Which of the following statements is incorrect?

The correct answer is Option (3) → The RNA can be removed by treatment with protease whereas proteins can be removed by treatment with ribonuclease.

During isolation of DNA, the other macromolecules are removed by enzymes that suit them. RNA is digested by ribonuclease and proteins by protease. The statement in option (3) swaps the two enzymes.

Option (1) A selectable marker helps in identifying and eliminating non-transformants and selectively permitting the growth of the transformants. — A correct statement. The selectable marker, usually an antibiotic resistance gene, lets transformants grow and removes non-transformants.

Option (2) A stirred-tank reactor is usually cylindrical or with a curved base to facilitate the mixing of the reactor contents. — A correct statement. The cylindrical shape or curved base of the stirred-tank reactor helps mix its contents.

Option (3) The RNA can be removed by treatment with protease whereas proteins can be removed by treatment with ribonuclease. — An incorrect statement. The enzymes are the wrong way round. RNA is removed by ribonuclease, and proteins are removed by protease.

Option (4) In genetic engineering, DNA from different sources are cut with the same restriction enzymes so that both DNA fragments have same kind of sticky ends. — A correct statement. Cutting both DNAs with the same restriction enzyme gives them the same kind of sticky ends, which DNA ligase can then join.

NCERT Extract:
Isolation of the Genetic Material (DNA) – “… You know that genes are located on long molecules of DNA interwined with proteins such as histones. The RNA can be removed by treatment with ribonuclease whereas proteins can be removed by treatment with protease.”
Cloning Vectors – “… the vector requires a selectable marker, which helps in identifying and eliminating non-transformants and selectively permitting the growth of the transformants.”
Obtaining the Foreign Gene Product – “… A stirred-tank reactor is usually cylindrical or with a curved base to facilitate the mixing of the reactor contents.”
Restriction Enzymes – “… When cut by the same restriction enzyme, the resultant DNA fragments have the same kind of ‘sticky-ends’ and, these can be joined together (end-to-end) using DNA ligases (Figure 9.2).”