GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Nucleus and Ribosomes

Study concepts, example questions & explanations for GRE Subject Test: Biochemistry, Cell, and Molecular Biology

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Example Questions

Example Question #1 : Nucleus And Ribosomes

What is the main function of the nucleolus in eukaryotic cells?

Possible Answers:

To promote the transcription, processing, and assembly of rRNA genes into ribosomes

To facilitate nuclear transport of proteins and mRNA from the nucleus to the cytoplasm

Storage site for transcription factors, RNA polymerases, DNA polymerases, and other transcriptional machinery until transcription occurs

Eukaryotic cells do not have a nucleolus

The site of microtubule assembly during mitosis

Correct answer:

To promote the transcription, processing, and assembly of rRNA genes into ribosomes

Explanation:

The correct answer is that the nucleolus is the site of ribsome synthesis and assembly. Chromosomes containing rRNA (ribosomal RNA genes) are clustered within the nucleolus, which facilitates rapid transcription and processing of rRNA genes and assembly of rRNA subunits into ribosomes. 

Example Question #1 : Nucleus And Ribosomes

The nuclear lamina is generally associated with __________.

Possible Answers:

euchromatin and transcriptionally inactive genes

heterochromatin and transcriptionally active genes

chromatin containing transcriptionally inactive genes

ribosome synthesis and assembly 

chromatin containing transcriptionally active genes

Correct answer:

chromatin containing transcriptionally inactive genes

Explanation:

The correct answer is chromatin and transcriptionally inactive genes. The nuclear lamina provides support and structure to the nucleus and binds transcriptionally inactive regions. Euchromatin is loosely packed chromatin and is, therefore, associated with transcriptionally active genes. Alternatively, heterochromatin is densely packed chromatin and implies inactive genes. Ribosome synthesis and assembly occurs in the nucleolus.

Example Question #1 : Help With Nuclear Transport

What is the function of RanGAP in relation to nuclear import and export?

Possible Answers:

Attach GTP to Ran

Maintain a gradient of GTP/GDP bound Ran 

Bind importins/exportins and bring them through the nuclear pore complex

Bind cargo proteins

Correct answer:

Maintain a gradient of GTP/GDP bound Ran 

Explanation:

RanGAP is notable for its ability to activate the GTPase function of the Ran protein. This allows for the hydrolysis of the third phosphate of GTP-bound Ran and the maintenance of a Ran-GDP/Ran-GTP gradient that is used to control nuclear import and export. RanGAP does not directly associate with cargo proteins or importins/exportins.

Example Question #2 : Nucleus And Ribosomes

Which of the following choices would have the biggest effect on nuclear transport?

Possible Answers:

A non-functional copy of the gene that encodes CRM1

An inhibitor that globally targets importins at their cargo binding sites

An inhibitor that globally targets exportins at their cargo binding sites

A mutated, non-functional copy of RanGAP

Correct answer:

A mutated, non-functional copy of RanGAP

Explanation:

Because the question is asking which will have the largest effect, it makes sense that the answer should affect import and export globally.

Importins and exportins only work in one direction. While mutations to either of these protein classes would greatly affect nuclear transport, each is only responsible for "half" or the transport across the membrane. CRM1 is a well characterized exportin.

RanGAP, however, is responsible for helping maintain the gradient of Ran bound to GTP or GDP. The gradient of RanGTP/GDP is what provides global directionality to nuclear import and export. Loss of RanGAP would affect both import and export, having the largest detrimental effect on nuclear transport.

Example Question #1 : Nucleus And Ribosomes

What is the site of ribosomal subunit synthesis in the cell?

Possible Answers:

Golgi apparatus

Smooth endoplasmic reticulum

Nucleolus

Nucleus

Correct answer:

Nucleolus

Explanation:

Ribosomal subunits are created in the nucleolus, a small area found inside of the nucleus.

Example Question #3 : Nucleus And Ribosomes

How many RNA molecules are found in the small ribosomal subunit.

Possible Answers:

6

1

2

3

0

Correct answer:

1

Explanation:

The small subunit of the ribosome contains only one RNA molecule; the 16S rRNA in prokaryotes and the 18S rRNA in eukaryotes.

All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources

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