Transcript The Road Not Taken

The Origin of
Eukaryotic
Cells
Nowadays we have known
that cells can be conveniently
divided into two groups :
prokaryotic and eukaryotic
cells.And people already
presumed that how the
prokaryotic cells start their
lives .
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The earliest prokaryotic
ancestors were
presumed to have been
anaerobic(厌氧的)
heterotrophic（异养的）
cells .
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• Anaerobic because they derived their
energy from food matter without
employing molecular oxygen .
• heterotrophic because they were
unable to synthesize organic
compounds from inorganic
precursors ,but instead had to obtain
preformed organic compounds from
their environment.
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But what is the
origin of the
eukaryotic cells?
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Most people have
been agreed that
prokaryotic cells arose
before eukaryotic cells
and gave rise to
eukaryotic cells .
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• This opinion can be verified
directly from the fossil(化石)
record ,which shows that
prokaryotic cells were present in
2.7 billion years ago .(It is 1 billion
years earlier than the eukaryotic
cells appear.)
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• This point is follow from the
the fact that 2 types of cells
have to be related to one
another because they share
many complex traits that could
not have enough
independently in different
organisms.
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About 1970,Lynn Margulis change
peoples concept through his work at
Boston University .He put forward a
hypothesis（假说） which is referred to
as the endosymbiont theory :The
certain organelles of a eukaryotic
cell—most notably the
mitochondria and chloroplasts —
had evolved from smaller
prokaryotic cells that had taken up
residence in the cytoplasm of a
larger host cell .
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• Step one :
A large ,anaerobic ,heterotrophic
prokaryote ingested a
small ,aerobic prokaryote.
• Step two :
Resisting digestion within the
cytoplasm ,the small aerobic
prokaryote took up residence as a
permanent endosymbiont ,and a
colony of these composite cell was
produced .
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• Step three :
The cell whose ancestors had
formed through the sequence of
symbiotic events just described
could have given rise to a line of
cells that evolved other basic
characteristics of eukaryotic
cells ,including a system of
membranes ,a complex
cytoskeleton ,and a mitotic（有丝
分裂） type of cell division .
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• Step four :
The cell which possessed these
various internal compartment
would have been an ancestor of a
heterotrophic eukaryotic cell .
• Step five :
The acquisition of another
endosymbiont could have
converted an early heterotrophic
eukaryote into an ancestor of
photosynthetic eukaryotes .
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About all of this ,we known
that groups of eukaryotes
either possess
mitochondria or show
definitive evidence they
have evolved from
organisms that possessed
these organelles .
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If we want to say that the modern
eukaryotes and most modern
prokaryotes evolved from two
different ancient groups,it is
conceivable under these
circumstances （情况）:a
particular group of living
prokaryotes —descendants of the
same group that gave rise to the
first prokaryote .
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How does one determine
evolutionary relationships
among organisms that
have been separated in
time for billions of years?
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Because the difference between
organisms in the sequence of
amino acid that make up a protein
or the sequence of nucleotides
that make up a nucleic acid are
the result of mutations of DNA that
have been transmitted to
offspring ,mutations can
accumulate in a given gene at a
relatively constant rate over long
periods of time .
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Consequently ,comparisons of
amino acid or nucleotide
sequences can be used to
determine how closely
organisms are related to one
another.
结论：我们可以通过对两种有
机体中氨基酸或者核苷酸序列
的比较，从 而得到它们之间亲
缘的远近关系 。
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