Introduction: The Molecules of Function and Information
Welcome to the second part of our exploration into biomolecules. In this lesson, we delve into two of the most vital classes of macromolecules: proteins, the versatile "workhorses" of the cell, and nucleic acids, the keepers of genetic information. Understanding their intricate structures is key to understanding their diverse functions.
Part 1: Proteins - The Architects of Function
1.1 Amino Acids: The Building Blocks
Proteins are polymers constructed from a set of 20 monomers called amino acids. All amino acids share a common structure: a central carbon atom (the alpha carbon) bonded to an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain called the R group. The unique chemical properties of the R group determine the specific characteristics of each amino acid.
Diagram: General Structure of an Amino Acid
1.2 The Peptide Bond: Linking Amino Acids
Amino acids are linked together by peptide bonds. This bond is formed through a dehydration reaction between the carboxyl group of one amino acid and the amino group of another. A chain of amino acids is called a polypeptide.
1.3 The Four Levels of Protein Structure
A functional protein is not just a random polypeptide chain. It is a precisely twisted, folded, and coiled molecule with a unique three-dimensional shape. This shape is determined by four hierarchical levels of structure.
Diagram: The Hierarchical Structure of Proteins
- Primary (1°) Structure: The unique, linear sequence of amino acids in a polypeptide chain. This sequence is determined by the genetic information in DNA.
- Secondary (2°) Structure: Coils and folds in the polypeptide backbone, stabilized by hydrogen bonds. The two major types are the α-helix (a delicate coil) and the β-pleated sheet (folded structure).
- Tertiary (3°) Structure: The overall three-dimensional shape of a single polypeptide, resulting from interactions between the R groups of the amino acids. These interactions include hydrogen bonds, ionic bonds, hydrophobic interactions, and strong covalent bonds called disulfide bridges.
- Quaternary (4°) Structure: The overall protein structure that results from the aggregation of two or more polypeptide subunits. Not all proteins have this level of structure. Hemoglobin is a classic example.
Part 2: Nucleic Acids - The Basis of Heredity
2.1 Nucleotides: The Monomers of Nucleic Acids
Nucleic acids are polymers made of monomers called nucleotides. Each nucleotide consists of three parts: a nitrogenous base, a five-carbon sugar (pentose), and one or more phosphate groups.
Diagram: Structure of a Nucleotide
2.2 Deoxyribonucleic Acid (DNA)
DNA is the molecule that stores the genetic blueprint for an organism. It consists of two polynucleotide strands that wind around each other to form a double helix. The sugar in DNA is deoxyribose. The two strands are held together by hydrogen bonds between pairs of nitrogenous bases: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C). The strands are antiparallel, meaning they run in opposite 5' to 3' directions.
2.3 Ribonucleic Acid (RNA)
RNA plays a crucial role in carrying out the instructions from DNA for protein synthesis. It differs from DNA in three main ways: its sugar is ribose, it is typically single-stranded, and the base Thymine (T) is replaced by Uracil (U).
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | Adenine (A), Guanine (G), Cytosine (C), Thymine (T) | Adenine (A), Guanine (G), Cytosine (C), Uracil (U) |
| Structure | Double-stranded helix | Typically single-stranded |
| Primary Role | Stores hereditary information | Protein synthesis, gene regulation |