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.
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.
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
Components
Centromere: Constricted region.
p arm: Short arm (“p” for petite).
q arm: Long arm.
Segregation
Independent Assortment
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