TS

CELL DIVISION- BIOS (copy)

Microscopy: Imaging and Resolution

  • Microscopy history and purpose

    • Microscopes let us see cells: invented in 1590; improved with advancing technology.
    • Modern standard in microbiology is the light microscope: uses visible light passed through the specimen and through glass lenses to magnify the image.
    • Two main families of microscopes discussed: light microscopes (visible light) and electron microscopes (electrons).
    • Electron microscopes were introduced in the 1950s to visualize viruses, ribosomes, proteins, lipids, and small molecules by using electron beams instead of light.

  • Light microscope basics

    • Light is focused by glass lenses; there are ocular (eyepiece) lenses and objective lenses (various types).
    • Lenses refract (bend) light to magnify the specimen image.
    • Two key parameters define microscopy:
    • Magnification: the ratio of an object's image size to its real size. M = \frac{I}{S} where I = image size, S = real size.
    • Resolution: the ability to distinguish two points as separate; the minimum resolvable distance between two points.

  • Limits of resolution and scale

    • Human eye resolution: about 0.2~\text{mm} (200 micrometers).
    • Light microscope resolution: down to about 0.2~\text{µm} = 200~\text{nm}.
    • Electron microscopes have far higher resolution because electron wavelengths are much shorter than visible light; typical electron-microscope resolution is about 0.002~\text{nm} (2 picometers).
    • This jump in resolution explains why electron microscopes can visualize viruses, ribosomes, and molecular features that light microscopy cannot.

  • Electron microscopy: two main types

    • Transmission electron microscope (TEM): electrons are shot through the specimen; requires thin slices and magnetic focusing.
    • Scanning electron microscope (SEM): electrons are scanned across the surface and used to form a 3D-like image of surfaces by scattering.
    • Both rely on the much shorter electron wavelength to achieve superior resolution.

  • Domains and cell types (big picture)

    • Three domains: Bacteria, Archaea, Eukaryota.
    • Prokaryotes include Bacteria and Archaea; typically single-celled and microscopic.
    • Eukaryotes include a wide variety of organisms; many are multicellular, but there are unicellular eukaryotes called protists.
    • Some scientists debate the exact number of kingdoms; domains are a higher-level grouping that helps organize evolutionary relationships.
    • Important distinction: Prokaryotes vs Eukaryotes in terms of cellular organization and compartments.

  • Historical classification notes

    • When there were five traditional kingdoms, Bacteria and Archaea were lumped as Monera due to similarities and differences in cell walls; later technologies clarified they are distinct groups.
    • Some sources now emphasize domains (Bacteria, Archaea, Eukaryota) rather than a single Monera kingdom for prokaryotes.

  • Cell structure: common features across all cells

    • Every cell has a boundary: a plasma membrane (often called the cell membrane) that acts as a selective barrier.
    • Cytosol: the semi-fluid interior within the membrane where organelles reside.
    • Chromosomes carry genes; all cells contain ribosomes for protein synthesis.

  • Major differences: prokaryotes vs eukaryotes

    • Prokaryotes: no membrane-bound organelles or nucleus; DNA usually in a nucleoid region; generally smaller; simpler organization.
    • Eukaryotes: membrane-bound organelles including a nucleus; larger; more complex organization.
    • Size ranges:
    • Smallest bacteria: about 0.11~\text{µm} in diameter (Mycoplasmas).
    • Most bacteria: 1-10~\text{µm} in diameter.
    • Eukaryotes: typically 10-100~\text{µm} in size.

  • Surface area to volume (SA:V) considerations

    • Smaller cells have higher SA:V, which facilitates exchange with the environment.
    • Example using a cube approximation:
    • For a cube of side length a=1~\text{µm}: SA = 6a^2 = 6(1)^2 = 6\;\text{µm}^2; V = a^3 = 1\;\text{µm}^3; SA:V = \frac{6}{1} = 6.
    • For a cube of side length a=5~\text{µm}: SA = 6a^2 = 6(25) = 150\;\text{µm}^2; V = a^3 = 125\;\text{µm}^3; SA:V = \frac{150}{125} = 1.2.
    • Implication: as cells get larger, SA:V tends to decrease; multicellularity mitigates this by dividing into smaller units with increased total surface area.
    • Real-world analogy: intestinal folds and alveoli increase surface area to volume to enhance nutrient absorption.
    • Conclusion: smaller cells have higher SA:V and thus more efficient exchange with the environment; this supports why many organisms are multicellular.

