Macromolecules: Structure and Function of DNA, RNA, and Proteins

Macromolecules: Structure and Function of DNA, RNA, and Proteins

Overview of Biological Macromolecules

Biological macromolecules are large, complex molecules essential for life. The primary macromolecules in living organisms include proteins and nucleic acids. These molecules are polymers, constructed from smaller subunits called monomers.

• Proteins: Polymers of amino acids, performing diverse cellular functions.

• Nucleic acids: Polymers of nucleotides, responsible for storing and transmitting genetic information.

Polymer Synthesis and Breakdown

Polymers such as proteins and nucleic acids are synthesized and degraded through specific chemical reactions:

• Dehydration Reaction (Synthesis): Monomers are joined together by covalent bonds, releasing a molecule of water for each bond formed.

• Hydrolysis (Breakdown): Polymers are broken down into monomers by the addition of water, which cleaves the covalent bonds.

Equation for Dehydration Reaction:

$\text{Monomer}_1 + \text{Monomer}_2 \xrightarrow{\text{Dehydration}} \text{Polymer} + \text{H}_2\text{O}$

Equation for Hydrolysis:

$\text{Polymer} + \text{H}_2\text{O} \xrightarrow{\text{Hydrolysis}} \text{Monomer}_1 + \text{Monomer}_2$

Nucleic Acids

Types and Functions

Nucleic acids are macromolecules that store, transmit, and help express hereditary information. There are two main types:

• Deoxyribonucleic acid (DNA): Stores genetic information and provides instructions for its own replication.

• Ribonucleic acid (RNA): Functions in the expression of genetic information by directing protein synthesis.

The process by which DNA directs the synthesis of proteins is called gene expression.

The Central Dogma of Molecular Biology

The flow of genetic information in cells follows the central dogma:

• DNA (gene) → RNA → Protein → Trait

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated into a protein.

Definition: A gene is a stretch of DNA that codes for a specific protein.

Structure of Nucleic Acids

Nucleic acids are polymers called polynucleotides. Each polynucleotide is made of monomers called nucleotides.

• Each nucleotide consists of:

  • A nitrogenous base (A, T, G, C in DNA; A, U, G, C in RNA)

  • A pentose sugar (deoxyribose in DNA, ribose in RNA)

  • One or more phosphate groups

DNA Structure and Base Pairing

DNA is typically double-stranded, forming a double helix. The two strands are held together by hydrogen bonds between complementary nitrogenous bases:

• Adenine (A) pairs with Thymine (T) (2 hydrogen bonds)

• Guanine (G) pairs with Cytosine (C) (3 hydrogen bonds)

This base pairing ensures the uniform width of the double helix and accurate replication.

Chargaff’s Rules

• The amount of adenine (A) equals thymine (T): $[A] = [T]$

• The amount of guanine (G) equals cytosine (C): $[G] = [C]$

• These rules allow prediction of base composition in double-stranded DNA.

RNA Structure and Function

• RNA is usually single-stranded, but can form complex secondary structures through internal base pairing.

• RNA contains uracil (U) instead of thymine (T).

• RNA molecules are more variable in shape and function than DNA.

Proteins

Structure and Function

Proteins are polymers of amino acids and perform a vast array of functions in cells, including catalysis, defense, transport, signaling, movement, and structural support.

• Monomer: Amino acid

• Polymer: Polypeptide (protein)

Structure of Amino Acids

Each amino acid has a central carbon atom (alpha carbon) bonded to:

• A hydrogen atom

• An amino group ($-NH_2$)

• A carboxyl group ($-COOH$)

• A variable side chain (R group) that determines the amino acid’s properties

Amino acids are joined by peptide bonds (covalent bonds) to form polypeptides.

Levels of Protein Structure

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

• Secondary structure: Local folding into alpha helices and beta sheets, stabilized by hydrogen bonds in the backbone.

• Tertiary structure: The overall 3D shape of a polypeptide, determined by interactions between side chains (R groups), including hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces, and disulfide bridges.

• Quaternary structure: The association of multiple polypeptide chains (subunits) into a functional protein complex.

Disulfide Bridges

Disulfide bridges are strong covalent bonds formed between the sulfur atoms of two cysteine residues, stabilizing protein structure at the tertiary and quaternary levels.

Protein Structure and Function Relationship

• The function of a protein is directly related to its structure.

• Even a single amino acid change (as in sickle-cell disease) can disrupt protein function.

• Protein structure can be altered (denatured) by changes in pH, temperature, or other environmental factors, leading to loss of function.

Summary Table: Macromolecules, Monomers, and Polymers

Macromolecule	Monomer	Polymer	Example
Nucleic acids	Nucleotide	Polynucleotide (DNA, RNA)	DNA, RNA
Proteins	Amino acid	Polypeptide	Hemoglobin, enzymes
Carbohydrates	Monosaccharide	Polysaccharide	Starch, cellulose
Lipids	Glycerol, fatty acids	Not true polymers	Fats, phospholipids

Summary Table: Key Functions of Proteins

Type	Example	Function
Enzymes	Amylase	Catalyze chemical reactions
Defensive proteins	Antibodies	Protect against disease
Transport proteins	Hemoglobin	Transport substances
Receptor proteins	Insulin receptor	Receive signals from outside the cell
Motor proteins	Myosin	Movement
Structural proteins	Collagen	Provide structural support

Example: Sickle-Cell Disease

A single amino acid substitution in the protein hemoglobin leads to sickle-cell disease, demonstrating the importance of primary structure in protein function.

Additional info: The notes also reference environmental effects on protein structure (denaturation), and the importance of side chain (R group) properties in determining protein folding and function.