Principles and processes of biotechnology
Read the following passage carefully and answer the given questions.
The DNA fragments separate according to their size through sieving effect provided by the agarose gel. Vectors used at present, are engineered in such a way that they help easy linking of foreign DNA and selection of recombinants from non-recombinants. If we are able to link an alien piece of DNA with bacteriophage or plasmid DNA, we can multiply its numbers equal to the copy number of the plasmid or bacteriophage. In the year 1963, the two enzymes responsible for restricting the growth of bacteriophage in Escherichia coli were isolated. One of these added methyl groups to DNA, while the other cut DNA. The latter was called restriction endonuclease. The first restriction endonuclease - Hind II, whose functioning depended on a specific DNA nucleotide sequence was isolated and characterised five years later. In PCR reaction, multiple copies of the gene of interest are synthesised in vitro using two sets of primers (small chemically synthesised oligonucleotides that are complementary to the regions of DNA). Modern biotechnology using genetically modified organisms was made possible only when man learnt to alter the chemistry of DNA and construct recombinant DNA. This key process is called recombinant DNA technology or genetic engineering.
Which one of the given enzymes is used for the extension or synthesis of the DNA template in polymerase chain reaction?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Taq polymerase
In PCR, a DNA segment is copied many times in the test tube. The DNA is heated so that the two strands separate. Primers then pair with the strands. A DNA polymerase extends the primers, using the nucleotides in the mixture and the DNA as the template. The enzyme must survive the high temperature of the next cycle. So a thermostable DNA polymerase is used. It was isolated from the bacterium Thermus aquaticus and is called Taq polymerase.
(1) RNA polymerase — Incorrect. This enzyme makes RNA from DNA in transcription. PCR copies DNA into DNA.
(2) Taq polymerase — Correct. It extends the primers and does not break down at the high temperature.
(3) DNA ligase — Incorrect. This enzyme joins DNA fragments, for example the insert and the vector. It does not extend a primer.
(4) Lysozyme — Incorrect. This enzyme breaks the cell wall of bacteria, and it is used to release DNA from them.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Amplification of Gene of Interest Using PCR – …
