Read the Passage carefully and answer the questions.
There are two kinds of nucleases - exonucleases and endonucleases. Exonucleases remove nucleotides from the ends of the DNA whereas, endonucleases make cuts at specific positions within the DNA. The cutting of DNA by restriction endonucleases results in the fragments of DNA. These fragments can be separated by a technique known as gel electrophoresis. Since DNA fragments are negatively charged molecules they can be separated by forcing them to move towards the anode under an electric field through a medium/matrix. Small volume cultures cannot yield appreciable quantities of products. To produce in large quantities, the development of bioreactors, where large volumes (100-1000 litres) of culture can be processed, was required. Thus, bioreactors can be thought of as vessels in which raw materials are biologically converted into specific products, individual enzymes, etc., using microbial plant, animal or human cells. PCR stands for Polymerase Chain Reaction. In this reaction, multiple copies of the gene (or DNA) of interest are synthesised in vitro using two sets of primers (small chemically synthesised oligonucleotides that are complementary to the regions of DNA) and the enzyme DNA polymerase.
Identify the incorrect fact about the PCR?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The repeated amplification in PCR is achieved by the use of a thermostable DNA polymerase, isolated from a bacterium, Thermus quatica.
PCR (polymerase chain reaction) makes many copies of a chosen piece of DNA in a test tube. Each cycle heats the DNA to separate its strands, lets the primers attach and lets DNA polymerase build the new strands.
Option (1) Primers are small chemically synthesised oligonucleotides that are complementary to the regions of DNA. — A correct fact. A primer is a short piece of DNA made chemically. It pairs with one end of the region to be copied, so DNA polymerase can start there.
Option (2) In PCR, multiple copies of the gene (or DNA) of interest is synthesised in vitro using two sets of primers. — A correct fact. Two primers are needed, one for each strand. “In vitro” means outside a living cell, in a tube.
Option (3) If the process of replication of DNA is repeated many times, the segment of DNA can be amplified to approximately billion times, i.e., 1 billion copies are made. — A correct fact. Each cycle doubles the number of copies. After 30 cycles there are 230, about 1.07 billion, copies.
Option (4) The repeated amplification in PCR is achieved by the use of a thermostable DNA polymerase, isolated from a bacterium, Thermus quatica. — An incorrect fact. The heat-stable DNA polymerase comes from the bacterium Thermus aquaticus, which lives in hot springs. NTA printed the name as “Thermus quatica”, and that change of a few letters makes the fact incorrect. One letter can turn a true line into a false one, so read every word before you mark the answer.
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 – “… PCR stands for Polymerase Chain Reaction. In this reaction, multiple copies of the gene (or DNA) of interest is synthesised in vitro using two sets of primers (small chemically synthesised oligonucleotides that are complementary to the regions of DNA) and the enzyme DNA polymerase. The enzyme extends the primers using the nucleotides provided in the reaction and the genomic DNA as template. If the process of replication of DNA is repeated many times, the segment of DNA can be amplified to approximately billion times, i.e., 1 billion copies are made. Such repeated amplification is achieved by the use of a thermostable DNA polymerase (isolated from a bacterium, Thermus aquaticus), which remain active during the high temperature induced denaturation of double stranded DNA.”