Notes on Essential Cell Biology: Common Features, Endosymbiosis, and Model Organisms

Common Features of All Living Cells

  • All living cells share a set of core features that distinguish them from non-living matter and viruses.

  • This course emphasizes the biochemical and biophysical processes that sustain, animate, propagate, coordinate, and integrate living cells into tissues, and the macromolecules that mediate these processes.

Five Essential Characteristics of All Living Cells

  • 1. Bounded by a lipid-rich plasma membrane

    • The membrane is a lipid barrier that defines the cell boundary and mediates transport and communication with the environment.

    • Phospholipid organization can form a phospholipid bilayer; some cells (notably certain Archaea) can have a phospholipid monolayer for membrane stability in extreme conditions. The classic depiction includes liposomes and the plasma membrane.

    • Key idea: membrane integrity is essential for maintaining the internal milieu.

  • 2. Contain nucleic acid that encodes at least one complete copy of a genome

    • Historically RNA-based in origin, but present-day cellular life uses DNA as the genome.

    • All cells replicate DNA by template-directed polymerization.

    • In replication, base pairing ensures fidelity; examples include a sequence with complementary base pairs (A pairs with T, G pairs with C).

    • Semi-conservative replication: each daughter DNA molecule contains one parental strand and one newly synthesized strand.

    • Important figures: template strand directing synthesis of a new strand; parent double helix gives rise to two new double helices.

    • Notation:
      extDNAreplication<br>ightarrowexttwoDNAmolecules,eachwithoneoldandonenewstrandext{DNA replication} <br>ightarrow ext{two DNA molecules, each with one old and one new strand}

  • 3. Capable of regulated metabolic activity

    • Cells carry out regulated metabolism, including conversion of nutrients to energy and biosynthetic building blocks.

    • Example pathways highlighted: fatty acyl CoA formation and turnover, and the associated co-factors and cycles (e.g., NADH/NAD⁺, FAD/FADH₂) involved in energy metabolism.

    • A depicted sequence shows the conversion of fatty acyl CoA through metabolic steps and cofactor cycles, illustrating regulation and flux control.

    • Through macromolecular catalysis, cells regulate metabolism via enzymes and molecular machines.

  • 4. Capable of protein synthesis

    • Central dogma components are conserved: DNA -> RNA -> Protein.

    • Transcription uses RNA polymerases to synthesize RNA from DNA templates; translation uses ribosomes and tRNA to synthesize proteins from mRNA templates.

    • The machinery of protein synthesis is highly conserved across all cells and is predominantly RNA-based in its components (e.g., ribosomal RNA and tRNA).

    • Illustrative chain:
      extDNA→transcriptionRNA→translationProteinext{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}

    • Key players shown include RNA polymerase II, 5S rRNA, ribosomes, and tRNAs.

  • 5. Capable of autonomous replication

    • Cells are capable of self-replication without external genetic material; organelles with genomes (like mitochondria and chloroplasts) are evidence of autonomous replication remnants.

    • Endosymbiotic theory supports that some organelles originated as free-living bacteria that became integrated.

    • Example: mitochondria replicate by fission, similar to bacterial division.

    • Evidence for autonomous replication is supported by the size, circular genomes, and ribosome-like machinery of organelles such as mitochondria.

DNA Replication and Central Dogma in Cells

  • DNA replication is template-directed and semi-conservative

    • Template strand guides synthesis of a new complementary strand.

    • Result: two DNA molecules, each containing one parental strand and one newly synthesized strand.

    • Notation:
      exttemplatestrand→new strandext{template strand} \rightarrow \text{new strand}
      extSemi−conservativereplicationext{Semi-conservative replication}

  • Transcription and RNA processing

    • Regions of the genome (genes) are transcribed into RNA, which can be processed into mature transcripts.

    • RNA serves as an expendable information carrier for protein synthesis.

    • Common steps depicted:

    • DNA → transcription → RNA

    • RNA → translation → protein

    • Notation:
      DNA→TRANSCRIPTIONRNA→TRANSLATIONProtein\text{DNA} \xrightarrow{\text{TRANSCRIPTION}} \text{RNA} \xrightarrow{\text{TRANSLATION}} \text{Protein}

    • Many identical RNA transcripts can be produced from a single gene locus.

  • The genetic code and translation machinery

    • The translation machinery is conserved among all cells and relies heavily on RNA components (ribosomal RNA and transfer RNA).

    • The ribosome is the molecular machine that reads mRNA and assembles amino acids into polypeptides with the help of tRNAs.

    • Key molecules shown: transfer RNA (tRNA), ribosomes, and various ribosomal subunits; protein synthesis is a central, conserved process.

Endosymbiosis and the Origin of Eukaryotic Cells

  • Endosymbiosis Hypothesis

    • Proposes that mitochondria (and chloroplasts in plants) originated from free-living bacteria that entered early archaeal/eukaryotic ancestors as endosymbionts.

