AG

Physical Basis of Heredity Notes

Basic Components of the Mammalian Cell

  • Plasma Membrane

    • Defines the cell boundary, separating it from the external environment.

    • Actively regulates movement of substances into and out of the cell.

  • Cell Wall

    • Present in plant cells.

    • Composed mainly of cellulose, a major polysaccharide.

  • Glycocalyx

    • Found in animal cells.

    • Covers the plasma membrane.

    • Composed of glycoproteins and polysaccharides.

    • Provides biochemical identity at the cell surface.

      • Examples: AB, Rh antigens, histocompatibility antigens

      • Elicits immune responses.

      • Acts as receptor molecules.

  • Nucleus

    • Membrane-bound structure housing DNA.

    • DNA complexed with acidic and basic proteins forms thin fibers.

    • In non-divisional phases of the cell cycle, these fibers are uncoiled, forming chromatin.

    • During cell division (mitosis or meiosis), chromatin fibers coil and condense into chromosomes.

  • Nucleolus

    • Amorphous component where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

    • Nucleolus organizing region (NOR) is a portion of DNA that encodes rRNA.

  • Nucleoid

    • Found in prokaryotes (e.g., Eubacteria).

    • Site where genetic material, a long, circular DNA molecule, is compacted.

    • Lacks a nuclear envelope and membrane organelles.

    • Two major groups of prokaryotes: Eubacteria and Archaea.

  • Cytoplasm

    • Extracellular nuclear organelles.

    • Cytosol: Nonparticulate, colloidal material surrounding organelles.

    • Cytoskeleton: Provides a lattice of support structures within the cell.

      • Microtubules: Derived from tubulin.

      • Microfilaments: Derived from actin.

      • Maintains cell shape, facilitates cell mobility, and anchors organelles.

  • Endoplasmic Reticulum (ER)

    • Compartmentalizes the cytoplasm.

    • Increases the surface area available for biochemical synthesis.

    • Smooth ER: Site for synthesizing fatty acids and phospholipids.

    • Rough ER: Studded with ribosomes.

  • Ribosomes

    • Sites where genetic information in messenger RNA (mRNA) is translated into proteins.

  • Mitochondria

    • Found in most eukaryotes, including animal and plant cells.

    • Sites of oxidative phases of cell respiration.

    • Generate adenosine triphosphate (ATP), an energy-rich molecule.

  • Chloroplasts

    • Found in plants, algae, and some protozoans.

    • Associated with photosynthesis, the major energy-trapping process on Earth.

  • Centriole

    • Pair of cytoplasmic bodies located in the centrosome.

    • Associated with the organization of spindle fibers for mitosis and meiosis.

    • In some organisms, derived from the basal body.

    • Basal bodies are associated with the formation of cilia and flagella, which propel cells or move materials.

  • Spindle Fibers

    • Organized by centrioles.

    • Important for chromosome movement during cell division.

    • Composed of microtubules consisting of polymers of the protein tubulin.

Human Cell Cycle

  • Cell Cycle Phases

    • G0: Nondividing cells.

    • Interphase: Includes G1, S, and G2 phases.

      • G1 phase.

      • S phase: DNA synthesis occurs.

      • G2 phase.

    • Mitosis: Cell division.

      • Prophase.

      • Prometaphase.

      • Metaphase.

      • Anaphase.

      • Telophase.

  • Cell Cycle Checkpoints

    • Cell division cycle (cdc) mutations affect cell cycle stages.

      • Involve kinases and cyclins.

      • Cyclins + kinase = cyclin-dependent kinases (CDKs).

      • Activated kinases phosphorylate target proteins to regulate cell cycle progression.

    • At least 3 cell cycle checkpoints.

      • Importance:

        • DNA damage: Cell cycle arrests to allow DNA repair.

          • If DNA damage is not repaired and the cell continues to divide, it can lead to genetically altered cells and potentially cancer.

          • Cell cycle arrest allows the cell to repair DNA damage or permanently stop dividing, preventing malignancy.

Cellular Division

  • Karyokinesis

    • Nuclear division.

    • Requires precision to partition genetic material into daughter cells with identical chromosome composition.

  • Cytokinesis

    • Cytoplasmic division.

