Evidence Much of the evidence for the endosymbiotic theory comes from the structure and handling of these organelles’ genetic codes.. In addition, mitochondria and plastid transcription
Trang 1themselves, and the plastid event about a half billion
years later
Evidence
Much of the evidence for the endosymbiotic theory comes
from the structure and handling of these organelles’ genetic
codes Both mitochondria and plastids have DNA sequences
in circles as that of bacteria Their DNA also lacks histones
(proteins that the DNA is wrapped around) which are present
in eukaryotes and some archea In addition, mitochondria
and plastid transcription begin with the amino acid fMet
(formylmethionine) as in bacteria, not Met (methionine) as in
eukaryotes
Ribosome sizes are questionable evidence for the
endo-symbiotic theory Bacteria usually have ribosomes of 70s
(Svedberg units) and eukaryotes usually have around 80s
in their cytoplasm While the mitochondrial and plastid
ribosomes are usually of around 70s, they do in fact vary among species from around 60s to 80s, thus overlapping both bacterial and cytoplasmic eukaryote ribosome sizes
Other evidence for the endosymbiotic theory comes from the two membranes usually surrounding these organelles The inner membrane belongs to that of the original bacteria and outer membrane presumably a result from the original engulfment The outer membrane has approximately a 1:1 protein–lipid ratio by dry weight, similar to many eukaryotic cytoplasmic membranes, while the inner membrane (which is made of two layers) has approximately a 3:1, similar to many bacteria These organelles and bacteria also both utilize elec-tron transport enzymes lacking elsewhere in eukaryotes Some of the best evidence for the endosymbiotic theory however comes from bioinformatics Phylogenetic analyses of various bacteria, mitochondria from various hosts from vari-ous kingdoms, and nuclear DNA from those hosts usually place mitochondria as most related to a group of bacteria
known as proteobacteria, often placed closest to Rickettsia
EK
) b ( )
a (
AZ
AR
f a
f a
AR
FLA
CH FLE M
Figure 1 The main competing theories of eukaryotic origin Schematic diagrams describing the Archezoa (a) and anti-Archezoa (b) hypotheses, and their archaeal (a) and fusion (f) versions as envisioned from genomic and biochemical perspectives AR, archaeon; BA, bacterium; CH, chimeric prokaryote; AZ, archezoon; EK, eukaryote; MAN, mitochondrial ancestor; FLA, free-living a-proteobacterium; RLE, rickettsia-like
endosymbiont; N, nucleus with multiple chromosomes; E, endomembrane system; C, cytoskeleton; M, mitochondria Reproduced from
Emelyanov VV (2003) Mitochondrial connection to the origin of the eukaryotic cell European Journal of Biochemistry 270(8): 1599–1618, with
permission from Wiley
Eukaryotes, Origin of 331