  • Plasma membrane and cell boundary details

    • Plasma membrane is a phospholipid bilayer: two layers; hydrophobic tails (water-fearing) face inward; hydrophilic heads face outward toward aqueous environments.
    • Proteins and carbohydrate side chains are embedded or attached to the membrane; many components are hydrophilic and interact with water on both sides of the membrane.
    • Function: selective barrier; allows passage of oxygen, nutrients, and wastes into and out of the cell; contains proteins that facilitate transport and signaling.
    • Plant cells have a cell wall in addition to the plasma membrane; bacterial cells also have cell walls.
    • Cytoskeleton: internal scaffolding (microfilaments, microtubules, and intermediate fibers) that maintains cell shape, anchors organelles, and aids in movement.

  • Endomembrane system and organelles (overview)

    • Endoplasmic reticulum (ER): network of tubular structures that partitions the cell into luminal and extraluminal spaces; two forms:
    • Rough ER: studded with ribosomes; synthesizes proteins destined for secretion or membranes.
    • Smooth ER: lacks ribosomes; involved in lipid synthesis and other metabolic processes.
    • Nucleus: contains nucleoplasm filled with genetic information (DNA) and ribosomes in the nucleus; nucleolus participates in ribosome production; nuclear envelope consists of an outer and inner membrane.
    • Golgi apparatus: flattened disc-like sacs responsible for modification, sorting, storage, and shipping of proteins and lipids to their destinations.
    • Ribosomes: two types
    • Free ribosomes: suspended in cytosol; synthesize cytosolic proteins.
    • Bound ribosomes: attached to the outer surface of the ER; synthesize proteins destined for membranes or secretion.
    • Mitochondria: generate cellular energy (ATP) for cellular processes.
    • Lysosomes: contain hydrolytic enzymes; digest proteins, lipids, carbohydrates, and nucleic acids; nicknamed "suicidal bags" due to degradative capability.
    • Cytoplasm: region between plasma membrane and nucleus; contains cytosol and organelles.

  • Nucleus, DNA, and chromosomes

    • Chromosomes are present in all eukaryotes; number varies by species.
    • Humans: 46 chromosomes (23 pairs).
    • Frogs: 26 chromosomes (13 pairs).
    • Mosquitoes and fruit flies: typically 6 chromosomes total (3 pairs).
    • Chromosome counts can influence metabolic function if abnormal.
    • Reproductive vs somatic cells:
    • Non-reproductive somatic cells are diploid with homologous chromosomes (two nearly identical copies).
    • Reproductive cells (gametes) are haploid; humans have 23 chromosomes in eggs and sperm (n = 23).
    • Fusion of egg and sperm yields a diploid zygote (2n = 46).
    • Homologous chromosomes: paired chromosomes with the same genes but possibly different alleles.
    • Sister chromatids: identical copies produced during DNA replication; held together at the centromere.
    • Centromere: constriction region where sister chromatids are joined.
    • Karyotype: a laboratory-produced image or arrangement of chromosomes ordered by size, banding pattern, and number to assess chromosomal integrity.
    • Historically done by manual staining and sorting; now often computer-assisted.

  • DNA packaging and chromosomal structure

    • DNA is highly charged negatively; histones are positively charged proteins that package DNA into compact chromatin.
    • Histones help wind DNA into chromosomes and regulate gene expression.
    • DNA replication during the cell cycle occurs in the S phase of interphase, producing sister chromatids for each chromosome.

  • DNA replication: origin of replication and replisome

    • DNA replication begins at a specific site called the origin of replication (Ori).
    • Replication is bidirectional: proceeds in two directions away from the origin.
    • Helicases unwind the double helix to form two replication forks.
    • A protein complex called the replisome assembles at the forks to coordinate DNA copying.
    • This process results in two identical copies of each chromosome (sister chromatids) until they are separated during mitosis.
    • In transcription and replication, histones are temporarily displaced and reassembled to allow access to DNA during replication and division.

  • The cell cycle: interphase and mitotic phase

    • Interphase encompasses normal cell growth and DNA replication in preparation for division; consists of G1, S, and G2 phases.
    • G1 (First gap): active biochemical activity; cell grows, accumulates building blocks of DNA and proteins, and stores energy (ATP) for later replication.
    • S phase (DNA synthesis): DNA replication occurs; chromosomes are replicated; centrosomes duplicate, giving rise to the mitotic spindle.
    • G2 (Second gap): energy stores replenished; proteins required for chromosome manipulation are synthesized; some organelles duplicate; cytoskeleton dismantled to provide resources for the mitotic spindle.
    • Mitotic phase (M phase): includes mitosis (nuclear division) and cytokinesis (cytoplasmic division) to produce two daughter cells.
    • G0 (Go) phase: some cells exit the cell cycle and do not actively divide; examples include most neurons and some cardiac muscle cells; they may re-enter the cycle in response to external signals, but often do not.

