Problem 4 - Campbell Biology

Question

Which of the following is not true of RNA processing? (A) Exons are cut out before mRNA leaves the nucleus. (B) Nucleotides may be added at both ends of the RNA. (C) Ribozymes may function in RNA spliceosomes. (D) RNA splicing can be catalyzed by spliceosomes.

Step-by-Step Solution

Answer

The correct answer is (A). Exons are not cut out; introns are.

Step 1 - Understand RNA Processing

RNA processing is a modification process where the initially synthesized RNA (pre-mRNA) is converted into mature RNA (mRNA) before it exits the nucleus. This process involves three main steps: capping, splicing, and polyadenylation.

Step 2 - Examine Option (A)

Option (A) states 'Exons are cut out before mRNA leaves the nucleus.' During RNA splicing, introns are the segments that are cut out, not exons. Exons are the coding sequences that remain in the processed mRNA. Therefore, option (A) is not true.

Step 3 - Verify Other Options

Let's check the other options to verify correctness. Option (B) refers to nucleotides being added at both ends of the RNA, which is true because a cap is added at the 5' end and a poly-A tail at the 3' end. Option (C) states that ribozymes may function in RNA spliceosomes, which is true because ribozymes are RNA molecules that can act as enzymes in the spliceosome. Option (D) mentions that RNA splicing can be catalyzed by spliceosomes, which is correct as spliceosomes are responsible for removing introns.

Key Concepts

mRNA Splicing

When a gene is transcribed, the resulting pre-mRNA contains both exons (coding regions) and introns (non-coding regions). mRNA splicing is the process where introns are removed, and exons are joined together to form a continuous coding sequence. This step is crucial for generating a mature mRNA molecule that can be translated into a functional protein.

Spliceosomes, which are large complexes of proteins and RNA molecules, orchestrate this process. They recognize the boundaries between introns and exons and facilitate the precise removal of introns.

During splicing, the pre-mRNA undergoes a series of cuts and rejoinings:

• An intron is cut at its 5' splice site.
• The cut end of the intron forms a lariat structure by joining to the branch point.
• The 3' splice site is then cut, and the intron is released.
• The exons on either side are joined together.

After splicing, the mRNA contains only exons, ready for translation into protein.

RNA Capping

RNA capping happens right after the initiation of transcription. This process involves adding a special guanosine triphosphate (GTP) to the 5' end of the growing RNA chain. The cap is then modified by methylation, forming what is called a 5' cap.

The 5' cap has multiple roles:

• It protects the mRNA from degradation.
• It assists in ribosome binding for translation initiation.
• It helps with the export of the mRNA from the nucleus to the cytoplasm.

Each of these roles is paramount to the survival and functionality of the mRNA, enabling accurate and efficient protein synthesis.

Polyadenylation

At the end of transcription, most eukaryotic mRNAs undergo polyadenylation, where a stretch of adenine nucleotides (poly-A tail) is added to the 3' end. This process begins with the cleavage of the 3' end of the pre-mRNA followed by the addition of about 200 adenine nucleotides.

Polyadenylation affects the mRNA in several ways:

• It enhances nuclear export.
• It increases mRNA stability and longevity in the cytoplasm.
• It also facilitates translation by aiding ribosome recruitment.

The poly-A tail works in conjunction with the 5' cap to protect the mRNA and ensure it is efficiently translated into protein. Mutations or errors in the polyadenylation signal or process can significantly impact gene expression and cellular function.