Final answer:
In splicing, the lariat structure is formed at an A residue branch site within the cell nucleus, where splicing removes non-coding introns from pre-mRNA. This process is carried out by the spliceosome, which is a complex made up of snRNPs.
Step-by-step explanation:
Splicing is a critical process that occurs within the cell’s nucleus during the production of mRNA. Specifically, the lariat structure is formed during the splicing of pre-mRNA to mature mRNA, involving the removal of introns. The loop of the lariat is formed using a sequence at an A residue branch site. Splicing is catalyzed by a complex called the spliceosome, which is composed of small ribonucleoproteins called small nuclear RNPs (snRNPs). These snRNPs play a significant role in identifying the splice sites, carrying out the cutting of the transcript at the intron borders, and ligating the exons together. Introns are non-coding regions, and their excision results in an mRNA transcript consisting only of exons, which are the coding regions that convey genetic information to direct protein synthesis.