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PCR or Polymerase Chain Reaction is a technique that is used in molecular biology and biotechnology labs in order to create many copies of a certain DNA segment. Developed in the year 1983 by an American biochemist named Kary Mullis, PCR has made it possible to create millions of copies of a small DNA segment. This technique is used on the basis of enzymatic DNA replication. In Polymerase Chain Reaction, PCR, a short DNA segment is amplified through the use of primer mediated enzymes. PCR contains the following three components that include DNA template, DNA Polymerase, Deoxyribonucleotide triphosphate, Buffer System, and Oligonucleotide Primers. PCR mainly involves three cyclic reactions:
- Denaturation: It occurs when the mixture of the reaction is heated to 94° C for up to 0.5 to 2 minutes. This helps in breaking the hydrogen bonds between the DNA strands by converting them to single-stranded DNA.
- Annealing: The temperature of the reaction is lowered to 54 to 60° C for about 20 to 40 seconds. In this step, the primers bind to the complementary sequences on the DNA template.
- Elongation: At this step of the reaction, the temperature is increased to 72 to 80° C. The bases are then added to the 3' end of the primer through the Taq polymerase enzyme.
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Ques 1. Polymer Chain reaction is used for which of the following?
- Constructing the RAPD maps
- Detecting the transgene presence in an organism
- Amplifying the gene of interest
- All of the above
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Ans. d) All of the above
Explanation: Polymer Chain Reaction is used to construct RAPD maps, amplify the gene of interest, and detect the presence of the transgene in an organism.
Ques 2. Who out of the following developed the PCR technique?
- Altman
- Milstein
- Kohler
- Kary Mullis
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Ans. d) Kary Mullis
Explanation: The Polymerase Chain Reaction technique or PCR was developed by Kary Mullis in the 1980s. It is based on utilizing the ability of DNA polymerase to synthesize a new DNA strand that is complementary to the offered template strand.
Ques 3. The polymerase chain reaction can be understood as:
- a DNA sequencing technique
- a DNA degradation technique
- a DNA amplification technique
- All of the above
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Ans. c) a DNA amplification technique
Explanation: The Polymerase Chain Reaction (PCR) is a DNA amplification technique that involves enzymatic amplification of the nucleic acid sequences through a series of repeated cycles of oligonucleotide annealing, denaturation, and DNA polymerase extension.
Ques 4. Out of the following, identify the first and most vital step of PCR:
- Annealing
- Denaturation
- Primer extension
- None of the above
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Ans. b) Denaturation
Explanation: Denaturation is the first step of the PCR technique that occurs when the mixture of the reaction is heated to a temperature of 94oC for a duration of 0.5 to 2 minutes. This helps in breaking the existing hydrogen bonds between 2 DNA strands to convert them to a single strand of DNA. This single strand of DNA is then used as a template for producing new DNA strands. To ensure the proper separation of the two strands, the temperature should be provided for a longer duration of time.
Ques 5. At what temperature does the DNA annealing and primer take place?
- 54° C
- 42° C
- 74° C
- 96° C
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Ans. a) 54° C
Explanation: During the annealing of DNA, the temperature is dropped to 54° C for about 20 to 40 seconds. In this step, the primers bind on the template DNA to their complementary sequences.
Ques 6. Identify the statement that is true about asymmetric PCR
- It is used to generate the single-stranded copies for the DNA sequence
- It is used to generate the double-stranded copies for the DNA sequence
- Both a) and b)
- None of the above
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Ans. a) It is used to generate the single-stranded copies for the DNA sequence
Explanation: Asymmetric PCR is a PCR variation that is used to amplify a single strand of the original DNA preferentially more than the other ones. It helps in sequencing and hybridization probing.
Ques 7. Polymerase used in the PCR technique is extracted from:
- Homo sapiens
- Thermus aquaticus
- Saccharomyces cerevisia
- Escherichia coli
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Ans. b) Thermus aquaticus
Explanation: Polymerase used in the PCR technique is derived from Thermus aquaticus. This organism survives in hot springs, therefore Taq polymerases are thermostable and hence they are resistant towards high temperatures.
Ques 8. How many DNA duplexes can be obtained from a DNA duplex after four cycles of PCR?
- 16
- 4
- 8
- 32
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Ans. a) 16
Explanation: After every cycle, the number of duplex doubles itself and hence after the first cycle, we have two DNA duplexes. Now, a duplex has 2 strands of DNA, thus after the second cycle we will have 4 Duplexes, after the third cycle, there will be 8 DNA duplexes. Finally, after the fourth cycle, there will be 16 duplexes obtained.
Ques 9. Why are the vent polymerase and Pfu comparatively more efficient than the Taq polymerase?
- Due to the more efficient polymerase activity
- Due to proofreading activity
- Both a) and b)
- None of the above
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Ans. b) Due to proofreading activity
Explanation: Vent polymerase and Pfu are more efficient in comparison to the Taq polymerase as they have a proofreading activity.
Ques 10. Primers that are used for the polymerase chain reaction process are:
- Single-stranded RNA oligonucleotide
- Double-stranded DNA oligonucleotide
- Double-stranded RNA oligonucleotide
- Single-stranded DNA oligonucleotide
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Ans. d) Single-stranded DNA oligonucleotide
Explanation: Two single-stranded DNA oligonucleotide is synthesized to be used as a primer. The primer that is produced is then bound to the denatured DNA and the polymerase synthesis starts in the 5’ to 3’ direction.
Previous Years Questions
- The successive nucleotides of RNA are…. [JIPMER 2001]
- The most abundant RNA in the cell is….[CUCET 2011]
- Which base is present in RNA but not in DNA….[BCECE 2004]
- The pentose sugar in DNA and RNA has the…
- The chemical nature of chromatin is as….
- Which of the following is not considered as a stop codon?
- When the centromere of a chromosome is located near the tip…
- The back bone of RNA consists of which of the….
- The name chromatin was coined by…. [KEAM]
- The codons UUU and UUC codes for phenylalanine only…. [KCET 2016]
- The length of one turn of the helix in...[NEET 2016]
- Purines found both in DNA and RNA are...[NEET 2019]
- what will be the percentage of Thymine, Guanine and Cytosine in..[NEET 2021]
- If the total amount of adenine and thymine in a double-stranded DNA is….[MET 2010]
- Mutations in plant cells can be induced by….[NEET 2021]
- The back bone of RNA consists of…
- Heterogeneous nuclear RNA is converted to… [AMUEEE 2014]
- DNA replication is semi-conservative because... [COMEDK UGET 2008]
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