Final answer:
The first step of intron removal in mRNA processing is the recognition of splice sites by spliceosomes, which is critical for the precise splicing of pre-mRNA.
Step-by-step explanation:
The first step of intron removal during mRNA processing is C. Recognition of the splice sites. Introns are noncoding sections of an RNA transcript, or the DNA encoding it, that are spliced out of the pre-mRNA before it becomes mature mRNA. In eukaryotic cells, the splicing of pre-mRNAs is conducted by complexes of proteins and RNA molecules called spliceosomes.
These spliceosomes recognize specific nucleotide sequences at the 5' and 3' ends of the intron - specifically, GU at the 5' end and AG at the 3' end. Recognition of these splice sites is critical to ensure precise removal of introns and joining of exons, which will encode the functional protein. The splicing process is highly precise, as even a single nucleotide error can shift the reading frame and result in a dysfunctional protein.