GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Help with RNA Structure and Modification

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Example Question #1 : Help With Rna Structure And Modification

Which of the following processes is not a type of RNA processing carried out by eukaryotes?

Possible Answers:

Capping of the 5' end

Polyadenylation of the 3' end

Nucleotide excision repair

Splicing and removal of introns

Correct answer:

Nucleotide excision repair

Explanation:

Processing of pre-mRNA occurs in the nucleus. After transcription, three crucial modification take place. A 7-methylguanosine molecule is added to the 5' end to form a cap. Polyadenylation is added to the 3' end to create a poly-A tail. Introns are spliced out by spliceosomes, removing the non-coding regions of the RNA. The final product after modifications is considered a mature mRNA; prior to this, the transcript is known as heteronuclear RNA (htRNA).

Nucleotide excision repair is a method of proofreading after DNA replication to reduce the frequency of mutation.

Example Question #2 : Help With Rna Structure And Modification

What are exons?

Possible Answers:

Intervening non-coding segments of DNA

Genetic information coding for an amino acid sequence that will form a functional protein

Set of genes that are adjacent to one another in the genome and are coordinately controlled

Site where repressor proteins bind

Correct answer:

Genetic information coding for an amino acid sequence that will form a functional protein

Explanation:

Exons contain the actual genetic information coding for protein. In contrast, introns are intervening non-coding segments of DNA. During the splicing process of pre-mRNA modification, introns are removed from the sequence. Alternative splicing patterns allow multiple exon sequences to be created from a single gene, resulting in multiple proteins.

An operon is a set of genes that are adjacent to one another in the genome and are coordinately controlled. The silencer region is a site of a gene where repressor proteins bind to regulate gene expression.

Example Question #3 : Help With Rna Structure And Modification

What are snRNPs (small nuclear ribonucleic particles)?

Possible Answers:

RNA protein complexes that combine with pre-mRNA and other proteins to form a spliceosome

RNA molecules that serve as the link between the nucleotide sequence of nucleic acids and the amino acid sequence of proteins

RNA molecules that convey genetic information from DNA to the ribosome

RNA component of the ribosome, essential for protein synthesis

Correct answer:

RNA protein complexes that combine with pre-mRNA and other proteins to form a spliceosome

Explanation:

snRNPs are RNA protein complexes that combine with pre-mRNA and other proteins to form a spliceosome. Spliceosomes remove introns from pre-mRNA. After final modifications, the spliced pre-mRNA is considered mature mRNA and can be exported to the cytoplasm for translation.

mRNA is an RNA molecule that conveys genetic material from DNA to the ribosome. tRNA is an RNA molecule that serves as the link between the nucleotide sequence of nucleic acids and the amino acid sequence of proteins. rRNA is the RNA component of the ribosome that is essential for protein synthesis in all living organisms.

Example Question #4 : Help With Rna Structure And Modification

Which of the following is not a type of RNA?

Possible Answers:

rRNA

cRNA

mRNA

tRNA

Correct answer:

cRNA

Explanation:

mRNA, tRNA, and rRNA are the most commonly recognized types of RNA, though there are several more divisions. Messenger RNA (mRNA) is the product of gene transcription and is used to carry genetic information to ribosomes for translation. Transfer RNA (tRNA) is used to transport amino acid residues to active ribosomes during translation and contains anticodon sequences to bind to mRNA. Ribosomal RNA (rRNA) forms part of the ribosomes structure.

Though cRNA is not a class of RNA molecule, complementary DNA (cDNA) is used to store and analyze genomes. cDNA is the non-coding complement to the template strand used for transcription, and can be used to analyze genetic sequences.

Example Question #5 : Help With Rna Structure And Modification

All of the follwing are types of alternative RNA splicing except __________.

Possible Answers:

exon reversal

alternative 3' splicing

dual splice site

skipped exon

Correct answer:

exon reversal

Explanation:

Skipped exon, dual splice site, and alternative 3' splicing are all types of alternative RNA splicing. Exon reversal is a nonsense term, and does not represent an actual biochemical process.

Example Question #6 : Help With Rna Structure And Modification

Which of the following is not a category of RNA?

