The Nobel Prize in Chemistry 2020 was awarded for the development of the CRISPR-Cas9 genome editing method to

The correct answer is B, Alec Jeffreys. Working at the University of Leicester in 1984, Alec Jeffreys found that certain short sequences, the minisatellites or variable number tandem repeats, repeat a different number of times in different people, and that a pattern made from them identifies an individual. His method cut the DNA with restriction enzymes, separated the pieces by gel electrophoresis and revealed them by Southern blotting.

Option A, Kary Mullis, invented the polymerase chain reaction in 1983, the amplification step that later made fingerprinting possible from a few cells, and he shared the 1993 Nobel Prize in Chemistry. Option C, Frederick Sanger, gave the chain-termination method of DNA sequencing, which reads the order of bases rather than comparing repeat patterns. Option D, Hamilton Smith, shared the 1978 Nobel Prize for restriction enzymes. In India the technique was established by Lalji Singh, who is called the father of DNA fingerprinting in India.

The correct answer is B, an endonuclease that cuts both strands of DNA. Cas9 is an enzyme that breaks the sugar-phosphate backbone of both DNA strands at the point where its guide RNA has bound, provided a short adjacent signal called the PAM is present. The cell then repairs the double-strand break, and that repair either knocks the gene out or, when a template is given, writes in the corrected sequence.

Option A describes DNA ligase, which seals fragments together and is used in making recombinant DNA. Option C describes a DNA polymerase such as Taq polymerase, which synthesises a new strand in the polymerase chain reaction; Cas9 copies nothing. Option D describes a vector: plasmids and the Ti plasmid of Agrobacterium tumefaciens carry genes into cells, whereas Cas9 is cargo, not carrier. The two parts of the system are worth separating in memory: the guide RNA chooses the address and Cas9 does the cutting.

The correct answer is B, towards the anode, because DNA is negatively charged. Each nucleotide of DNA carries a phosphate group, and the phosphate backbone gives the whole molecule a net negative charge at the pH used in the laboratory. In an electric field the fragments therefore migrate to the positive electrode, the anode, and the agarose gel slows the longer fragments more than the shorter ones, so the pieces sort themselves by size.

Option A and Option C both place DNA at the cathode, the negative electrode, which would require a positive or neutral molecule; proteins can behave that way at some pH values, but DNA does not. Option D is wrong because separation is exactly what the gel achieves; the dye, such as ethidium bromide, only makes the separated bands visible under ultraviolet light. The small-moves-faster rule is often asked: shorter fragments travel farther from the well.

The correct answer is C, 2003. The Human Genome Project was launched in 1990 as an international public effort coordinated in the United States by the National Institutes of Health and the Department of Energy, with laboratories in several countries taking part. A working draft was announced in 2000 and the project was declared complete in April 2003, the fiftieth year after the double helix was described. It established that the human genome has about three billion base pairs and roughly twenty thousand protein-coding genes.

Option A, 1990, is the year the project began, and it is the commonest wrong choice. Option B, 1998, is the year a private company began a competing sequencing effort, which hurried the public project along. Option D, 2010, belongs to no milestone of the project. India's own genome programmes came later: IndiGen from 2019 and the Genome India Project from January 2020.

The correct answer is B, Taq polymerase. The polymerase chain reaction, invented by Kary Mullis in 1983, repeats three steps: the DNA is denatured near ninety-five degrees Celsius, short primers anneal at a lower temperature, and the strand is extended at about seventy-two degrees. An ordinary polymerase would be destroyed by the heating step, so the reaction uses Taq polymerase from Thermus aquaticus, a bacterium of hot springs, which stays active at these temperatures.

Option A, DNA ligase, joins fragments and is used in recombinant DNA work, not in amplification. Option C, EcoRI, is a restriction enzyme that cuts DNA at a specific sequence; cutting is the opposite of what the reaction needs. Option D, reverse transcriptase, makes DNA from RNA and is used first when the starting material is RNA, as in the reverse transcription test for RNA viruses, but the amplification that follows still needs a heat-stable polymerase.