  • Mitosis: five stages and nuclear division

    • Overall goal: duplicate chromosomes align, separate, and move to opposite poles; culminates in two genetically identical daughter nuclei.
    • Prophase: chromosomes condense (appear as spaghetti-like threads); nuclear envelope begins to break into vesicles; Golgi and ER fragment and disperse; nucleolus disappears; centrosomes move to opposite poles; mitotic spindle begins to form as microtubules extend between poles.
    • Prometaphase: nuclear envelope fragments completely; spindle apparatus attaches to chromosomes via kinetochores; chromosomes begin moving toward the center.
    • Metaphase: chromosomes align on the metaphase plate (equatorial plane) with centrosomes at opposite poles; kinetochores attach to spindle fibers; chromosomes are maximally condensed.
    • Anaphase: sister chromatids separate and are pulled toward opposite poles; the cell elongates as nonkinetochore microtubules slide past one another.
    • Telophase: two daughter nuclei begin to form; chromosomes de-condense; nuclear envelope re-forms around each set of chromosomes.

  • Cytokinesis: division of the cytoplasm

    • Animal cells: cytokinesis begins during anaphase with formation of a contractile actin ring just inside the plasma membrane at the former metaphase plate, creating a cleavage furrow that contracts and splits the cell into two.
    • Plant and fungal cells: due to rigid cell walls, cytokinesis is different; a cell plate forms between the daughter cells to build a separating cell wall rather than a cleavage furrow.

  • Checkpoints and variations

    • Not all cells comply with a strict cell cycle; some enter G0 and remain quiescent until an external signal triggers re-entry into the cycle.
    • Cardiac muscle cells and neurons are examples that may remain in G0 permanently, making them less capable of regeneration after injury.

  • Connecting ideas and broader implications

    • The distinction between prokaryotes and eukaryotes underpins many practical applications (e.g., antibiotic targets often exploit differences in cell membranes and ribosomes).
    • The SA:V concept provides a physical constraint on cell size and explains why multicellularity is advantageous for nutrient uptake and exchange.
    • Karyotyping and chromosome counts are essential in medical genetics, prenatal screening, and cancer diagnostics.
    • The debate about how many kingdoms exist touches on philosophy of science: how we classify life versus how we infer evolutionary relationships.

  • Quick reference of key terms and concepts mentioned

    • Magnification: M = \frac{I}{S}
    • Resolution: ability to distinguish two points as separate
    • SA:V ratio: for a cube of side length a, SA = 6a^2, V = a^3, so SA:V = \frac{6}{a}
    • Smallest bacteria: \approx 0.11~\µm diameter (mycoplasmas)
    • Typical bacteria: 1-10~\µm; typical eukaryotes: 10-100~\µm
    • Nucleoplasm, nucleolus, nuclear envelope (outer and inner membranes); ribosomes; rough ER (ribosomes) and smooth ER (no ribosomes)
    • Centrosomes with two centrioles; mitotic spindle; kinetochores
    • Replisome: multi-protein complex at replication forks coordinating DNA synthesis
    • Origin of replication (Ori); bidirectional replication; replication forks
    • DNA packaging with positively charged histones; DNA is negatively charged
    • Haploid (n) vs Diploid (2n); human gametes are haploid (n = 23); zygote is diploid (2n = 46)
    • Karyotype: ordered image of chromosomes used for analysis
    • Prophase, Prometaphase, Metaphase, Anaphase, Telophase (mitotic stages)
    • Cytokinesis: cleavage furrow in animals vs cell plate formation in plants

  • Note on terminology and classroom context

    • The speaker sometimes mixed statements about plant/bacteria cell walls; standard biology: plants and bacteria have cell walls; animal cells do not.
    • Health-related relevance: understanding karyotypes and chromosome numbers is foundational for diagnosing certain genetic conditions and for understanding development and cell division disorders.

  • See-also: related topics in genetics and cell biology often covered in later lectures

    • Phylogeny and evolutionary relationships among domains and kingdoms
    • More detailed exploration of the cell cycle checkpoints (G1/S, G2/M, etc.) and their regulatory proteins
    • Genetic expression control via histone modification and chromatin remodeling
    • Differences in cytokinesis among plant, animal, and fungal cells at the structural level