Final answer:
To create blunt ends on DNA fragments with sticky ends, DNA polymerase fills in the recessed 3' ends, followed by DNA ligase permanently joining the DNA fragments.
Step-by-step explanation:
A "fill-in" reaction is used to create blunt ends on fragments created by cleavage with restriction enzymes that leave 5' overhangs, also known as sticky ends. These sticky ends can anneal by forming hydrogen bonds with complementary sequences on single strands to form double-stranded DNA. However, to fill in the recessed 3' ends, an enzyme such as DNA polymerase fills in the gaps, and then DNA ligase will permanently join the DNA fragments, effectively creating blunt ends that no longer have overhangs. This is essential for the cloning of DNA fragments into vectors that require blunt ends for ligation or for various other molecular biology techniques.