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 : Help With Nuclear Structure
What is the main function of the nucleolus in eukaryotic cells?
To facilitate nuclear transport of proteins and mRNA from the nucleus to the cytoplasm
Eukaryotic cells do not have a nucleolus
Storage site for transcription factors, RNA polymerases, DNA polymerases, and other transcriptional machinery until transcription occurs
The site of microtubule assembly during mitosis
To promote the transcription, processing, and assembly of rRNA genes into ribosomes
To promote the transcription, processing, and assembly of rRNA genes into ribosomes
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 #2 : Help With Nuclear Structure
The nuclear lamina is generally associated with __________.
ribosome synthesis and assembly
chromatin containing transcriptionally inactive genes
heterochromatin and transcriptionally active genes
euchromatin and transcriptionally inactive genes
chromatin containing transcriptionally active genes
chromatin containing transcriptionally inactive genes
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 : Nucleus And Ribosomes
What is the function of RanGAP in relation to nuclear import and export?
Maintain a gradient of GTP/GDP bound Ran
Attach GTP to Ran
Bind cargo proteins
Bind importins/exportins and bring them through the nuclear pore complex
Maintain a gradient of GTP/GDP bound Ran
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?
A non-functional copy of the gene that encodes CRM1
An inhibitor that globally targets exportins at their cargo binding sites
A mutated, non-functional copy of RanGAP
An inhibitor that globally targets importins at their cargo binding sites
A mutated, non-functional copy of RanGAP
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 #3 : Nucleus And Ribosomes
What is the site of ribosomal subunit synthesis in the cell?
Smooth endoplasmic reticulum
Nucleolus
Nucleus
Golgi apparatus
Nucleolus
Ribosomal subunits are created in the nucleolus, a small area found inside of the nucleus.
Example Question #1 : Help With Ribosome Structure
How many RNA molecules are found in the small ribosomal subunit.
1
6
0
3
2
1
The small subunit of the ribosome contains only one RNA molecule; the 16S rRNA in prokaryotes and the 18S rRNA in eukaryotes.
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