Genetic Material and Gene Expression

Genes and Nucleic Acids

Genes are the fundamental units of inheritance, encoding the amino acid sequences of polypeptides. The information in genes is stored and transmitted via nucleic acids, which are polymers made of nucleotide monomers.

• Gene: A segment of DNA that programs the amino acid sequence of a polypeptide.

• Nucleic Acid: Macromolecules (DNA and RNA) composed of nucleotide monomers.

• Types of Nucleic Acids:

  • Deoxyribonucleic Acid (DNA): Stores genetic information, directs its own replication, and controls RNA synthesis.

  • Ribonucleic Acid (RNA): Functions in gene expression, acting as a messenger and regulator.

• Gene Expression: The process by which DNA directs RNA synthesis and, through RNA, controls protein synthesis (DNA → RNA → Protein).

Structure of Nucleic Acids

• Polynucleotide: A polymer of nucleotides; both DNA and RNA are polynucleotides.

• Nucleotide: The monomer unit, consisting of a nitrogenous base, a pentose sugar, and a phosphate group.

• Nitrogenous Bases:

  • Pyrimidines: Single-ring structures (cytosine, thymine, uracil).

  • Purines: Double-ring structures (adenine, guanine).

• Sugars:

  • Deoxyribose: Found in DNA.

  • Ribose: Found in RNA.

• Double Helix: DNA consists of two antiparallel polynucleotide strands twisted into a double helix.

• Antiparallel: The two DNA strands run in opposite directions (5' to 3' and 3' to 5').

Cell Division and the Cell Cycle

Overview of Cell Division

Cell division is essential for growth, development, and reproduction. It ensures the continuity of life by distributing genetic material to daughter cells.

• Genome: The complete set of genetic information in a cell.

• Chromosome: DNA molecules packaged with proteins.

• Chromatin: The complex of DNA and proteins forming chromosomes.

• Somatic Cells: Non-reproductive body cells (diploid in humans).

• Gametes: Reproductive cells (sperm and eggs; haploid in humans).

• Sister Chromatids: Identical copies of a duplicated chromosome, joined at the centromere.

• Centromere: The region where sister chromatids are most closely attached.

Phases of the Cell Cycle

• Mitotic (M) Phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division); shortest phase.

• Interphase: Accounts for ~90% of the cycle; includes cell growth and DNA replication.

  • G1 Phase: First gap; cell grows.

  • S Phase: Synthesis; DNA is replicated.

  • G2 Phase: Second gap; preparation for mitosis.

Mitosis and Cytokinesis

• Mitosis: Divided into five phases:

  1. Prophase

  2. Prometaphase

  3. Metaphase

  4. Anaphase

  5. Telophase

• Mitotic Spindle: Structure of microtubules and proteins that segregates chromosomes.

• Centrosome: Microtubule organizing center in animal cells.

• Kinetochore: Protein structure on chromatids where spindle fibers attach.

• Metaphase Plate: Imaginary plane where chromosomes align during metaphase.

• Cytokinesis: Division of cytoplasm; involves cleavage furrow in animals and cell plate in plants.

• Binary Fission: Prokaryotic cell division; DNA replication begins at the origin of replication.

Cell Cycle Regulation

• Cell Cycle Control System: Molecular system that triggers and coordinates cell cycle events.

• Checkpoints: Control points where stop/go signals regulate the cycle.

• G0 Phase: Non-dividing state entered if cell does not receive go-ahead signal.

• Growth Factor: Protein that stimulates cell division.

• Density-Dependent Inhibition: Crowded cells stop dividing.

• Anchorage Dependence: Cells must be attached to a substrate to divide.

• Transformation: Cells acquire ability to divide indefinitely (cancer-like behavior).

• Benign Tumor: Abnormal cells remain at original site.

• Malignant Tumor: Invade tissues and metastasize to form new tumors.

DNA Replication and Repair

Mechanisms of DNA Replication

• Semiconservative Model: Each daughter DNA molecule has one old and one new strand.

• Origins of Replication: Specific sites where DNA replication begins, forming replication bubbles.

• Replication Fork: Y-shaped region where parental DNA is unwound.

• Helicase: Enzyme that unwinds DNA at the replication fork.

• Single-Strand Binding Proteins: Stabilize unwound DNA strands.

• Topoisomerase: Relieves strain ahead of replication fork by breaking and rejoining DNA strands.

• Primer: Short RNA segment synthesized by primase to initiate DNA synthesis.

• DNA Polymerases: Enzymes that add nucleotides to the 3' end of a preexisting chain.

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized discontinuously away from the fork as Okazaki fragments.

• Okazaki Fragments: Short DNA segments on the lagging strand.

• DNA Ligase: Joins Okazaki fragments into a continuous strand.

DNA Repair Mechanisms

• Mismatch Repair: Enzymes correct errors in base pairing.

• Nuclease: Cuts out damaged DNA segments.

• Nucleotide Excision Repair: Damaged DNA is removed and replaced by DNA polymerase and ligase.

• Telomeres: Repetitive sequences at chromosome ends; protect against DNA loss.

• Telomerase: Enzyme that extends telomeres in germ cells.

Gene Regulation in Prokaryotes and Eukaryotes

Operon Model in Bacteria

• Operon: Cluster of genes under coordinated control, including promoter, operator, and structural genes.

