Problem 4 - Campbell Biology

Question

Which of the following is true of RNA processing? (A) Exons are cut out before mRNA leaves the nucleus. (B) Nucleotides are added at both ends of the RNA. (C) Ribozymes may function in the addition of a 5 ' cap. (D) RNA splicing adds a poly-A tail to the mRNA.

Step-by-Step Solution

Answer

Option (B) is correct.

Step 1: Understanding RNA Processing

First, understand that RNA processing involves several modifications before mRNA is functional for translation.

Step 2: Identify the Role of Exons

Exons are sequences that remain in the mRNA after processing. Since they are spliced together, option (A) is incorrect because exons are not cut out.

Step 3: Nucleotide Addition at RNA Ends

It's necessary to know that a 5' cap and a poly-A tail are added to the ends of the RNA. Therefore, option (B) is correct since nucleotides are added at both ends of the RNA.

Step 4: Ribozymes and 5' Cap

Ribozymes are RNA molecules that can catalyze certain biochemical reactions. However, they are not involved in adding the 5' cap. Thus, option (C) is incorrect.

Step 5: Role of RNA Splicing

RNA splicing removes introns and splices exons together, but it does not add a poly-A tail. Hence, option (D) is incorrect.

Key Concepts

RNA splicing

RNA splicing is a crucial step in the processing of eukaryotic pre-mRNA. Initially, the pre-mRNA transcript contains both exons (coding sequences) and introns (non-coding sequences). The primary role of splicing is to remove these introns. The remaining exons are then joined together to form a continuous sequence that will code for a functional protein. This process occurs in the nucleus and is facilitated by a complex called the spliceosome.

Key Elements of RNA Splicing:

• Spliceosome: A complex composed of small nuclear RNAs (snRNAs) and proteins.
• Exons: Sequences that remain in the mRNA after splicing and code for proteins.
• Introns: Non-coding sequences that are removed during splicing.

RNA splicing increases genetic diversity through a process called alternative splicing, where different combinations of exons are joined together to produce various proteins from a single gene.

By understanding these basic elements, you'll appreciate that RNA splicing does not involve the addition of nucleotides or the poly-A tail.

5' cap addition

The addition of a 5' cap is one of the essential modifications that occur during RNA processing in eukaryotes. This process happens shortly after the initiation of transcription.

The 5' cap is a modified guanine nucleotide added to the 5' end of the nascent mRNA transcript. This cap protects the mRNA from degradation by exonucleases and aids in ribosome binding during translation.

Functions of the 5' cap:

• Protection: Shields mRNA from enzymatic degradation.
• Initiation of Translation: Helps recruit the ribosome to the mRNA.
• Export: Facilitates the transport of mRNA from the nucleus to the cytoplasm.

It's also important to note that ribozymes, which are RNA molecules with catalytic properties, do not play a role in adding the 5' cap. This task is carried out by specific capping enzymes.

poly-A tail addition

The poly-A tail is another critical modification added to pre-mRNA during RNA processing. Located at the 3' end of the mRNA transcript, this tail consists of a string of adenine nucleotides.

Steps in Poly-A Tail Addition:

• Cleavage: The pre-mRNA is first cleaved downstream of an AAUAAA consensus sequence.
• Addition: Poly-A polymerase then adds around 200 adenine nucleotides to the newly formed 3' end.

Functions of the Poly-A Tail:

• Stability: Protects the mRNA from degradation by exonucleases.
• Translation: Enhances the efficiency of translation by aiding in the recruitment of ribosomes.
• Export: Assists in the transport of mature mRNA from the nucleus to the cytoplasm.

Unlike RNA splicing, which removes introns and joins exons, the addition of the poly-A tail is specifically for stabilizing and preparing the mRNA for translation. This crucial modification ensures that the mRNA remains intact long enough to be translated into a protein.