A eukaryotic cell has( )

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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1.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

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2.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

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3.单项选择题在下列组织中, 紧密连接特别重要的有( )

A．Stomach(胃) and kidneys
B．Smooth muscle
C．Heart muscle
D．Intestinal epithelial tissue(肠表皮组织)

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4.单项选择题钙泵的作用主要是( )。

A．降低细胞质中Ca2+的浓度；
B．提高细胞质中Ca2+的浓度；
C．降低内质网中Ca2+的浓度；
D．降低线粒体中Ca2+的浓度

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5.单项选择题The hormone glucagons(胰高血糖素) stimulates the breakdown of stored glycogen in liver and muscle cells by the following enzymes. Which is the first enzyme that must be activated ( )

A．Protein kinase A
B．Phosphorylase kinase
C．Glycogen phosphorylase
D．Protein phosphatase
E． Protein phosphatase inhibitor-1

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6.单项选择题Which one of the following is incorrect for the biogenesis of eukaryotic ribosomes( )

A．25-50% of the pre-rRNA is degraded
B． Pre-rRNA binds to ribosomal protein
C． Pre-rRNA is cut to form three classes of rRNA
D．Pre-rRNA is methylated
E．The rRNA gene for the 40S subunit is transcribed separately

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