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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