Correct answer: (2) separation of DNA fragments on the basis of size

Gel electrophoresis is a laboratory technique used to separate DNA fragments according to size as they move through the gel matrix using electricity. The DNA fragments are attracted towards the other end of the matrix, and the smaller molecules move faster than the large ones. This technique is used in DNA profiling to identify possible genetic relationships of organisms.

Incorrect answers:

(1) Synthesis of new DNA strands from subunits

Prior to electrophoresis, DNA fragments could be amplified using polymerase chain reaction (PCR). It is important to make copies of the DNA to help the researcher in other different molecular biology laboratory techniques.

(3) production of genetically engineered DNA molecules

Gel electrophoresis is only a molecular biology technique that may be used to know the presence of genes to be used for genetic engineering. It is not the process used to produce genetically engineered DNA. Instead, genetic engineering can be carried out through recombinant DNA technology.

(4) removal of defective genes from DNA

Gel electrophoresis is a process of classifying DNA molecules according to their sizes. It is not used for deletion of unwanted genes from the DNA. Unwanted genes can be "turned off" using genetic engineering techniques.