Possible Answers:

Short interfering RNAs

MicroRNAs

Piwi-interacting RNAs

Major RNAs

Long non-coding RNA

Correct answer:

Major RNAs

Explanation:

Major RNAs are not a category of RNAs. Long non-coding RNAs are non-protein coding transcripts typically longer than 200 base pairs (bp) and play a role in regulating gene expression and epigenetic regulation. MicroRNAs are small RNAs (~20 bp) and play a role in RNA silencing and post-transcriptional regulation of gene expression. Short interfering RNAs are double-stranded (20-25 bp) and play a role in post-transcriptional gene silencing. Piwi-interacting RNAs are small non-coding RNAs that interact with piwi proteins in epigenetic and post-transcriptional silencing of genetic elements such as retroposons. 

Example Question #7 : Help With Rna Structure And Modification

What approximate percentage of total RNA content in a cell is messenger RNA (mRNA)?

Possible Answers:

Correct answer:

Explanation:

Only 2-5% of the total RNA content in a cell is mRNA. Approximately 10% is transfer RNA (tRNA), and approximately 85% is ribosomal RNA (rRNA).

Example Question #8 : Help With Rna Structure And Modification

Why is the post-transcriptional poly-A tail an important addition to a mRNA molecule?

Possible Answers:

The 3' tail signals the end of transcription by the RNA polymerase.

The poly-A tail is a part of the protein translated from the mRNA.

The poly-A tail facilitates the splicing of introns.

The poly-A tail facilitates the binding of ribosomes.

The tail adds stability to the transcript, controlling the time of degradation.

Correct answer:

The tail adds stability to the transcript, controlling the time of degradation.

Explanation:

The poly-A tail adds to the stability of the mRNA transcript. The tail becomes shorter and shorter over time due to exonucleases, eventually signaling an enzyme to break down the mRNA to stop further translation. The 5' methylated guanosine cap facilitates binding of ribosomes, and specific splicing sequences signal removal of introns, not the poly-A tail. The tail cannot signal the end of transcription by the RNA polymerase because the poly-A tail is a post-transcriptional change, meaning it is not present during transcription. Lastly, the poly-A tail lies at the end of the 3' UTR (untranslated region), and is not included in the protein product of the mRNA.

Example Question #9 : Help With Rna Structure And Modification

Researchers first identified parts of genes that are spliced out of mRNA and not included in the final protein product by observing that not all of the original gene hybridizes to the cognate mRNA. What are these regions called?

Possible Answers:

Introns

Internal transcribed spacers

Micro RNAs

Inter genic regions

Exons

Correct answer:

Introns

Explanation:

Introns are regions included in genes that are not actually part of the final protein generated. Scientists first observed that some areas of genes are removed before mRNA translation by visualizing that not all of a gene hybridizes with its cognate mRNA, and hence there are pieces that are spliced out and not used. Note that splicing of introns, like all other post-transational modifications, only occurs in eukaryotes. The function of intron regions is thought to be mostly regulatory.

Example Question #10 : Help With Rna Structure And Modification

How is splicing regulated at the level of cis-acting RNA sequence elements? 

Possible Answers:

Splicing repressors bind splicing silencers and decrease nearby splice junction activity. Splicing activators bind splicing enhancers and increase likelihood of proximal sites as splice junction. 

Splicing repressors bind splicing silencers and decrease nearby splice junction activity. Splicing activators bind splicing promoters and increase likelihood of proximal sites as splice junction. 

Splicing repressors bind splicing silencers and increase nearby splice junction activity. Splicing activators bind splicing enhancers and decrease likelihood of proximal sites as splice junction. 

Insulators bind splicing silencers and decrease nearby splice junction activity. Splicing activators bind splicing enhancers and increase likelihood of proximal sites as splice junction. 

Heterochromatin prevents alternative splicing whereas euchromatin promotes alternative splicing

Correct answer:

Splicing repressors bind splicing silencers and decrease nearby splice junction activity. Splicing activators bind splicing enhancers and increase likelihood of proximal sites as splice junction. 

Explanation:

The correct answer is repressors bind splicing silencers and increase nearby splice junction activity. Activators bind splicing enhancers and decrease likelihood of proximal sites as splice junction. These enhancer sites can be in the intron or exon of the nascent RNA molecule and are most commonly bound by serine and arginine (SR) proteins. Moreover, the relative abundance of these bound RNA regulatory elements in proximity to a splice junction confers differential splicing activity. 

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