    • Evidence summarized in slides:

    • Mitochondria and chloroplasts have similarities to bacteria: similar size, circular genomes, ribosomes, and t-RNAs, and aerobic metabolism in mitochondria.

    • Mitochondria divide by fission, like bacteria.

    • Asgard archaea and endosymbiont associations provide possible evolutionary links to mitochondria and nuclear membrane development.

    • Plastids (chloroplasts) arose from photosynthetic bacterial endosymbionts; chloroplasts have outer and inner membranes and photosynthetic machinery.

    • Historical note: Lynn Margulis proposed the Endosymbiosis Hypothesis in 1966; initially controversial and rejected by some journals, but now widely accepted.

  • Key lines of evidence

    • Similar size between organelles and certain bacteria

    • Circular genomes in mitochondria/chloroplasts

    • Prokaryotic-like ribosomes within organelles

    • Unique organelle t-RNAs

    • Aerobic metabolism evidenced in mitochondria

  • Illustrative detail from the slides

    • Mitochondrial genomes and replication show bacterial-like features: fission for reproduction; mtDNA is separate from nuclear DNA.

    • Archæal endosymbionts and ectosymbionts illustrate evolutionary steps toward organelle formation and the development of the nucleus and internal membranes.

  • Chloroplasts and photosynthesis

    • Chloroplast outer and inner membranes emerged as photosynthetic bacteria became endosymbionts; chloroplasts are the descendants of these endosymbiotic organisms.

    • The chloroplast genome, its membranes, and photosynthetic apparatus reflect the endosymbiotic origin.

  • Domain tree of life

    • The three-domain model situates Bacteria, Archaea, and Eukaryotes as distinct domains.

    • Eukaryotes arose from an archaeal lineage that acquired bacterial endosymbionts (mitochondria) and later chloroplasts in plants/algae.

  • Implications

    • Endosymbiosis explains the origin of organelles, cellular complexity, and the evolution of eukaryotic cells with compartmentalized functions.

Model Organisms and Their Roles in Cell Biology

  • Escherichia coli (E. coli) – Prokaryote

    • The best-characterized prokaryote: common, easy to grow, robust, and ideal for cloning experiments.

  • Saccharomyces cerevisiae (budding yeast) – Eukaryote

    • Simplest eukaryote; useful for studying cell cycle, carbohydrate metabolism (fermentation), and temperature-sensitive mutants.

    • Features include a Golgi apparatus, mitochondrion, and vacuole.

  • Arabidopsis thaliana – Plant model

    • Widely used for plant molecular biology; genome ~110 Mb; short life cycle (~8-10 weeks); representative of higher plants.

  • Caenorhabditis elegans – Animal model

    • A nematode with a fixed cell lineage: exactly 959 somatic cells; complete developmental lineage is known; excellent for genetic analysis.

  • Drosophila melanogaster (fruit fly) – Animal model

    • Over 90 years of use; complete genome available; broad utility in development, neuroscience, gene expression, clocks, and transgenics; notable for polytene chromosomes.

  • Xenopus species (X. tropicalis, X. laevis) – Vertebrate development

    • Used to study vertebrate development, neuroscience, and genetics; embryos are suitable for manipulation and observation.

  • Zebrafish (Danio rerio) – Vertebrate model

    • Vertebrate genetics and development; transparent embryos; useful for studying development, neuroscience, and behavior; transgenics available.

  • GloFish – Fluorescent fish (model demonstration)

    • Not a standard model for deep biology; used for education and display; demonstrates genetic modification in vertebrates.

  • Mus musculus (mouse) – Mammalian model

    • Mouse is a standard model for mammalian biology; transgenic lines are relatively accessible; complete genome sequence available; extrapolation to human biology is common.

  • Summary of model organism usage

    • Each organism provides a different balance of simplicity, genetic tractability, and relevance to human biology.

Subcellular Architecture: Organelles and the Cytoskeleton

  • Cytoskeleton – Three major components

    • Actin filaments: contribute to cell shape and motility; vesicle delivery and mitosis support; mechanical stability.

    • Microtubules: provide tracks for vesicle movement; essential for mitosis and chromosome segregation.

    • Intermediate filaments: provide mechanical stability.

    • Overall function: mechanical support, intracellular transport, and organization of organelles.

  • Nucleus and nuclear envelope

    • Eukaryotes sequester DNA into a distinct nuclear compartment.

    • Nuclear envelope with nuclear pore complexes regulates traffic; the nucleus houses the genome and transcriptional machinery.

    • The nucleolus is involved in ribosome biogenesis.

    • Nuclear dynamics during cell division involves stages: interphase, prophase, metaphase, with chromosomes condensing during mitosis.

  • Endomembrane system

    • Endoplasmic reticulum (ER): rough (with ribosomes) and smooth (lipid synthesis, calcium storage).

    • Functions include membrane protein synthesis, protein surveillance, and N-glycosylation; calcium homeostasis.

    • Golgi apparatus: processing and distribution center for proteins and lipids; contains cis and trans faces; processing includes glycosylation and quality control/surveillance.

