Unit 3/4 Biology: Cellular Processes, Genetic Expression, and Regulation
Cell Theory: All living things are composed of cells, the smallest functional units of life. All cells originate from pre-existing cells via division.
Prokaryotes: Simple single-celled organisms (Bacteria and Archaea). They lack membrane-bound organelles and a defined nucleus, replicating through binary fission.
Eukaryotes: Complex organisms (Protists, plants, fungi, animals) containing membrane-bound organelles and a nucleus. They replicate through mitosis and meiosis.
Cell Size Dynamics: Smaller cells maintain a higher surface-area-to-volume ratio (SA:V), facilitating efficient exchange of nutrients and wastes with the extracellular environment and allowing quicker intracellular transport. As cell size increases, SA:V decreases.
The Plasma Membrane and Transport
Fluid-Mosaic Model: The membrane is a dynamic, semipermeable phospholipid bilayer embedded with proteins, cholesterol, and carbohydrates (glycocalyx).
Passive Transport:
Simple Diffusion: Movement of small, nonpolar molecules down a concentration gradient.
Facilitated Diffusion: Movement of polar or larger molecules through specific protein channels or carrier proteins.
Osmosis: Diffusion of water across a semipermeable membrane from low to high solute concentration, assisted by aquaporins.
Active Transport: Requires energy (ATP) to move substances against a concentration gradient via protein pumps (e.g., Na+/K+ pump).
Bulk Transport:
Endocytosis: Importing material by folding the membrane into vesicles (phagocytosis for solids, pinocytosis for liquids).
Exocytosis: Exporting material by fusing internal vesicles with the plasma membrane.
Nucleic Acid Structure and Function
Nucleotides: Composed of a phosphate group, a five-carbon sugar, and a nitrogenous base.
DNA: Double-stranded helix using deoxyribose sugar and bases Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). Strands are antiparallel (5' to 3' and 3' to 5').
RNA: Typically single-stranded using ribose sugar and Uracil (U) instead of Thymine. Types include mRNA (messenger), tRNA (transfer), and rRNA (ribosomal).
Base Pairing: Purines (A, G) pair with Pyrimidines (T, C, U) through hydrogen bonds. A pairs with T (or U in RNA); G pairs with C.
Gene Expression: Transcription and Translation
The Genetic Code: Universal, unambiguous, non-overlapping, and degenerate (multiple codons can code for one amino acid).
Transcription: RNA polymerase binds to the promoter and constructs a pre-mRNA strand complementary to the DNA template strand.
RNA Processing (Eukaryotes only): Addition of a 5' methyl-G cap and a 3' poly-A tail, and splicing to remove non-coding introns and join coding exons.
Alternative Splicing: Different combinations of exons are joined to allow one gene to code for multiple proteins.
Translation: Ribosomes read mRNA codons; tRNA molecules with matching anticodons deliver specific amino acids to form a polypeptide chain.
Gene Regulation: The Trp Operon
Regulatory vs. Structural Genes: Regulatory genes produce proteins (repressors/activators) that control the expression of structural genes.
Trp Operon Repression: When tryptophan levels are high, it binds to a repressor protein, which then attaches to the operator to block RNA polymerase.
Attenuation (Prokaryotes): A mechanism where transcription is aborted prematurely if tryptophan levels are high. It relies on the formation of a terminator hairpin loop in the leader mRNA sequence due to the coupling of transcription and translation.
Protein Structure and Secretion
Proteome: The complete set of proteins expressed by an organism.
Hierarchical Structure:
Primary: Linear sequence of amino acids.
Secondary: Local folding into α-helices or β-pleated sheets.
Tertiary: Overall 3D conformation stabilized by R-group interactions.
Quaternary: Association of multiple polypeptide subunits (e.g., Hemoglobin).