单项选择题

A eukaryotic cell has( )

A．One copy of each ribosomal transcription unit.
B．Thousands of copies of the 5S rRNA gene.
C．Two copies of each gene for rRNA and ribosomal proteins.
D．Two copies of each gene for rRNA and one copy of each ribosomal proteingene.

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单项选择题

Which one of the following was used to determine the structure of the DNA molecule ( )

A．Transmission(透射) electron microscope
B．Scanning electron microscope
C．Differential centrifugatioin(差速离心)
D．X-ray crystallography

单项选择题

Plant guard cells(保卫细胞) have a higher [C1-] than the outside environment. The most likely explanation for this is( ).

A． Water leaves the cell by osmosis.
B． C1- is diffusing through transmembrane channels.
C． C1- diffusion is aided by a mobile carrier.
D．An active transport symport transports C1-/H+.
E．None of the above