BIL 160 - Lecture 13

Domain Eukarya

Eukaryotes are unicellular or multicellular organisms made up of cells containing a membrane-bound nucleus and organelles.

Plantae Fungi Animalia

a ridiculous diversity of unicellular and colonial protists

Cytoskeleton

The eukaryotic cytoskeleton consists of
• tubulin-based microtubules
• actin-based microfilaments

Flagella

The axoneme is the central strand of the flagellum (pl. flagella). Visible in cross section are
• nine peripheral micotubule doublets
• one central micotubule doublet
• "nine surrounding two")

A cilium (pl. cilia) is a very short flagellum. Its axoneme structure is identical to that of a flagellum.

Endomembrane System

The endomembrane system defines the
• endoplasmic reticulum
• Golgi bodies
• vacuoles
• lysosomes
• peroxisomes
• nuclear envelope

Linear Chromosomes

The primary genome of eukaryotes is contained on multiple, linear chromosomes.

This is distinct from the single, circular chromosome of prokaryotes.

(NOTE: the prokaryotic genome above is from a lysed cell; the DNA doesn't actually look like that in a live bacterium. This is just a more direct way of visualizing the prokaryotic DNA strand.)

Mitochondria

Mitochondria are energy-transducing organelles bounded by a double membrane.

They are derived from ancient, symbiotic prokaryotes.

Derived Ribosomes

Eukaryotic cells have characteristic 80S ribosomes, each consisting of four molecules of RNA complexed with many proteins.

Ribosomal RNA sequences and protein identities differ between prokaryotes and eukaryotes.

Which is handy when you want to hit prokarotes with ribosome-targeting antibiotics.

Mitosis

Eukaryotic cells can reproduce asexually via mitosis.

Prokaryotic cells undergo the less complex asexual process of fission.

Lacking linear chromosomes, prokaryotes do not undergo mitosis or meiosis.

Click on the Bonus Icon.

The Origin of Eukaryotes

Two processes contributed to the origin of eukaryotic cells.

Each occurred more than once in our evolutionary history.

• autogeny
  Extensive inpocketing of the external plasma membrane formed a complex internal network of membranes.

• endosymbiosis
  First proposed by Lynn Margulis, this model describes how small, energy-transducing prokaryotes
  • were ingested as prey, but not digested
  • took up residence as internal symbionts

Eventually, host and internal resident became inextricably linked in a mutually beneficial symbiotic relationship.