The Origin of Eukaryotic Cells. Free Template from Nowadays we have known that cells can be...

The Origin of Eukaryotic Cells

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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 .


The earliest prokaryotic ancestors were presumed to have been anaerobic( 厌氧的 ) heterotrophic （异养的） cells .


• 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.


But what is the origin of the eukaryotic cells?


Most people have been agreed that prokaryotic cells arose before eukaryotic cells and gave rise to eukaryotic cells .


• 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.)


• 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.


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 .



• 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 .


• 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 .


• 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 .


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 .


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 .


How does one determine evolutionary relationships among organisms that have been separated in time for billions of years?


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 .


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。

Date post:	14-Dec-2015
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