Central Dogma, Gene Expression, and Cellular Biology: Comprehensive Study Notes

Central Dogma: From DNA to Protein

Transfer RNA (tRNA) and the Mechanism of Translation

The process of translation converts genetic information from messenger RNA (mRNA) into functional proteins. Transfer RNA (tRNA) molecules are essential for decoding the mRNA sequence into a polypeptide chain.

• tRNA Structure: Each tRNA carries a specific amino acid at its 3' end and possesses an anticodon at the opposite end. The anticodon is a triplet nucleotide sequence complementary to an mRNA codon, ensuring accurate translation.

• Folding: tRNA molecules fold into a cloverleaf secondary structure stabilized by hydrogen bonds, which further folds into an L-shaped three-dimensional form.

• Function: The amino acid attachment site is at the top of the tRNA, while the anticodon interacts with the mRNA within the ribosome.

Ribosomes and Protein Synthesis

Ribosomes are the molecular machines responsible for protein synthesis, composed of a large and a small subunit.

• Subunit Functions: The small subunit binds to mRNA for decoding, while the large subunit catalyzes peptide bond formation.

• Structural Differences: Ribosomes differ between bacteria, archaea, and eukaryotes, which is exploited by antibiotics targeting bacterial ribosomes.

• Clinical Relevance: Antibiotics can disrupt beneficial bacteria, leading to side effects.

Ribosomal Binding Sites: A, P, and E Sites

• A site (Aminoacyl site): Holds the incoming tRNA with the next amino acid.

• P site (Peptidyl site): Contains the tRNA attached to the growing polypeptide chain.

• E site (Exit site): Where empty tRNAs exit the ribosome.

• Mechanism: The ribosome moves along the mRNA (5' to 3'), shifting tRNAs through the A → P → E sites, transferring the polypeptide chain during elongation.

Stages of Translation

• Initiation: The small ribosomal subunit binds to the mRNA (at the 5' cap in eukaryotes), scans for the start codon (AUG), and the initiator tRNA binds to the P site. The large subunit then joins.

• Elongation: Sequential addition of amino acids:

  • Codon recognition: Correct tRNA enters the A site.

  • Peptide bond formation: Polypeptide is transferred from the P-site tRNA to the amino acid on the A-site tRNA.

  • Translocation: Ribosome shifts, moving tRNAs through the sites.

• Termination: When a stop codon (UAA, UAG, UGA) enters the A site, a release factor binds, triggering hydrolysis and release of the polypeptide. Ribosomal subunits dissociate.

Post-Translational Events

• Newly synthesized polypeptides may fold, undergo modifications, or assemble into complexes.

• Chaperone proteins assist in proper folding, essential for protein function.

Gene Expression Overview and mRNA Processing

Transcription and mRNA Maturation

Transcription is the synthesis of RNA from a DNA template, producing precursor mRNA (pre-mRNA) in eukaryotes.

• Capping: Addition of a 5' methylguanosine cap protects RNA and aids ribosome binding.

• Polyadenylation: Addition of a poly-A tail at the 3' end enhances stability and export from the nucleus.

• Splicing: Removal of introns and joining of exons by the spliceosome allows for alternative splicing and proteomic diversity.

• Export: Mature mRNA exits the nucleus to be translated in the cytoplasm.

Codon-Anticodon Interaction

• Codons are three-nucleotide sequences on mRNA (e.g., UGG, UUU, AUG).

• tRNAs have complementary anticodons (e.g., anticodon for AUG is UAC).

• The start codon AUG codes for methionine and initiates translation.

Mutations and Genetic Diseases

Point Mutations

• A point mutation is a change in a single nucleotide pair within a gene.

• Even a single base substitution can alter protein structure and function.

• Example: Sickle Cell Anemia

  • Normal DNA: CTC → mRNA: GAG → glutamic acid.

  • Mutated DNA: CAC → mRNA: GUG → valine.

  • Valine substitution causes hemoglobin to polymerize under low oxygen, distorting red blood cells into a sickle shape.

  • Consequences: Poor circulation, organ damage, pain crises, reduced red blood cell lifespan.

CRISPR-Cas9 Gene Editing

• CRISPR-Cas9 is a gene-editing technology derived from a bacterial immune system.

• It uses a guide RNA to direct the Cas9 enzyme to a specific DNA sequence for precise modification.

• Applications include correcting disease-causing mutations (e.g., sickle cell anemia).

• Clinical Example: Edited hematopoietic stem cells restored normal hemoglobin in sickle cell patients, with 95% showing symptom absence.

• Ethical Considerations: Human germline editing is highly regulated and restricted.

Cumulative Biology Review

Domains of Life

• Bacteria: Prokaryotic, unicellular, lack membrane-bound organelles.

• Archaea: Prokaryotic, biochemically/genetically distinct from bacteria.

• Eukarya: Includes plants, animals, fungi, protists; have membrane-bound organelles and a nucleus.

• Prokaryote: Refers to organisms without a nucleus (Bacteria and Archaea).

Cellular Structure

• Prokaryotic Cells: Lack a nucleus; DNA is in a nucleoid region; no membrane-bound organelles.

• Eukaryotic Cells: Have a nucleus and various organelles (e.g., mitochondria, ER).

Evolution and Natural Selection

• Charles Darwin proposed natural selection as the mechanism of evolution.

• Individuals with advantageous traits are more likely to survive and reproduce.

• Examples: Adaptive radiation in Galápagos finches and tortoises.

Chemical Foundations of Life

• Major Elements: Carbon (C), Oxygen (O), Hydrogen (H), Nitrogen (N).

• Other Essential Elements: Calcium, phosphorus, potassium, sulfur, sodium, chlorine, magnesium, trace elements (e.g., iodine).

