CHAPTER 22THE ORIGIN OF EUKARYOTES

QUICK REVIEW

Prokaryotes Simple Lack nucleus or any membrane enclosed

organelles Eukaryotes

Complex Contain nucleus and membrane enclosed

organelles

ORIGIN OF EUKARYOTES

Arose during the Proterozic eon Not fully understood how first cell originated DNA sequences examined Studies show that nuclear, mitochondrial

and chloroplast genomes are derived from cells that were once separate but then came together All eukaryotes contain DNA in nucleus and

mitochondria Plants have DNA in chloroplasts as well

QUESTION #1

ORIGIN OF NUCLEAR GENOME

Derived from ancient bacteria and archea Bacteria

Lipid biosynthesis and genes encoding proteins for metabolic pathways

Archea Genes related to transcription and translation

Possibilities for relationship between bacteria and archea Symbiotic Endosymbiotic

SYMBIOSIS

2 different species live in direct contact with each other

Hypothesis Ancient bacterium and archaeon formed a

symbiotic relationship A fusion event combined the genetic material

of the organisms Natural selection favored retention of genes

involved with translation and transcription, metabolism and lipid biosynthesis

ENDOSYMBIOSIS

Relationship in which a smaller organism (endosymbiont) lives within a larger organism (host)

Hypothesis An ancient archeon engulfed a small

bacterium into its cytoplasm Possibly occurred via endocytosis Eventually genes were transferred from the

bacterium to the host archeon resulting in the nuclear genome

SYMBIOSIS & ENDOSYMBIOSIS

ORIGIN OF MITOCHONDRIAL AND CHLOROPLAST GENOMES

Russian botanist suggested an endosymbiotic origin Discovery of genetic material in chloroplasts

and mitchondria supported this theory Mitochondria in eukaryotes likely derived

from bacterial species similar to a-proteobacteria Bacteria that utilizes oxidative

phosphorylation to make ATP

MITOCHONDRIAL AND CHLOROPLAST GENOMES, CONT.

2 possible hypotheses An endosymbiotic event involving an a-

proteobacteria resulted in the first eukaryotic cell and the first mitochondrian was a by-product

ORA symbiotic or endosymbiotic event produced the first eukaryote, and then a subsequent endosymbiosis produced the first mitochondria

DNA SEQUENCING

DNA sequencing of mitochondrial and prokaryotic genomes has been used to determine which hypothesis is correct Data from DNA sequencing has indicated

that chloroplasts originate from an endosymbiotic event between a eukaryote and a cyanobacterium

IS IT POSSIBLE?

In 2001, an endosymbiotic relationship between two different proteobacteria was reported

Mealybugs Bacteria survive within cytoplasm of large

host cells of bacteriome Due to this observation, it is now known

that endosymbiosis can occur between two different prokaryotic species