Genes, Proteins, Enzymes, and Human Disease: Molecular Genetics Study Guide

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Genes and Human Disease

Historical Discovery of Genetic Diseases

Mutations in genes can cause human diseases, as first described by Archibald Garrod in 1902 with the disease alkaptonuria. This disease is characterized by black urine and arthritis, resulting from the accumulation of homogentisic acid (alkapton), which turns black upon exposure to air. Garrod termed this an 'inborn error of metabolism,' indicating a genetic rather than infectious origin.

Key Point: Alkaptonuria is inherited and not caused by germs.
Key Point: Garrod suggested it was a recessive Mendelian trait.
Example: The accumulation of homogentisic acid in urine is a direct result of a defective enzyme in the phenylalanine breakdown pathway.

Urine samples showing normal and black coloration due to alkaptonuria

What is a Gene?

Genetic and Biochemical Definitions

Genes have been defined both genetically and biochemically. Genetically, a gene controls some aspect of an organism’s phenotype and resides on chromosomes, segregating in defined ways. Biochemically, a gene is a segment of DNA containing the information to express a protein that performs a function in the cell or body.

Key Point: Genes encode proteins, which determine cellular and organismal functions.
Key Point: Most genes contain instructions to make proteins; some proteins are enzymes.

Proteins: Structure and Function

Amino Acids and Protein Structure

Proteins are polymers made of amino acids, each with a unique sequence. There are 20 common amino acids, grouped by their R side chains into categories: nonpolar, polar, acidic, and basic. The sequence and chemical properties of amino acids determine how a protein folds and functions.

Key Point: Amino acids are joined by peptide bonds, forming polypeptide chains with an amino terminus (start) and carboxyl terminus (end).
Key Point: Hydrophilic amino acids are generally found on the outside of proteins, while hydrophobic amino acids are found inside.
Example: The general structure of amino acids and their grouping by R side chains.

Amino acids grouped by R groups

Levels of Protein Structure

Proteins fold into precise three-dimensional structures, which determine their function. There are four levels of protein structure:

Primary structure: Linear sequence of amino acids joined by covalent bonds.
Secondary structure: Local structures formed by hydrogen bonds (e.g., alpha helix, beta sheet).
Tertiary structure: Final 3D shape formed by long-range interactions and hydrophobic/hydrophilic properties.
Quaternary structure: Multiple polypeptides working together (e.g., hemoglobin has 2 alpha and 2 beta subunits).

Diagram of primary, secondary, and tertiary protein structure Diagram of quaternary protein structure

Genes, Proteins, and Enzymes

Enzymes and Biochemical Pathways

Enzymes are proteins that catalyze specific chemical reactions. Mutations in genes encoding enzymes can disrupt biochemical pathways, leading to the accumulation of substrates and disease.

Key Point: Enzymes have active sites where reactions occur.
Key Point: Biochemical pathways consist of multiple steps, each controlled by a specific enzyme.
Example: Alkaptonuria results from a defective enzyme in the phenylalanine breakdown pathway.

Protein structure showing active site

Beadle and Tatum: One Gene-One Enzyme Hypothesis

Model Organisms and Genetic Screens

Beadle and Tatum used the bread mold Neurospora crassa to show that one gene controls one enzyme at a specific step in a biochemical pathway. They exposed spores to mutagens, creating auxotrophic mutants that could not synthesize specific vitamins or amino acids.

Key Point: Prototrophs can grow on minimal media; auxotrophs require supplementation.
Key Point: Mutations in single genes can block specific steps in biochemical pathways.
Example: Mutants unable to synthesize methionine or arginine can be identified by supplementing media.

Scanning electron micrograph of Neurospora crassa

Human Genetic Diseases Caused by Enzyme Defects

Phenylketonuria (PKU)

PKU is caused by mutations in the gene encoding phenylalanine hydroxylase (PAH). Without PAH, phenylalanine accumulates and is broken down into harmful products, leading to severe mental retardation if untreated.

Key Point: More than 400 recessive mutations affecting PAH have been identified.
Key Point: Early detection and dietary management prevent disease symptoms.

Tay-Sachs Disease

Tay-Sachs disease is caused by a defect in the lysosomal enzyme N-acetylhexosaminidase A (HEXA), leading to substrate accumulation and neuronal degeneration. It is most common among Jewish people of European ancestry.

Key Point: Affected children suffer from severe neurological symptoms and early death.
Key Point: Genetic testing has reduced prevalence in high-risk populations.

Human Genetic Diseases Caused by Non-Enzyme Protein Defects

Sickle Cell Anemia

Sickle cell anemia is caused by mutations in the gene for beta hemoglobin. Hemoglobin is composed of two alpha and two beta chains. A single amino acid change (E6V: glutamic acid to valine) alters the protein’s structure, causing red blood cells to adopt a sickle shape under low oxygen conditions.

Key Point: Sickle cells are fragile and can block capillaries, leading to reduced oxygen transport and severe symptoms.
Key Point: The disease is common in populations with historical exposure to malaria.
Example: Linus Pauling described sickle cell anemia as a 'molecular disease' based on protein gel electrophoresis.

Scanning electron micrograph of sickle-shaped red blood cells Amino acid sequence comparison of normal and sickle cell hemoglobin

Sickle Cell Anemia and Malaria: Population Genetics

Heterozygote Advantage

Individuals heterozygous for the sickle cell mutation (HbA/HbS) have increased fitness in malaria-endemic regions. The presence of mutant hemoglobin shortens red blood cell lifespan, disrupting the malaria parasite’s life cycle.

Key Point: The sickle cell allele persists in populations where malaria is common due to the heterozygote advantage.
Example: The sickle cell allele spread throughout central Africa following the rise of agriculture and persists in descendants in the Americas.

Summary Table: Amino Acid Properties

The chemical nature of the R side group is used to group amino acids into categories. This classification is important for understanding protein folding and function.

Category	Amino Acids	Properties
Nonpolar	Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Tryptophan, Phenylalanine, Proline	Hydrophobic, found inside proteins
Polar	Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine	Hydrophilic, found outside proteins
Acidic	Aspartic acid, Glutamic acid	Negatively charged
Basic	Lysine, Arginine, Histidine	Positively charged

Amino acid classification table

Key Equations and Concepts

Peptide Bond Formation:
Central Dogma of Molecular Biology:
One Gene-One Polypeptide Hypothesis:

Additional info: The notes cover Ch. 4 (Gene Interaction), Ch. 10 (Eukaryotic Chromosome Abnormalities and Molecular Organization), and related sections on protein structure, gene function, and genetic diseases, making them highly relevant for a college genetics course.