17._Introduction_to_eukaryotes_mini_17

Unifying Characteristics of Eukaryotes

• Geological Record:

  • Eukaryotes present approximately 2.1 billion years ago (BYA).

  • Prokaryotes date back to about 3.8 BYA (ex: Stromatolites).

• Monophyletic Group:

  • Eukaryotes form a single clade descended from a common ancestor.

• Characteristics of the Last Common Ancestor:

  • Cells containing nuclei.

  • Nuclear envelope with nuclear pores.

  • Presence of mitochondria.

  • Cytoskeleton composed of microtubules and microfilaments.

  • Motility organelles such as flagella or cilia (present in some lineages).

  • Chromosomes organized by histones.

  • Capability for mitosis.

  • Mechanism of sexual reproduction.

  • Some lineages retain cell walls, while many have lost them.

Prokaryote Metabolism & Endosymbiosis

• Aerobic Respiration:

  • Present in all eukaryotic lineages, utilizing mitochondria.

  • Aerobic metabolism is efficient, producing significant amounts of ATP.

• Endosymbiosis Theory:

  • Eukaryotic cells arose from one cell engulfing another, leading to a symbiotic relationship.

  • Co-evolution of engulfed and engulfing organisms.

  • The timing of endosymbiosis in relation to the development of the nucleus remains unclear.

  • Mitochondria = derived from bacteria; nuclear genes = derived from archaea.

Mitochondria

• Presence:

  • Exclusive to eukaryotic cells (numbers per cell range from 1 to over 1,000).

• Structure:

  • Resemble proteobacteria and have two membranes.

• Genome:

  • Contains its own circular chromosome.

• Ribosomes and tRNA:

  • Similar to prokaryotes, indicating a shared ancestry.

• Gene Transfer:

  • Some mitochondrial genes have been transferred to the nucleus.

• Division:

  • Mitochondria divide independently through a process similar to binary fission.

• Anaerobic Eukaryotes:

  • Represent an evolutionary reversal from aerobic to anaerobic metabolism.

Plastids

• Overview:

  • Like mitochondria, plastids include the photosynthetic chloroplasts.

• Symbiotic Origin:

  • Plastids originated from cyanobacteria through endosymbiosis (occurred twice).

  • First occurrence = led to Archaeplastida, ancestors of most modern plants.

  • Second occurrence = resulted in plastids in a few select species.

• Genome:

  • Plastids have a circular chromosome similar to that of mitochondria.

• Secondary Endosymbiosis:

  • Characteristic of major groups of algae (protozoa), where secondary plastids are surrounded by three membranes.

• Evolutionary Connections:

  • Important illustrations and discussion can be found on page 575.