Cells come in two main flavors: prokaryotic and eukaryotic. Prokaryotes, like bacteria, are simpler and lack a nucleus. Eukaryotes, including plants and animals, have a nucleus and other fancy parts. These differences shape how they work and evolve.
Prokaryotes and eukaryotes organize their genes differently. Prokaryotes have a single DNA loop, while eukaryotes pack DNA into chromosomes inside a nucleus. This affects how they make proteins and adapt to their environment.
Prokaryotic vs Eukaryotic Cells
Structural Differences
- Prokaryotic cells lack membrane-bound nucleus and organelles while eukaryotic cells possess these structures
- Prokaryotes measure 0.1-5 μm compared to eukaryotes at 10-100 μm with simpler internal organization
- Prokaryotes include bacteria and archaea whereas eukaryotes encompass protists, fungi, plants, and animals
- Eukaryotic cells contain a cytoskeleton (microfilaments, intermediate filaments, microtubules) less developed or absent in prokaryotes
- Prokaryotes typically reproduce through binary fission
- Involves replication of single circular chromosome
- Cell elongates and splits into two identical daughter cells
- Eukaryotes undergo mitosis and meiosis for cell division and reproduction
- Mitosis produces two genetically identical daughter cells
- Meiosis produces four genetically diverse haploid cells (gametes)
Genetic Material Organization
- Prokaryotes contain single circular DNA molecule in nucleoid region
- Eukaryotes possess linear DNA organized into multiple chromosomes within membrane-bound nucleus
- Eukaryotic DNA associates with histone proteins forming chromatin structure
- Allows for compaction and regulation of gene expression
- Chromatin can condense further into chromosomes during cell division
- Prokaryotic DNA typically lacks histone association
- Prokaryotes often contain plasmids (small circular DNA molecules) separate from main chromosome
- Plasmids can carry genes for antibiotic resistance or virulence factors
- Can be transferred between bacteria through conjugation
- Eukaryotes generally lack plasmids with some exceptions in certain protists (mitochondrial DNA)
Genetic Material Organization
Gene Structure and Expression
- Prokaryotic genes often organized in operons transcribed as single mRNA
- Example: lac operon in E. coli controls lactose metabolism
- Allows for coordinated regulation of related genes
- Eukaryotic genes typically individually transcribed and contain introns and exons
- Introns removed during RNA splicing
- Exons can be alternatively spliced to produce different protein isoforms
- Prokaryotic transcription and translation coupled in cytoplasm
- Ribosomes can begin translating mRNA while it's still being transcribed
- Eukaryotic transcription occurs in nucleus, translation in cytoplasm
- Spatial separation allows for additional regulation and processing
Post-transcriptional Modifications
- Eukaryotic genes undergo extensive post-transcriptional processing
- Splicing removes introns and joins exons
- 5' capping adds modified guanine nucleotide to protect mRNA
- 3' polyadenylation adds poly-A tail for stability and export
- Prokaryotic mRNA generally lacks post-transcriptional modifications
- Some exceptions exist (tRNA processing, rRNA modifications)
- Eukaryotic mRNA processing enables additional layers of gene regulation
- Alternative splicing can produce multiple protein isoforms from a single gene
- RNA editing can alter the nucleotide sequence of mRNA
Unique Features of Prokaryotes
Cell Envelope and External Structures
- Prokaryotic cells possess peptidoglycan cell wall for structural support and protection
- Gram-positive bacteria have thick peptidoglycan layer
- Gram-negative bacteria have thin peptidoglycan layer with outer membrane
- External structures aid in various functions
- Flagella enable motility (example: H. pylori uses flagella to move through stomach mucus)
- Pili facilitate adhesion and genetic exchange (example: F pilus in E. coli for conjugation)
- Capsules provide protection and enhance virulence (example: Streptococcus pneumoniae capsule resists phagocytosis)
Specialized Internal Features
- Mesosomes involve specialized membrane invaginations
- Participate in DNA replication and cell division
- Aid in energy production and protein secretion
- Gas vesicles regulate buoyancy in aquatic prokaryotes
- Example: Cyanobacteria use gas vesicles to position themselves in water column for optimal light exposure
- Magnetosomes enable magnetotaxis in certain bacteria
- Example: Magnetospirillum magnetotacticum orients itself along Earth's magnetic field
- Prokaryotic ribosomes (70S) found free in cytoplasm
- Smaller than eukaryotic ribosomes (80S)
- Composed of 30S and 50S subunits
Metabolic Diversity and Adaptations
- Prokaryotes exhibit remarkable metabolic diversity
- Photosynthesis in cyanobacteria and purple bacteria
- Chemosynthesis in sulfur-oxidizing bacteria (example: Thiobacillus)
- Anaerobic respiration using alternative electron acceptors (nitrate, sulfate)
- Endospore formation allows survival in extreme conditions
- Example: Bacillus anthracis spores can survive harsh environments for years
- Prokaryotes adapt to various ecological niches
- Thermophiles thrive in hot springs (example: Thermus aquaticus)
- Halophiles live in high-salt environments (example: Halobacterium)
Complexity of Eukaryotic Cells
Membrane-bound Organelles
- Nucleus contains genetic material and regulates gene expression
- Nuclear pores control transport between nucleus and cytoplasm
- Nuclear lamina provides structural support
- Mitochondria generate ATP through oxidative phosphorylation
- Contain own DNA and ribosomes (endosymbiotic origin)
- Cristae increase surface area for ATP production
- Endoplasmic reticulum (ER) synthesizes and modifies proteins and lipids
- Rough ER studded with ribosomes for protein synthesis
- Smooth ER involved in lipid synthesis and detoxification
- Golgi apparatus modifies, sorts, and packages proteins for secretion or transport
- Consists of stacked cisternae (flattened membrane sacs)
- Forms vesicles for protein transport
- Lysosomes contain hydrolytic enzymes for cellular digestion
- Break down cellular debris, damaged organelles, and engulfed particles
- Maintain cellular homeostasis through autophagy
Cytoskeleton and Intracellular Transport
- Complex cytoskeleton provides structural support and enables cell movement
- Microfilaments (actin) involved in cell shape and muscle contraction
- Intermediate filaments provide mechanical strength
- Microtubules facilitate intracellular transport and form mitotic spindle
- Sophisticated mechanisms for intracellular trafficking
- Vesicle-mediated transport moves proteins between organelles
- Motor proteins (kinesin, dynein) transport cargo along microtubules
- Signal sequences target proteins to specific organelles (example: nuclear localization signal)
Compartmentalization and Regulation
- Spatial and temporal regulation of cellular processes
- Compartmentalization allows for specialized environments (example: low pH in lysosomes)
- Separation of transcription and translation enables additional regulation
- Endomembrane system interconnects organelles
- Includes nuclear envelope, ER, Golgi, lysosomes, and vesicles
- Facilitates protein synthesis, modification, and transport
- Semi-autonomous organelles support endosymbiotic theory
- Mitochondria and chloroplasts contain own DNA and ribosomes
- Evolved from ancient prokaryotic endosymbionts