Lecture 21. Basic Architecture of the Eukaryotic Cell, Symbioses, Early Eukaryote Fossils. reading: Chapter 5

Lecture 21. Basic Architecture of the Eukaryotic Cell, Symbioses, Early

Eukaryote Fossils.

reading: Chapter 5

Basic Structure of the Prokaryotic Cell

Prokaryotes = Archaea + BacteriaProkaryotes- lack nucleus/nuclei

Basic Structure of the Eukaryotic Cell

Eukaryote (“true nucleus”)are much more complexDNA containing organelles (“little organs”)

nucleusmitochondrion - respirationchloroplast - photosynthesis}were once free-living prokaryotes

often have multiple chromosomes(linear chromosomes)lots more geneslots of “junk DNA” in their genes

Eukaryotes are Typically Larger than Prokaryotes

Organelles (“little organs”)

Found only in eukaryotes.

Were once free-living Bacteria.

How do we know this?-have their own genomes-genomes are circular-have 16S rRNA genes chloroplast branches in Cyanobacteria - oxygenic photosynthesis mitochondria branches in Proteobacteria near Rickettsia- aerobic respiration-have lipid membranes (often two sets)-one species of alga still has peptidoglycan cell wall around its chloroplast

Endosymbiosis

Host cell engulfed a prokaryote cell:-many protozoa ingest prokaryote cells as a food source

(endocytosis and then digest cells)-maybe they didn’t digest the cells one day-stably maintained endosymbiontsymbiosis - a mutually beneficial relationship

Endosymbiosis, cont.

mitochondria and chloroplasts contain two or more sets of membranes

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Serial Endosymbiosis

Algae with Primary Endosymbionts

Green Algae(& Plants)

Red AlgaeCyanophora

Algae with Secondary Endosymbionts

Diatoms

Dinoflagellates

Golden Brown Algae

Brown Algae/Kelps

Yellow Algae

Cryptomonads

Chlorarachniophytes

OrganellesMitochondria:- nearly all eukaryotes have them- some have modified them so they no longer do aerobic respiration- some have lost them- eukaryote ancestor likely had mitochondria- engulfment likely happened once

Chloroplasts:- most eukaryotes don’t have them- independent lineages have them- acquired late- original primary engulfment likely happened once- secondary engulfments occurred- some have lost their chloroplasts



chloroplast of a golden brownalgae showing >2 membranes

Ancient Lipids

2.7 GaEukaryote lipids (steranes) and Cyanobacterial lipids.Need oxygen to make steranes.

What does this suggestabout the presence of O2

2.7 Ga?

Early Fossil Record of Eukaryotes

Acritarchs:spherical microfossilsthought to be unicellular algaelarge diameters (20 - 150 µm)thick organic cell walls - easily preservedfirst appear ~1.7 Ga







Later Acritarchs More Diverse



Cambrian 560 Maspiny acritarch



Upper Ordovician





Cyst broken - dormant cell released

all images: copyright Cushman Foundation for Foraminiferal Research



Grypania spiralis

1.3 Ga ~0.5m in length.Found in shales and slates from Montana, China, and India.Not known what organism this once was.Animal? Plant? Algae? Extinct kingdom?

Fossil Red Algae (?)

1.2 GaDistinctive cell division patterns found only

in the red algae.First occurrence of complex multicellularity

in the fossil record.Also first evidence of sexual reproduction.





reading: Chapter 5





Lecture 22. Evolution of Multicellularity, Colonization of Land.

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Lecture 21. Basic Architecture of the Eukaryotic Cell, Symbioses, Early Eukaryote Fossils. reading: Chapter 5