Chapter 3: Nucleic Acids and Proteins

Key Concepts

• 3.1: Nucleic Acids Are Informational Macromolecules

• 3.2: Proteins Are Polymers with Important Structural and Metabolic Roles

Nucleic Acids

Nucleic Acids as Informational Macromolecules

Nucleic acids are polymers that store, transmit, and express hereditary (genetic) information. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Their monomers are called nucleotides.

• DNA: Stores genetic information.

• RNA: Involved in gene expression and protein synthesis.

Nucleotide Structure

Each nucleotide consists of three components:

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

• Nitrogenous base (Adenine, Thymine, Cytosine, Guanine in DNA; Adenine, Uracil, Cytosine, Guanine in RNA)

• Phosphate group

Nucleosides are composed of a pentose sugar and a nitrogenous base, without the phosphate group.

Types of Nitrogenous Bases

• Pyrimidines: Single-ring structures (Cytosine, Thymine, Uracil)

• Purines: Double-ring structures (Adenine, Guanine)

The sugar in DNA is deoxyribose; in RNA, it is ribose.

Formation of Nucleic Acid Polymers

Nucleotides are joined by phosphodiester bonds through condensation reactions. The bond forms between the 5' carbon of one sugar and the 3' carbon of the next, resulting in a sugar-phosphate backbone. Nucleic acids grow in the 5' to 3' direction.

Oligonucleotides and Polynucleotides

• Oligonucleotides: Short chains (up to 20 monomers), important in DNA replication and gene expression.

• Polynucleotides: Long chains (DNA and RNA), the largest polymers in living organisms.

Distinguishing RNA from DNA

Complementary Base Pairing

• In DNA: A-T (Adenine-Thymine), G-C (Guanine-Cytosine)

• In RNA: A-U (Adenine-Uracil), G-C (Guanine-Cytosine)

Base pairs are held together by hydrogen bonds, which are relatively weak and allow the strands to separate during replication and transcription.

Structure of DNA and RNA

• DNA: Usually double-stranded, forming a right-handed double helix. The two strands are antiparallel (run in opposite directions).

• RNA: Usually single-stranded, but can fold into complex 3D structures via complementary base pairing.

Genetic Information and Gene Expression

• DNA encodes information in the sequence of its bases.

• Functions of DNA:

  • Replication: Copying genetic information for cell division.

  • Gene expression: Information is transcribed into RNA and translated into proteins.

• Genome: The complete set of DNA in a living organism.

DNA Replication and Transcription

• Replication: The entire DNA molecule is copied so each new cell receives a complete set.

• Transcription: Specific genes are transcribed into RNA.

Example of base pairing during transcription:

• DNA: 5'-TACGCA-3'

• RNA: 5'-UACGCA-3'

Evolutionary Relationships

DNA base sequences can be compared to reveal evolutionary relationships among species. Closely related species have more similar DNA sequences.

Proteins

Functions of Proteins

• Enzymes: Catalyze biochemical reactions.

• Defensive proteins: e.g., antibodies.

• Hormonal and regulatory proteins: Control physiological processes.

• Receptor proteins: Receive and respond to molecular signals.

• Storage proteins: Store amino acids.

• Structural proteins: Provide stability and movement.

• Transport proteins: Carry substances (e.g., hemoglobin).

• Genetic regulatory proteins: Regulate gene expression.

Amino Acids: The Building Blocks of Proteins

Proteins are polymers of amino acids. Each amino acid has a central (α) carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable R group (side chain).

• There are 20 common amino acids, classified by the properties of their R groups.

Properties of Amino Acids

• Glycine: Smallest side chain (hydrogen), fits into tight corners.

• Proline: Rigid structure, limits rotation, stabilizes bends.

• Cysteine: Can form covalent disulfide bridges with other cysteine residues.

Peptides and Polypeptides

• Oligopeptides (peptides): Short polymers (20 or fewer amino acids).

• Polypeptides: Long polymers, may consist of one or more chains.

• Proteins can range from small (e.g., insulin, 51 amino acids) to very large (e.g., muscle protein titin, over 34,000 amino acids).

Peptide Bond Formation

Amino acids are linked by peptide bonds formed in condensation reactions. The bond forms between the amino group of one amino acid and the carboxyl group of another, releasing water.

Levels of Protein Structure

• Primary structure: The sequence of amino acids in a polypeptide chain.

• Secondary structure: Regular, repeated patterns stabilized by hydrogen bonds. Includes:

  • α (alpha) helix: Right-handed coil.

  • β (beta) pleated sheet: Two or more sequences extended and aligned.

Summary Table: Distinguishing RNA from DNA

Summary Table: Major Functions of Proteins

Key Equations and Concepts

• Phosphodiester bond formation:

• Peptide bond formation:

Additional info:

• Protein structure is further organized into tertiary (3D folding) and quaternary (multiple polypeptide chains) levels, though these are not detailed in the provided slides.

• Gene expression involves both transcription (DNA to RNA) and translation (RNA to protein).