After digestion by restriction endonucleases DNA strands can be joined again by which of the following enzymes?
**Core Concept**
The process of joining DNA strands involves **enzymatic ligation**, which is crucial in molecular biology for cloning and genetic engineering. This process requires specific enzymes that can form a phosphodiester bond between the 5' end of one DNA fragment and the 3' end of another. **DNA ligase** is the key enzyme in this process.
**Why the Correct Answer is Right**
DNA ligase is responsible for sealing the gaps between DNA fragments by forming a phosphodiester bond. This enzyme is essential for the replication and repair of DNA, as well as for artificial DNA manipulation techniques like cloning. The action of DNA ligase allows for the joining of DNA strands that have been cut by **restriction endonucleases**, enabling the creation of recombinant DNA molecules.
**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not specify an enzyme involved in the ligation of DNA strands.
**Option B:** Similarly, this option does not correctly identify the enzyme responsible for joining DNA fragments.
**Option C:** This choice is also incorrect as it does not correspond to the enzyme that seals the gaps between DNA strands.
**Clinical Pearl / High-Yield Fact**
Remember that DNA ligase requires **NAD+** or **ATP** as a cofactor to function, depending on the type of ligase. This is a critical point for understanding the efficiency and conditions required for ligation reactions in molecular biology experiments.
**Correct Answer:** D. DNA ligase.