    • Vesicles and endosomes: transit and sorting compartments; endosomes distribute material acquired by endocytosis; secretory vesicles transport cargo to the plasma membrane for exocytosis.

    • Lysosomes: degradation and recycling compartments; peroxisomes: lipid metabolism and reactive oxygen species detoxification; mitochondria: energy production and apoptosis.

    • Nuclear envelope, ER, Golgi, lysosomes, endosomes, peroxisomes, and mitochondria together form the eukaryotic endomembrane system.

  • Mitochondrion

    • Double-membrane organelle responsible for aerobic metabolism.

    • Key processes include the citric acid cycle and oxidative phosphorylation, leading to ATP production, and involvement in apoptosis.

    • Mitochondria replicate by fission; they possess their own DNA (mtDNA) and ribosomes, reflecting their endosymbiotic origin.

  • Peroxisomes

    • Involved in lipid metabolism and detoxification; contain enzymes for peroxide metabolism.

  • Nucleolus and nucleus

    • The nucleus houses genomic DNA and transcriptional machinery; the nucleolus is the site of rRNA synthesis and ribosome assembly.

  • Endoplasmic reticulum and Golgi connection

    • ER synthesizes proteins and lipids; Golgi processes and sorts them for delivery to various destinations, including the plasma membrane and secretory pathways.

  • Primary cilium

    • Most differentiated cells have a single primary cilium, a specialized sensory organelle that participates in signaling.

    • In vertebrates, highly modified cilia function as principal sensors in specialized sensory organs.

  • Cytoplasm and cellular crowding

    • The cytoplasm is a crowded, crowded space where much of the water is bound; macromolecular crowding influences diffusion and biochemical reactions.

Endomembrane System and Intracellular Transport

  • Endoplasmic reticulum (ER)

    • Rough ER: ribosome-studded region dedicated to protein synthesis and membrane protein integration.

    • Smooth ER: lipid synthesis and calcium storage; participates in detoxification processes.

    • ER morphology and protein processing are integrated with the secretory pathway.

  • Golgi apparatus

    • Receives proteins and lipids from ER via vesicles; cis face receives cargo, trans face dispatches to destinations.

    • Glycosylation and processing occur within Golgi cisternae.

    • Golgi also participates in protein surveillance and packaging for secretion.

  • Endosomes and lysosomes

    • Endosomes sort internalized material and direct it to appropriate destinations; lysosomes degrade biomolecules and recycle components.

    • Secretory vesicles along the secretory pathway deliver cargo to the plasma membrane.

The Cytoplasm, Organelles, and Cellular Architecture in Context

  • Cytoplasm and cytosol

    • Cytosol is the aqueous matrix inside the cell excluding organelles; the cytoplasm includes cytosol plus organelles.

    • The cytoplasm is a crowded space; many essential reactions occur within this milieu.

  • Cytoskeletal architecture and function

    • The cytoskeleton provides cell shape, intracellular transport tracks, and structural integrity necessary for cellular processes like mitosis and vesicle trafficking.

Virus vs. Cells and the Human Microbiome

  • Not all biological particles are cells; viruses are subcellular Infectious agents that require host cells to replicate.

  • The human microbiome

    • The body hosts a diverse community of microorganisms (bacteria, archaea, fungi) contributing to vitamin production, nutrient metabolism, detoxification, and protection against pathogens.

    • A representative map shows dominant groups across body sites (e.g., gut, skin, oral cavity, nasal passages, etc.).

    • Prominent groups include Actinobacteria, Firmicutes (e.g., Lactobacillus), Bacteroidales, and others listed in the microbiome map.

Practical and Ethical Contexts

  • Model organisms as tools for discovery

    • The use of E. coli, yeast, Arabidopsis, C. elegans, Drosophila, Xenopus, zebrafish, mice, and other models enables controlled studies of gene function, development, and disease.

    • Ethical considerations accompany animal research and genetic manipulation; models provide insights while reducing reliance on higher mammals.

Upcoming Topics

  • Next time: Proteins (Chapters 3/4)

  • Anticipated themes: protein structure, function, folding, signaling, and interactions within the cell.

Quick Reference: Key Terms and Concepts

  • Lipid bilayer vs monolayer membranes in different domains

  • Nucleic acids: DNA as genome, RNA transcripts, transcription, translation

  • Central dogma: DNA → RNA → Protein

  • Endosymbiosis: mitochondria and chloroplasts origins; circular genomes; bacterial-like ribosomes

  • Three-domain system: Bacteria, Archaea, Eukaryotes

  • Organelles: nucleus, ER (rough/smooth), Golgi, endosomes, lysosomes, peroxisomes, mitochondria, chloroplasts (in plants)

  • Cytoskeleton: actin, microtubules, intermediate filaments

  • Primary cilium: sensory organelle

  • Model organisms: E. coli, S. cerevisiae, A. thaliana, C. elegans, D. melanogaster, Xenopus spp., zebrafish, mouse

  • Microbiome: gut and body-site microbial communities and their roles