Final answer:
The first base after the SfiI site represents the start of the inserted cDNA because it is conventionally considered as the +1 nucleotide following the initiation site in cloning. This site is crucial for the correct expression in eukaryotes, following Kozak's rules for efficient translation initiation.
Step-by-step explanation:
The cDNA insert is cloned into the SfiI site as part of a molecular cloning process that involves inserting a gene or DNA fragment into a vector. This SfiI site is a recognition sequence where the restriction enzyme cuts the DNA to create a space for the DNA insert. The first base after the site indicates the start of the insert because, by convention, the nucleotide at the initiation site is considered the +1 nucleotide and is the point from which downstream nucleotides are numbered positively. Upstream nucleotides are numbered negatively. This is important for keeping track of the sequence order in DNA cloning experiments.
In eukaryotic translation, the ribosome selects the start codon by recognizing the 7-methylguanosine cap at the 5' end of the mRNA and scanning for the AUG start codon in accordance with Kozak's rules. For vectors intended for eukaryotic expression, the cDNA would typically be inserted such that the Kozak consensus sequence surrounds the AUG start codon to ensure efficient translation initiation. Therefore, when cDNA is inserted right after the SfiI site, it is expected to begin with the start codon or be proximal to it, depending on additional regulatory elements present in the vector or the designed construct.