After digestion by restriction endonucleases DNA strands can be joined again by
**Core Concept**
DNA restriction and modification play a crucial role in genetic engineering. Restriction endonucleases cleave DNA at specific recognition sites, resulting in DNA fragments with sticky or blunt ends. To rejoin these fragments, an enzyme is required to form a covalent bond between the DNA strands.
**Why the Correct Answer is Right**
DNA ligase is the enzyme responsible for sealing the gaps between DNA fragments by forming a phosphodiester bond between the 3'-hydroxyl group of one nucleotide and the 5'-phosphate group of another. This process is essential in cloning, gene editing, and other molecular biology techniques. DNA ligase can seal both blunt and sticky ends, allowing researchers to join DNA fragments from different sources.
**Why Each Wrong Option is Incorrect**
* **Option A:** DNA polymerase is an enzyme involved in DNA replication and repair, but it is not responsible for joining DNA fragments after restriction digestion. DNA polymerase synthesizes new DNA strands by adding nucleotides to the 3'-end of a primer.
* **Option B:** RNA ligase is an enzyme that joins RNA molecules together, but it is not typically used for DNA joining. RNA ligase is used in RNA splicing, gene expression studies, and other RNA-related applications.
* **Option C:** DNA helicase is an enzyme that unwinds double-stranded DNA into single strands, allowing other enzymes to access the DNA. While important in DNA replication and repair, DNA helicase does not join DNA fragments.
**Clinical Pearl / High-Yield Fact**
When working with DNA, it's essential to remember that restriction endonucleases can introduce unwanted mutations or alter gene expression. To minimize these effects, researchers often use restriction enzymes with overlapping recognition sites or employ alternative cloning strategies.
**Correct Answer:** D. DNA ligase.