine in DNA, Uracil in RNA)

ATP (Adenosine Triphosphate) is a multifunctional nucleotide, serving as the primary energy currency in cells.

Components of Nucleotides

Energy in Nucleotides

The energy stored in nucleotides like ATP is found in the phosphate bonds. Hydrolysis of ATP releases energy:

• ATP → ADP + Pi (most energy released)

• ADP → AMP + Pi (less energy released)

Nucleic Acid Polymers and Structure

Nucleic acids are formed by covalent bonds between the sugar and phosphate groups of nucleotides, creating a sugar-phosphate backbone. DNA is typically double-stranded and forms a double helix, while RNA is usually single-stranded.

• Double helix: Two strands held together by hydrogen bonds between complementary bases.

• Antiparallel: DNA strands run in opposite directions.

Complementary Base Pairing

Properties of DNA Double Helix

• Strands are antiparallel

• Strands are not complementary in direction

• Base stacking interactions stabilize the helix

• Hydrogen bonds hold the two strands together

Types of RNA

• mRNA (messenger RNA): Carries genetic code from DNA to ribosomes

• tRNA (transfer RNA): Brings amino acids to ribosomes during protein synthesis

• rRNA (ribosomal RNA): Forms the core of ribosome structure and catalyzes protein synthesis

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information:

• DNA → RNA → Protein

• Transcription: DNA is copied into RNA

• Translation: RNA is used to synthesize proteins

• Replication: DNA is copied to produce identical DNA molecules

Proteins

Gene Expression and Protein Synthesis

Gene expression is the process by which information from a gene is used to synthesize a functional gene product, typically a protein. This involves transcription (DNA to RNA) and translation (RNA to protein).

Protein Functions

• Enzymes: Catalyze biochemical reactions (e.g., digestive enzymes)

• Transport proteins: Move substances across membranes (e.g., hemoglobin)

• Storage proteins: Store amino acids (e.g., casein in milk)

• Structural proteins: Provide support (e.g., collagen, keratin)

• Contractile and motor proteins: Movement (e.g., actin, myosin)

• Defensive proteins: Protect against disease (e.g., antibodies)

• Hormonal proteins: Coordinate activities (e.g., insulin)

Amino Acids and Polypeptides

Proteins are polymers built from amino acid monomers. There are 20 different amino acids, each with a unique side chain (R group) that determines its properties.

• Hydrophobic amino acids: Nonpolar side chains

• Hydrophilic amino acids: Polar or charged side chains

• Essential amino acids: Cannot be synthesized by the body and must be obtained from the diet

Peptide Bonds and Polypeptide Chains

Amino acids are linked by peptide bonds to form polypeptide chains. Each polypeptide has an N-terminus (amino end) and a C-terminus (carboxyl end).

Levels of Protein Structure

Protein Denaturation

Proteins can lose their structure (denature) due to changes in pH, temperature, or chemical exposure. Denatured proteins are usually inactive.

Summary Tables

Comparison of DNA and RNA

Vocabulary

• Genome

• Nucleic acid

• Deoxyribose

• Ribose

• Polynucleotide

• Nucleotide

• Gene expression

• Transcription

• Translation

• Replication

Additional info:

• Chaperonins are proteins that assist in the proper folding of other proteins.

• Sickle-cell disease is caused by a single amino acid substitution in hemoglobin, affecting quaternary structure.

• Base stacking interactions contribute to the stability of the DNA double helix.