• Atom: Smallest unit of an element; composed of protons (+), neutrons (0), electrons (-).

• Iodine Deficiency: Causes goiter due to impaired thyroid hormone production.

Valence Electrons and Bonding

• Fluorine: 9 electrons (2 in first shell, 7 in valence shell).

• Types of Bonds:

  • Ionic: Electron transfer between atoms.

  • Covalent: Electron sharing (e.g., in water).

  • Hydrogen Bonds: Weak attraction between H (partially +) and electronegative atoms (e.g., O); important for water and DNA structure.

  • Van der Waals Forces: Temporary dipoles; important in molecular interactions.

Water and Solutions

• Cohesion: Water molecules stick to each other (hydrogen bonds).

• Adhesion: Water sticks to other substances.

• Solution Components: Solute (dissolved substance), solvent (dissolving medium).

• pH Scale: 0 (acidic) to 14 (basic), 7 is neutral.

  • Acids: High H+ concentration.

  • Bases: High OH- concentration.

Macromolecules

• Carbohydrates: Sugars and polymers (e.g., glucose, starch); energy and structure.

• Lipids: Hydrophobic molecules (fats, phospholipids, steroids).

  • Saturated Fats: No double bonds, solid at room temperature (animal-derived).

  • Unsaturated Fats: Double bonds, liquid at room temperature (plant-derived).

• Proteins: Polymers of amino acids; structure, enzymes, signaling, transport.

• Nucleic Acids: DNA and RNA; genetic information storage and transmission.

• Polymerization: Formed by dehydration synthesis (removal of water), broken by hydrolysis (addition of water).

Cell Membrane and Transport

• Fluid Mosaic Model: Plasma membrane is a phospholipid bilayer with embedded proteins and cholesterol.

• Diffusion: Movement of small, nonpolar molecules down their concentration gradient.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Tonicity:

  • Isotonic: Equal solute concentration; no net water movement.

  • Hypertonic: Higher solute outside; cell shrinks (crenation).

  • Hypotonic: Lower solute outside; cell swells or bursts (lysis).

Energy and Metabolism

• Anabolic Pathways: Build complex molecules; require energy (endergonic). E.g., protein synthesis, DNA replication.

• Catabolic Pathways: Break down molecules; release energy (exergonic). E.g., cellular respiration.

• Laws of Thermodynamics:

  • First law: Energy cannot be created or destroyed.

  • Second law: Energy transformations increase entropy.

• Enzymes: Biological catalysts that lower activation energy and speed up reactions without being consumed.

Cellular Respiration and Photosynthesis

Cellular Respiration

• Overall Equation:

  $\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP}$

• OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

• Steps:

  1. Glycolysis (cytoplasm): Glucose → 2 pyruvate + net 2 ATP + 2 NADH.

  2. Pyruvate oxidation (mitochondria): Pyruvate → acetyl-CoA + CO2 + NADH.

  3. Citric Acid Cycle (Krebs cycle, mitochondrial matrix): Generates ATP, NADH, FADH2, CO2.

  4. Oxidative phosphorylation: Electron transport chain (ETC) and chemiosmosis.

     • Electrons from NADH/FADH2 move through ETC, pumping H+ into intermembrane space.

     • H+ gradient drives ATP synthase to produce ATP.

     • Final electron acceptor: O2 → H2O.

     • Produces ~26-34 ATP per glucose.

• Anaerobic Conditions:

  • Alcohol fermentation: Pyruvate → ethanol + CO2 (brewing, bread making).

  • Lactic acid fermentation: Pyruvate → lactate (muscle cells, yogurt production).

Photosynthesis

• Overall Equation:

  $\mathrm{6CO_2 + 6H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2}$

• Location: Chloroplasts (thylakoids and stroma).

• Stages:

  1. Light-dependent reactions (thylakoid membranes):

     • Light excites electrons in photosystems II and I.

     • Water is split, releasing O2 and electrons.

     • ETC generates ATP and NADPH.

  2. Calvin cycle (stroma):

     • Uses ATP and NADPH to fix CO2 into glucose.

     • Steps: Carbon fixation → reduction → regeneration of RuBP.

• Importance: Produces oxygen and organic compounds essential for life.

Cell Division and Chromosomes

Chromatin and Chromosome Structure

• In non-dividing cells, DNA is in the form of chromatin (DNA + histone proteins).

• During cell division, chromatin condenses into visible chromosomes.

Cell Cycle

• Interphase:

  • G1 phase: Cell growth and metabolism.

  • S phase: DNA replication.

  • G2 phase: Preparation for mitosis; organelles duplicated.

• M phase: Mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis

• Phases:

  1. Prophase: Chromosomes condense; nuclear envelope breaks down.

  2. Prometaphase: Spindle fibers attach to kinetochores.

  3. Metaphase: Chromosomes align at metaphase plate.

  4. Anaphase: Sister chromatids separate to opposite poles.

  5. Telophase/Cytokinesis: Nuclear envelopes reform; cytoplasm divides.

Meiosis

• Reductive division producing gametes (sperm and egg).

• Meiosis I: Homologous chromosomes pair and exchange genetic material (crossing over), then separate (diploid to haploid).

• Meiosis II: Sister chromatids separate (like mitosis).

Chromosome Types

• Sex Chromosomes: Determine sex (XX = female, XY = male).

• Autosomes: Other 22 pairs of chromosomes (not involved in sex determination).

Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (nucleoid region)	Present
Membrane-bound Organelles	Absent	Present (e.g., mitochondria, ER)
Cell Division	Binary fission	Mitosis/meiosis
Examples	Bacteria, Archaea	Plants, animals, fungi, protists

Additional info:

• Graduate students presented metagenomics research projects, highlighting the importance of scientific inquiry and interdisciplinary learning.