Question

The cutting of DNA at specific locations became possible with the discovery of-
A. Ligase
B. Restriction enzymes
C. Probes
D. Selectable markers

Answer 1

A protein that recognises a unique, short sequence of nucleotides and cleaves DNA into fragments inside the molecule at or near unique recognition sites. The bacteria that produce them have isolated more than 400 such proteins.

Complete answer:
Option A is incorrect. A ligase is an enzyme that, by creating a new chemical bond, can catalyse the combining of two large molecules, typically with the subsequent hydrolysis on one of the larger molecules of a small pendant chemical group, or the enzyme that catalyses the bonding of two compounds, e.g., enzymes that catalyse C-O, C-S, C-N, etc. Ligase can connect two nucleic acid complementary fragments and repair single stranded breaks that occur during replication in double stranded DNA.
Option B is correct. An enzyme that cleaves DNA into fragments at or close unique recognition sites within the molecule known as restriction sites is a limitation enzyme or restriction endonuclease. Enzyme restrictions are one class in the larger endonuclease category of enzymes. Usually, restriction enzymes are classified into five classes that differ in their structure and whether their DNA substrate is cut at the recognition site or whether the recognition and cleavage sites are different from each other.
Option C is incorrect. A probe is a single-stranded DNA or RNA sequence used in a sample genome to search for its complementary sequence. Under conditions that allow the probe sequence to hybridise with its complementary sequence, the probe is brought into contact with the sample.
Option D is incorrect. A selectable marker is a gene inserted into a cell that confers a trait suitable for artificial selection, especially a bacterium or cells in culture. It regulates a protein-encoding gene that helps transformed plants to live in toxic conditions, whereas cells and tissues that are not transformed die, e.g. Kanamycin, Ampicillin is considered to be a valuable selectable marker for E. coli.

So, the correct answer is Option B.

Note: Restriction enzymes play a very important role in the construction of recombinant DNA molecules, used during gene cloning and protein synthesis experiments to help insert genes into plasmid vectors, digesting genomic DNA by Southern Blot for gene analysis. They can also be used by precisely identifying single base variations in DNA known as single-nucleotide polymorphisms (SNPs) to identify gene alleles.