Foundations of Biology: Chemistry of Life, Macromolecules, and Molecular Genetics

Introduction: Evolution and the Foundations of Biology

Levels of Biological Organization

Biology studies life across a hierarchy of structural levels, from the biosphere to molecules. Understanding these levels is essential for grasping how complex biological systems function.

• Biosphere: All environments on Earth inhabited by life.

• Ecosystems: Communities of living organisms and their physical environments.

• Communities: Different populations living together in a defined area.

• Populations: Groups of individuals of the same species.

• Organisms: Individual living entities.

• Organs and Organ Systems: Structures composed of tissues that perform specific functions.

• Tissues: Groups of similar cells performing a common function.

• Cells: Basic units of life.

• Organelles: Functional components within cells.

• Molecules: Chemical structures consisting of two or more atoms.

The Chemical Context of Life

Atoms, Elements, and Compounds

All matter is composed of elements, which are substances that cannot be broken down by chemical means. Atoms are the smallest units of elements, and compounds are substances consisting of two or more elements combined in a fixed ratio.

• Atoms: Composed of protons, neutrons, and electrons.

• Isotopes: Atoms of the same element with different numbers of neutrons.

• Energy Levels: Electrons occupy specific energy levels or shells around the nucleus.

Chemical Bonds

Chemical bonds are attractions that hold atoms together in molecules or compounds. They are classified as strong or weak based on their stability and energy requirements.

• Covalent Bonds: Atoms share pairs of electrons. Can be polar (unequal sharing, e.g., H2O) or nonpolar (equal sharing, e.g., O2).

• Ionic Bonds: Atoms transfer electrons, forming charged ions (e.g., NaCl in dry form).

• Hydrogen Bonds: Weak attractions between a hydrogen atom and an electronegative atom (e.g., H2O-H2O).

• Van der Waals Interactions: Weak, distance-dependent attractions between molecules.

Water: Structure and Properties

Water is essential for life due to its unique chemical structure and properties, which arise from its polar covalent bonds and ability to form hydrogen bonds.

• Cohesion and Adhesion: Water molecules stick to each other (cohesion) and to other substances (adhesion), aiding processes like water transport in plants.

• Expansion Upon Freezing: Ice is less dense than liquid water, allowing it to float and insulate aquatic life.

• Temperature Moderation: High specific heat allows water to buffer temperature changes.

• Versatility as a Solvent: Water dissolves many substances due to its polarity.

Carbon and the Molecular Diversity of Life

Organic Molecules and Functional Groups

Life is carbon-based, and organic molecules are characterized by carbon atoms bonded to hydrogen and other elements. Functional groups attached to carbon skeletons determine the chemical behavior of organic molecules.

• Hydrocarbons: Molecules consisting only of carbon and hydrogen.

• Functional Groups: Specific groups of atoms (e.g., hydroxyl, carboxyl, amino) that confer distinct properties.

Macromolecules: Structure and Function

Carbohydrates

Carbohydrates are sugars and polymers of sugars, serving as energy sources and structural materials.

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides joined by a glycosidic bond (e.g., sucrose, lactose).

• Polysaccharides: Long chains of monosaccharides; serve as storage (starch, glycogen) or structural (cellulose, chitin) molecules.

Lipids

Lipids are hydrophobic molecules that include fats, phospholipids, and steroids. They are important for energy storage, membrane structure, and signaling.

• Fats (Triglycerides): Glycerol + 3 fatty acids; store energy efficiently.

• Saturated Fats: No double bonds, solid at room temperature, found in animal products.

• Unsaturated Fats: One or more double bonds, liquid at room temperature, found in plants and fish oils.

• Phospholipids: Glycerol + phosphate group + 2 fatty acids; form cell membranes.

• Steroids: Four fused carbon rings; include cholesterol and hormones.

Proteins

Proteins are polymers of amino acids that perform a vast array of functions, including catalysis, transport, structure, and signaling.

• Amino Acids: Building blocks of proteins; 20 different types with distinct side chains (R groups).

• Polypeptides: Chains of amino acids linked by peptide bonds.

• Protein Structure: Four levels—primary (sequence), secondary (α-helix, β-sheet), tertiary (3D folding), quaternary (multiple polypeptides).

Nucleic Acids

Nucleic acids (DNA and RNA) store and transmit genetic information. They are polymers of nucleotides, each consisting of a sugar, phosphate group, and nitrogenous base.

• DNA: Double-stranded, deoxyribose sugar, bases A, T, C, G.

• RNA: Single-stranded, ribose sugar, bases A, U, C, G.

The Molecular Basis of Inheritance

Structure of DNA

DNA is a double helix with antiparallel strands held together by complementary base pairing (A=T, G≡C). The sequence of bases encodes genetic information.

• Antiparallel Strands: One strand runs 5'→3', the other 3'→5'.

• Base Pairing: Adenine pairs with thymine, guanine pairs with cytosine via hydrogen bonds.

Experimental Evidence for DNA as Genetic Material

Key experiments established DNA as the hereditary material.

• Griffith's Transformation Experiment: Showed that a substance from dead pathogenic bacteria could transform non-pathogenic bacteria.

• Hershey-Chase Experiment: Demonstrated that DNA, not protein, is the genetic material in phages.

Chargaff's Rules and DNA Structure

Erwin Chargaff discovered that DNA from any cell of any organism has a 1:1 ratio of purines to pyrimidines (A=T, G=C), which was explained by the double helix model.

DNA Replication

DNA replication is semiconservative: each new DNA molecule consists of one old strand and one new strand. The process is highly accurate and involves multiple enzymes.

• Semiconservative Model: Supported by the Meselson-Stahl experiment.

• Replication Fork: Site where DNA is unwound and new strands are synthesized.

• Enzymes: Helicase, primase, DNA polymerase, ligase, and others coordinate replication.

Gene Expression: From Gene to Protein

Central Dogma: Transcription and Translation

Gene expression involves two main processes: transcription (DNA to RNA) and translation (RNA to protein). The genetic code is universal and specifies how nucleotide sequences are converted into amino acid sequences.

• Transcription: Synthesis of RNA from a DNA template by RNA polymerase.

• Translation: Synthesis of a polypeptide at the ribosome, using mRNA as a template and tRNA for amino acid delivery.

Mutations

Mutations are changes in the genetic material that can affect protein structure and function. They can be point mutations (substitutions, insertions, deletions) and may result in silent, missense, or nonsense effects.

• Silent Mutation: No effect on protein sequence.

• Missense Mutation: Changes one amino acid.

• Nonsense Mutation: Introduces a stop codon, truncating the protein.

• Frameshift Mutation: Alters the reading frame, usually with severe consequences.