What happens to a newly transcribed mRNA molecule before it leaves the nucleus? A Introns are removed as exons are spliced together B Nothing, the mRNA is ready to leave the nucleus immediately C Stop signal at the end of mRNA must be removed D Exons are removed, introns are connected together
The correct answer and explanation is:
Correct Answer: A. Introns are removed as exons are spliced together
Before a newly transcribed mRNA molecule leaves the nucleus, it undergoes several important modifications known as RNA processing. These modifications ensure that the mRNA is properly prepared for translation into a protein in the cytoplasm. The primary transcript, also called pre-mRNA, contains both exons and introns. Exons are the coding regions that contain the information to build a protein, while introns are non-coding regions that need to be removed.
The first major step in mRNA processing is splicing, where introns are removed from the pre-mRNA, and the remaining exons are joined together to form a continuous coding sequence. This task is carried out by a complex molecular machine called the spliceosome. Proper splicing is critical because errors in this process can lead to the production of nonfunctional or harmful proteins.
In addition to splicing, two other modifications occur. A 5′ cap is added to the beginning of the mRNA, which helps protect it from degradation and assists in ribosome binding during translation. A poly-A tail, which is a long sequence of adenine nucleotides, is added to the 3′ end of the mRNA. This tail also helps stabilize the mRNA and regulates its export from the nucleus to the cytoplasm.
Option B is incorrect because multiple processing steps must occur before mRNA is functional. Option C is incorrect because the stop codon is a necessary signal for translation and is not removed. Option D is incorrect because exons are retained and introns are removed, not the other way around.
Therefore, the correct answer is A. Introns are removed as exons are spliced together.