Final answer:
Noncoding sequences called introns are removed during mRNA processing, while exons remain. The mRNA also receives a 5' cap and a 3' poly(A) tail, which are essential for its function but are not removed. The correct option is a. exons
Step-by-step explanation:
During mRNA processing in eukaryotic cells, noncoding sequences known as introns are removed from the pre-mRNA. These introns do not code for proteins and are spliced out by a complex called the spliceosome. The remaining sequences, called exons, are coding sequences that are expressed in the form of proteins.
The pre-mRNA also undergoes additional processing where a 5' cap, typically 5'-methylguanosine, is added, and a poly(A) tail is appended to the 3' end. The 5' cap and poly(A) tail are crucial for mRNA stability, nuclear export, and translation efficiency. In summary, exons remain in the mRNA, while introns are spliced out, and both the 5' cap and 3' poly(A) tail are added components, not removed. The correct option is a. exons