• Operator: DNA segment acting as a switch for gene expression.

• Repressor: Protein that binds operator to block transcription.

• Regulatory Gene: Encodes the repressor protein.

• Corepressor: Small molecule (e.g., tryptophan) that activates the repressor.

• Repressible Operon: Usually on; can be turned off (e.g., trp operon).

• Inducible Operon: Usually off; can be turned on by an inducer (e.g., lac operon).

• Inducer: Inactivates the repressor to allow transcription.

• Repressible Enzymes: Function in anabolic pathways; synthesis repressed by end product.

• Inducible Enzymes: Function in catabolic pathways; synthesis induced by substrate.

Gene Regulation Mechanisms

• Negative Control: Operons switched off by active repressors.

• Activator (CRP): When glucose is scarce, cAMP binds CRP, activating transcription.

• Differential Gene Expression: Different cell types express different genes from the same genome.

• Histone Acetylation: Acetyl groups added to histone tails, promoting transcription.

• DNA Methylation: Addition of methyl groups (usually to cytosine), often silencing genes.

• Epigenetics: Heritable changes in gene expression not involving DNA sequence changes.

• Control Elements: Noncoding DNA segments serving as binding sites for transcription factors.

• Enhancers: Distal control elements that can increase transcription rates.

• Alternative RNA Splicing: Different mRNAs produced from the same transcript by varying exon/intron selection.

Cancer and the Cell Cycle

Oncogenes and Tumor Suppressor Genes

• Oncogenes: Mutated genes that cause cancer by promoting uncontrolled cell growth.

• Proto-oncogenes: Normal genes that stimulate cell growth and division.

• Tumor Suppressor Genes: Encode proteins that inhibit cell division and prevent tumor formation.

• Ras Gene: Mutations can lead to constant cell division signals.

• p53 Gene: Mutations prevent cell cycle arrest and apoptosis, contributing to cancer.

Cell Communication and Signal Transduction

Cell Signaling Overview

• Hormones: Secreted chemicals that transmit signals over long distances.

• Reception: Detection of a signaling molecule by a receptor.

• Transduction: Conversion of the signal to a cellular response via a signal transduction pathway.

• Response: Cellular activity triggered by the signal.

• Ligand: A molecule that specifically binds to a receptor.

Types of Receptors and Pathways

• G Protein-Coupled Receptors (GPCRs): Work with G proteins that bind GTP; diverse functions.

• Ligand-Gated Ion Channels: Open or close in response to ligand binding, allowing ion flow.

• Protein Kinases: Enzymes that phosphorylate proteins, often forming phosphorylation cascades.

• Protein Phosphatases: Remove phosphate groups (dephosphorylation).

• Second Messengers: Small, nonprotein molecules (e.g., cAMP) that relay signals inside cells.

• Cyclic AMP (cAMP): Common second messenger; mediates effects of hormones like epinephrine.

Animal Physiology: Endocrine and Nervous Systems, Homeostasis

Stimulus and Response

• Stimulus: Internal or external factor triggering a change in activity.

• Response: The change in activity resulting from a stimulus.

Endocrine and Nervous Systems

• Endocrine System: Uses hormones released into the bloodstream to signal target cells throughout the body.

• Nervous System: Uses neurons to transmit signals along specific routes.

• Endocrine Glands: Ductless organs that secrete hormones directly into interstitial fluid and blood.

• Hypothalamus: Brain region controlling neuroendocrine signaling.

• Pituitary Gland: Consists of posterior (stores/releases hypothalamic hormones) and anterior (synthesizes/secretes its own hormones) parts.

• Posterior Pituitary Hormones: Oxytocin (milk release, positive feedback), Antidiuretic hormone (ADH/vasopressin).

• Epinephrine: Adrenal hormone increasing blood glucose, airflow, and altering blood flow patterns.

Homeostasis and Thermoregulation

• Homeostasis: Maintenance of a stable internal environment (e.g., temperature, pH, glucose).

• Set Point: The target value for a physiological variable.

• Sensor: Detects deviations from the set point.

• Control Center: Compares sensor input to set point and directs effectors.

• Effector: Acts to return variable to set point.

• Negative Feedback: Response reduces the stimulus (most common in physiology).

• Positive Feedback: Response amplifies the stimulus (less common; e.g., oxytocin in milk release).

• Regulator: Maintains internal stability despite external changes.

• Conformer: Internal condition changes with external environment.

• Interstitial Fluid: Fluid surrounding body cells; important for solute balance.

Thermoregulation

• Thermoregulation: Maintaining internal temperature within a normal range.

• Endotherms: Generate heat metabolically (birds, mammals).

• Ectotherms: Gain heat from environment (most invertebrates, fishes, amphibians, reptiles).

• Countercurrent Exchange: Mechanism to reduce heat loss by transferring heat between fluids flowing in opposite directions (e.g., blood in arteries and veins).

Table: Comparison of Endotherms and Ectotherms

Key Equation: Phosphorylation Reaction

Protein kinases transfer phosphate groups from ATP to proteins:

Additional info: These notes integrate and expand upon the provided content, ensuring coverage of all major concepts relevant to molecular genetics, cell division, gene regulation, and animal physiology as outlined in a typical college biology curriculum.