Question

In the cellular turnover of RNA, A. repair is more active than degradation. B. regions of extensive base pairing are more susceptible to cleavage. C. a small double-stranded region of mRNA could lead to cleavage of the mRNA if one strand has perfect complementarity with a region of the mRNA. D. the products are always nucleotides with a phosphate at the \(5^{\prime}-\mathrm{OH}\) group. E. all species except rRNA are cleaved.

Short answer

A. Repair is more active than degradation. B. Regions of extensive base pairing are more susceptible to cleavage. C. A small double-stranded region of mRNA could lead to cleavage of the mRNA if one strand has perfect complementarity with a region of the mRNA. D. The products are always nucleotides with a phosphate at the \(5^{\prime}-\mathrm{OH}\) group. E. All species except rRNA are cleaved. Answer: C. A small double-stranded region of mRNA could lead to cleavage of the mRNA if one strand has perfect complementarity with a region of the mRNA.

Step-by-step solution

Option A

Repair is more active than degradation. In the cellular turnover of RNA, degradation is generally more active than repair. RNA molecules have a short lifespan and are continuously synthesized and degraded to control gene expression levels. Therefore, this statement is false.

Option B

Regions of extensive base pairing are more susceptible to cleavage. Regions with extensive base pairing are generally less susceptible to cleavage because they form stable secondary structures that protect the RNA from degradation. This statement is false.

Option C

A small double-stranded region of mRNA could lead to cleavage of the mRNA if one strand has perfect complementarity with a region of the mRNA. This statement is true. If a small double-stranded region forms within mRNA due to perfect complementarity, it can signal the presence of an incorrect or foreign mRNA sequence. This could lead to cleavage of the mRNA by cellular RNA degradation machinery, such as RNase enzymes.

Option D

The products are always nucleotides with a phosphate at the \(5^{\prime}-\mathrm{OH}\) group. The statement is false. During RNA degradation, cleaved RNA products can have either a phosphate group at the \(5^{\prime}\) end or a hydroxyl group at the \(3^{\prime}\) end, depending on the type of enzyme responsible for the cleavage.

Option E

All species except rRNA are cleaved. This statement is false. Although rRNA is more stable than other RNA species, it can be cleaved under specific conditions, such as cellular stress or damage. Therefore, it is not accurate to say that only rRNA is exempt from cleavage. Based on the analysis of each option, the correct answer is: C. A small double-stranded region of mRNA could lead to cleavage of the mRNA if one strand has perfect complementarity with a region of the mRNA.

Key concepts

RNA Turnover

RNA turnover refers to the process by which RNA molecules are synthesized and degraded within cells. This process is crucial for maintaining the correct balance of RNA types required for various cellular functions.

RNA molecules, such as messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), have varying life spans and play key roles in protein synthesis. For example, mRNA is relatively short-lived, allowing for the dynamic regulation of gene expression in response to the cellular environment.

Turnover involves both the degradation and synthesis of RNA molecules. Degradation is typically more active than repair because RNA is not repaired in the same manner as DNA. Instead, RNA that is no longer needed or that has been damaged is rapidly degraded and its nucleotide components are recycled for the synthesis of new RNA molecules.

mRNA Cleavage

mRNA cleavage is a critical step in the degradation of messenger RNA. It marks the beginning of the decay process, which ultimately results in the breakdown of mRNA into its constituent nucleotides.

Cleavage can occur at specific sites within an mRNA molecule, often triggered by small double-stranded regions that form when sequences within the mRNA are perfectly complementary. These structures signal that mRNA may be faulty or part of a regulatory mechanism. Enzymes called endonucleases recognize these structures and cleave the mRNA, leading to its degradation. It's a misconception that regions with extensive base pairing are more prone to cleavage; in fact, they often confer stability and protect RNA from rapid degradation.

RNase Enzymes

RNase enzymes play a pivotal role in RNA metabolism, particularly in the degradation of RNA molecules. They are a diverse group of enzymes that catalyze the cleavage of RNA.

Depending on their specificity and mode of action, RNases can be classified as endonucleases or exonucleases. Endonucleases cut RNA at internal sites, while exonucleases remove nucleotides one at a time from the RNA's ends. The RNases recognize certain nucleotide sequences or structural motifs within RNA molecules, making them essential for quality control and regulation of RNA levels within the cell. For example, RNase H specifically degrades RNA in RNA-DNA hybrids, and RNase P is involved in tRNA processing.

Nucleotide Structure

Nucleotide structure is fundamental to the understanding of RNA metabolism. Each nucleotide is composed of three parts: a phosphate group, a five-carbon sugar (ribose in the case of RNA), and a nitrogenous base.

The nitrogenous bases in RNA are adenine (A), guanine (G), cytosine (C), and uracil (U), which pair with their complementary bases through hydrogen bonds (A pairs with U, G pairs with C). This base pairing is crucial for the formation of RNA secondary structures and for RNA's interaction with proteins and other molecules. The linkage between nucleotides involves a phosphate group connected to the 5' carbon of one ribose and the 3' carbon of the adjacent ribose, creating a backbone with directionality, from the 5' to the 3' end.

RNA Base Pairing

RNA base pairing is a process by which nucleotides within the RNA molecule form bonds with complementary bases, which is similar to the principles of DNA base pairing but involves uracil instead of thymine.

This pairing is crucial for the structure and function of RNA. In mRNA, correct base pairing ensures proper translation into proteins, while in tRNA and rRNA, it helps form the correct three-dimensional structure necessary for their roles in protein synthesis. Additionally, the concept of RNA base pairing extends to interactions between RNA molecules, such as the formation of double-stranded regions that can signal for the cleavage and turnover of mRNA, as mentioned in Option C from the original exercise. Moreover, RNA-RNA interactions are also involved in the regulation of gene expression and the function of non-coding RNAs.