    • Partitions the volume into two parts and encloses each new cell in a distinct plasma membrane.

    • Cytoplasm is reconstituted.

    • Organelles replicate themselves or are synthesized de novo from existing membrane structures.

  • Mitosis

    • Zygote development: Foundation for growth and development.

    • Adult: Basis for wound healing and tissue replacement.

    • Somatic cells of a species have the same number of chromosomes (diploid number, 2n).

      • Except sex chromosomes, chromosomes exist in pairs (homologous chromosomes).

      • Bacteria and viruses have only one chromosome.

    • Diploid: 2n = 46 chromosomes.

    • Haploid: n = 23 chromosomes.

    • Genome: Genetic information contained in the haploid set.

    • Locus: Gene sites located along homologous chromosomes.

    • Biparental inheritance: One member of each pair from the maternal parent (ovum) and the other from the paternal parent (sperm).

    • Interphase: Chromosomes are extended and uncoiled, forming chromatin.

    • Prophase: Chromosomes coil up and condense; centrioles divide and move apart.

    • Prometaphase: Chromosomes are clearly double structures; centrioles reach opposite poles; spindle fibers form.

    • Metaphase: Centromeres align on the metaphase plate.

    • Anaphase: Centromeres split, and daughter chromosomes migrate to opposite poles.

    • Telophase: Daughter chromosomes arrive at the poles; cytokinesis commences.

  • Meiosis

    • Produces gametes or spores with a haploid set of chromosomes.

    • During sexual reproduction, haploid gametes combine at fertilization to reconstitute the diploid complement of parental cells.

    • Meiosis ensures each gamete or spore contains precisely one member of each homologous pair.

    • Maintains genetic continuity from generation to generation.

    • Underlying basis of Mendel’s principles of segregation and independent assortment.

    • Crossing over: Genetic exchange between members of homologous pairs, creating chromosomes that are mosaics of maternal and paternal homologs.

    • Converts diploid number of chromosomes to haploid number; 2n \rightarrow n.

    • Following fusion of two gametes at fertilization, the diploid number is reestablished.

    • Exception to homologous chromosomes: Sex-determining chromosomes (one pair) are not homologous in size, centromere placement, arm ratio, or genetic content.

      • Female: XX.

      • Male: XY.

  • Meiosis I

    • Prophase I

    • Metaphase I

    • Anaphase I

    • Telophase I

    • Cytokinesis

  • Meiosis II

    • Prophase II

    • Metaphase II

    • Anaphase II

    • Telophase II

    • Haploid gametes are the result

  • Key Structures in Meiosis

    • Chromomeres

    • Bivalent

    • Tetrad

    • Chiasma

    • Terminalization

Basic Structure of a Human Chromosome

  • Components

    • Centromere: Constricted region.

    • p arm: Short arm (“p” for petite).

    • q arm: Long arm.

Mendel’s Law of Inheritance

  • Segregation

  • Independent Assortment

Gametogenesis and Fertilization

  • Spermatogenesis

    • Process: Occurs entirely in testes.

    • Meiotic Divisions: Equal division of cells.

    • Gamete Production: Is involved in gamete production.

    • Gamete Size: Sperm smaller than spermatocytes.

    • Number Produced: Four.

    • Timing: Uninterrupted process.

    • Onset: Begins at puberty.

    • Release: Continuous.

    • End: Lifelong (but reduces with age).

  • Oogenesis

    • Process: Occurs mostly in ovaries.

    • Meiotic Divisions: Unequal division of cytoplasm.

    • Gamete Production: Is not involved in gamete production.

    • Gamete Size: Ova larger than oocytes.

    • Number Produced: One (plus 2-3 polar bodies).

    • Timing: In arrested stages.

    • Onset: Begins in the fetus (prenatal).

    • Release: Monthly from puberty (menstrual cycle).

    • End: Terminates with menopause.

  • Cellular Events

    • Spermatogonium -> Primary Spermatocyte -> Secondary Spermatocytes -> Spermatids -> Spermatozoa

      • Involves growth/maturation, meiosis I and II, and differentiation.

    • Oogonium -> Primary Oocyte -> Secondary Oocyte -> Ootid -> Ovum

      • Involves meiosis I and II