Final answer:
Nick translation is the in vitro reaction that specifically requires full-length DNA polymerase I, as it utilizes both the polymerization and the 5'→3' exonuclease activities of the enzyme.
Step-by-step explanation:
Requirement for Full-Length DNA Polymerase I
Among the given options, full-length DNA polymerase I, or DNA pol I, is required for nick translation. This is because DNA pol I has both polymerization activity and 5'→3' exonuclease activity, which are necessary for removing RNA primers and replacing them with DNA nucleotides. This process of nick translation is essential for repairing the phosphodiester backbone during molecular cloning, which will then be sealed by DNA ligase. In DNA sequencing and the creation of a blunt end by exonuclease activity, the Klenow fragment of DNA pol I, which lacks the 5'→3' exonuclease activity, may be utilized rather than the full-length enzyme. The use of the Klenow fragment allows maintenance of the necessary polymerization and proofreading functions without the 5'→3' exonuclease activity. Lastly, for filling-in blunt ends, DNA pol I is not specifically required; other DNA polymerases can be used for this purpose as long as they possess polymerization activity.