Prokaryotes vs Eukaryotes – Study Notes

Prokaryotes

• Prokaryotes are organisms whose cells lack a nucleus. Etymology: 'pro' = before; 'karyon' = nucleus.

• Distinguishing features to compare with Eukaryotes:

  • Nucleus: absent in prokaryotes; DNA is located in a region of the cytoplasm called the nucleoid.
  • DNA form: circular DNA; chromosome sometimes complemented by smaller, autonomous DNA molecules called plasmids (horizontal gene transfer).
  • Ribosomes: present for protein synthesis; prokaryote ribosome = $70S$.
  • Cell envelope: cell wall often present and composed of peptidoglycan in many bacteria; some have a slime capsule for protection.
  • Motility and attachment: flagella for movement; pili for adherence to surfaces and for conjugation (sex pili).
  • Reproduction: primarily asexual via binary fission.

• Domain-level classification (three domains of life): Bacteria, Archaea, Eukarya.

• Common classification framing (Woese): three-domain system arising from analyses of rRNA genes; domains reflect major evolutionary lineages.

• Prokaryotes can be broadly categorized into two major groups:

  • Domain Bacteria (often referred to as Eubacteria): traditional bacteria including many known pathogens (e.g., $E. coli$, $S. aureus$).
  • Domain Archaea: often extremophiles found in extreme environments (high temperature, salinity, pH).

• In many diagrams, the six kingdom system is shown under the three-domain framework:

  • Bacteria, Archaea, Eukarya are the three domains.
  • Within Eukarya, the six kingdoms (historical view) are Protista, Plantae, Fungi, Animalia, plus Bacteria and Archaea are sometimes listed separately as domains rather than kingdoms.
  • The slide notes explicitly show: Protist, Plantae, Fungi, Animalia as part of the Eukarya domain, and Bacteria (Eubacteria) and Archaea as separate domains within the 6-kingdom framing.

Prokaryotic cell components and features

• Cytoplasm: internal fluid component of the cell.
• Nucleoid: region of the cytoplasm where the DNA is located.
  • DNA is circular and called a genophore.
• Plasmids: autonomous circular DNA molecules that may be transferred between bacteria (horizontal gene transfer).
• Ribosomes: complexes of RNA and protein responsible for polypeptide synthesis; prokaryote ribosome = $70S$.
• Cell wall: rigid outer covering made of peptidoglycan; maintains shape and prevents bursting (lysis).
• Slime capsule: thick polysaccharide layer for protection against desiccation and phagocytosis.
• Flagella: long, slender projections containing a motor protein that enables movement (singular: flagellum).
• Pili: hair-like extensions enabling adherence to surfaces (attachment pili) or mediating bacterial conjugation (sex pili).

Prokaryotic reproduction: binary fission

• Prokaryotic cells reproduce asexually by binary fission.
  • A replication signal triggers copying of circular DNA.
  • The two DNA loops attach to the membrane.
  • The membrane elongates and pinches off (cytokinesis), forming two daughter cells.

Eukaryotes: overview

• Eukaryotic cells contain a nucleus ("eu" = good/true; "karyon" = nucleus).
• They are more complex and are believed to have evolved from prokaryotic cells via endosymbiosis.
• Eukaryotic cells are compartmentalized by membrane-bound organelles that perform specialized roles.
• The cell includes an extensive endomembrane system and cytoskeletal components.
• Major organelles and structures (illustrative list):
  • Endoplasmic reticulum (ER): Rough ER and Smooth ER.
  • Golgi apparatus.
  • Lysosome.
  • Mitochondria.
  • Nucleus with nuclear envelope, chromatin (DNA), and nucleolus.
  • Nucleoplasm and cytoplasm.
  • Nucleolus, ribosomes (80S in eukaryotes).
  • Cytoplasm.
  • Cell (plasma) membrane.
  • Microfilaments, intermediate filaments, and microtubules (cytoskeleton).
  • Peroxisome and vacuole.

Endomembrane system and organelles (diagrammatic terms)

• Endoplasmic reticulum: Rough ER (studded with ribosomes) and Smooth ER (lipid synthesis and detoxification).
• Golgi apparatus: modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.
• Lysosome: contains hydrolases for digestion of macromolecules.
• Mitochondria: powerhouse of the cell; site of cellular respiration; may contain their own DNA and ribosomes.
• Nucleus: houses chromatin (DNA); contains nucleolus where ribosomal RNA synthesis occurs.
• Nuclear envelope: double lipid bilayer surrounding the nucleus.
• Cytoplasm: all cellular content outside the nucleus; site of many metabolic processes.
• Ribosomes: in eukaryotes, typically 80S; in mitochondria and chloroplasts, 70S-like ribosomes.
• Cytoskeleton: microfilaments, intermediate filaments, and microtubules provide shape, support, and transport.
• Peroxisome: contains enzymes for oxidative reactions.
• Vacuole: storage and maintaining turgor (larger in plant cells).

Endosymbiotic origin of Eukaryotes

• Eukaryotic cells are believed to have evolved from prokaryotic cells through endosymbiosis.
• This theory explains the presence of mitochondria (and chloroplasts in plants and algae) with their own circular DNA and a ribosome of bacterial type, suggesting a bacterial ancestry.

Kingdoms and classification (based on Woese 3-domain system)

• Three Domains of Life:

  • Domain Bacteria
  • Domain Archaea
  • Domain Eukarya

• Within the Domain Eukarya, the traditional six kingdoms are often listed as:

  • Protista
  • Plantae
  • Fungi
  • Animalia
  • (Historically) plus two other realms within the Bacteria and Archaea domains depending on schemes, but the primary three-domain framework places Bacteria, Archaea, and Eukarya as the three domains.

• Three-domain classification and common ancestor:

  • All three domains share a common cellular ancestor (root of the tree of life).

The six kingdoms (as presented in the slide context)

• Bacteria (Eubacteria): traditional bacteria; includes many pathogenic forms (e.g., $E. coli$, $S. aureus$).
• Archaea: extremophiles found in environments with extreme temperatures, salinity, or pH.
• Protista: unicellular organisms or simple multicellular organisms without specialized tissues.
• Plantae: cells have a cell wall made of cellulose; obtain nutrition autotrophically via photosynthesis.
• Fungi: typically have cell walls made of chitin and obtain nutrients via heterotrophic absorption; generally non-photosynthetic.
• Animalia: multicellular organisms lacking a cell wall; obtain nutrients via heterotrophic ingestion.

Important notes on content alignment from the slide

• Some wording on the Protista entry in the slide text appears garbled (e.g., reference to a chitin cell wall and heterotrophic absorption in Protista). In standard biology, fungi have chitin in their cell walls and fungi are heterotrophs; Protists vary widely and may lack rigid cell walls. Treat this slide content as a point of study material that may require cross-checking with standard references.

Summary of key terms to remember

• Nucleoid vs Nucleus
• Genophore (circular bacterial chromosome)
• Plasmids and horizontal gene transfer
• 70S ribosome in prokaryotes; 80S ribosome in most eukaryotes
• Peptidoglycan in bacterial cell walls
• Slime capsule, flagella, pili
• Binary fission as prokaryotic reproduction
• Endosymbiosis and derivation of mitochondria/chloroplasts
• The three-domain system: Bacteria, Archaea, Eukarya
• The four main eukaryotic kingdoms within Eukarya: Protista, Plantae, Fungi, Animalia