Origin of Eukaryotes

Features of Eukaryotic Cells

1. Presence of a nucleus and linear chromosomes

2. Membrane-enclosed organelles

3. Larger than prokaryotes

4. Presence of cytoskeleton and cellular dynamics

5. Sexual reproduction (Meiosis)

Cytoskeleton and Cellular Dynamics

Cytoskeleton - network of proteins that facilitate cell shape, motility (amoeboid movement), transportation of substances in the cell, dynamic membrane for feeding activities (like endocytosis).

Cellular Dynamics - Refers to the mechanisms that govern the mechanical movement and interactions of cells over time

Sexual Reproduction

Sexual reproduction promotes genotypic diversity by introducting new variations (via recombination, independent assortment and random fertilizations).
Unicellular Eukaryotes - alternate between haploid and diploid life cycles

Multicellular Eukaryotes - have distinct organs and stages of cell cycle for gametic production

Endosymbiosis

Relationship between two species in which one organism lives inside the cell or cells of another organism. Typically mutualistic and common among unicellular organisms.

Plastids

Refer to chloroplasts and related organelles

Endosymbiont Theory

Proposes that mitochondria and plastids originated from small prokaryotes residing within larger host cells. Eukaryotic cells likely originated from serial endosymbiosis

Steps of Serial Endosymbiosis

1. Plasma membrane formed endomembrane through infoldings, giving rise to structures (ER, nuclear envelope)

2. Endosymbiotic relationship established between host prokaryote and an aerobic heterotrophic prokaryotic symbiont.

   • Likely an alpha-proteobacterium, later becoming the mitochondrion, marking the origin of ancestral heterotrophic eukaryote

3. Additional endosymbionts occurred later on, cyanobacterium engulfed, later becoming the chloroplast

Types of Endosymbiosis

Primary Endosymbiosis - Process in which a prokaryotic cell is engulfed by another organism

Secondary Endosymbiosis - Process in which a eukaryotic cell is engulfed by another eukaryotic cell

Evidence for Endosymbiotic Theory

1. Mitochondria and plastids both have a wall with double membranes

2. Inner membrane of mitochondria/chloroplasts are homologous to plasma membrane of the cell

3. DNA in mitochondria and plastids are circular

4. Organelles similar size to prokaryotes

5. Genomes within mitochondria and plastids replicate without interference of nuclear DNA

6. Proteins can be synthesized within mitochondria and plastids

Benefits of Multicellularity

1. Specialization of cells for distinct functions

2. Size and complexity of organisms are increased (multiple cells can reduce or eliminate physiological limitation from surface-to-volume ratio)

3. Life span of organism is extended

4. Coordinated activities of multiple cells provide a defensive advantage (like the immune system)

Phylogeny of Basal Eukrayotes

Ribosomal RNA suggests mitochondria and chloroplasts placed within monophyletic bacteria. Eukarya closer to Archaea than Bacteria;

• nuclear genome in eukaryotes contain genes shared with archaea and bacteria, also has unique genes seperating them as a distinct lineage

Early Phylogeny (Tangled Web) Explanation

1. Evidence is scarce as events took place deep into evolutionary history

2. Horizontal transfer introduced information unrelated to common ancestry