The region of DNA shown below is to be amplified by PCR. The appropriate pair of primers to use is which of the following?
(A) B and C
(B) A and D
(C) A and C
(D) B and D
Correct Answer: ✅ (A) B and C
Explanation
PCR (Polymerase Chain Reaction) is a widely used technique to amplify a specific region of DNA. Successful PCR requires a pair of primers — a forward primer and a reverse primer — that bind to opposite strands of the target DNA in a specific orientation.
🔬 1. How PCR Primers Work
- Forward Primer: Binds to the 5′ end of the sense strand (template strand).
- Reverse Primer: Binds to the 3′ end of the antisense strand (complementary strand).
- DNA polymerase extends the primers in the 5′ → 3′ direction, producing new DNA strands.
✅ Why the Answer is (A) B and C
- Primer B – Acts as the forward primer, binding to the 5′ end of the template strand.
- Primer C – Acts as the reverse primer, binding to the 3′ end of the complementary strand.
- Both primers face each other, allowing DNA polymerase to extend them and produce the desired amplified product.
❌ Why Other Options Are Incorrect
| Option | Explanation | Correct/Incorrect |
|---|---|---|
| (A) B and C | Correct orientation for primer binding and amplification. | ✅ Correct |
| (B) A and D | Both primers face away from each other, leading to no amplification. | ❌ Incorrect |
| (C) A and C | Both primers are on the same strand, preventing amplification. | ❌ Incorrect |
| (D) B and D | Both primers face away from each other, preventing extension. | ❌ Incorrect |
How to Choose the Right PCR Primers
✅ 1. Correct Orientation:
- Forward primer binds to the 5′ end of the template strand.
- Reverse primer binds to the 3′ end of the complementary strand.
✅ 2. Proper Length:
- Ideal primer length: 18–24 nucleotides
- Short primers → Low specificity
- Long primers → Low efficiency
✅ 3. Melting Temperature (Tm):
- Ideal Tm: 50°C–60°C
- Tm can be calculated using the formula:
Tm=(4×G+C)+(2×A+T)\text{Tm} = (4 \times G + C) + (2 \times A + T)
✅ 4. GC Content:
- Ideal GC content: 40%–60%
- High GC content → Stronger binding and higher stability
✅ 5. Avoid Primer-Dimer Formation:
- No complementary regions within the primers
- No secondary structure formation
Steps in PCR Amplification
🧪 1. Denaturation:
- Heating to 94–98°C → Breaks hydrogen bonds → Single-stranded DNA templates
🧬 2. Annealing:
- Cooling to 50–65°C → Primers bind to target DNA sequences
🔎 3. Extension:
- DNA polymerase extends the primers at 72°C → Formation of new DNA strands
Cycle-Wise DNA Amplification
| Cycle Number | Number of Double-Stranded DNA Molecules |
|---|---|
| 0 (Initial) | 1 |
| 1st cycle | 2 |
| 2nd cycle | 4 |
| 3rd cycle | 8 |
| 4th cycle | 16 |
Common Errors in PCR Primer Design
🔴 1. Wrong Orientation:
- Primers should face each other → Incorrect orientation = No amplification
🔴 2. Primer-Dimer Formation:
- Complementary sequences in primers → Leads to unwanted products
🔴 3. Incorrect Tm:
- Low Tm → Non-specific binding
- High Tm → Weak primer binding
Applications of PCR
✅ 1. Genetic Diagnosis:
- Detecting genetic mutations and infections
✅ 2. Cloning and Sequencing:
- Amplifying genes for functional analysis
✅ 3. Forensic Science:
- DNA fingerprinting and criminal identification
✅ 4. Evolutionary Studies:
- Amplification of ancient DNA for phylogenetic analysis
Summary
- PCR primers must bind in opposite orientations on complementary DNA strands.
- Primer B binds to the 5′ end of the template strand, and Primer C binds to the 3′ end of the complementary strand.
- This ensures correct amplification of the target region.
- ✅ Correct Answer: (A) B and C
76 Comments
Akshay mahawar
March 17, 2025Done 👍
Ujjwal
March 17, 2025Done sir
Suman bhakar
March 17, 2025Ok
Nisha
March 17, 2025Best website for preparation
Arushi
March 17, 2025👍👍
Parul
March 22, 2025Easy one. Done sir
Roopal Sharma
August 24, 2025Correct orientation forward amd reverse primer at 5 end and 3 end option a
Soniya Shekhawat
August 24, 2025Forward primer is bind to 5 ‘and reverse primer is bind 3’ and so forward primer is a b and reverse primer is a c so option is a first is right.
Mohd juber Ali
August 24, 2025Ans is (a)
Bcz primer A not bind bcz dna 5′ to 3′ direction me new strand produce karta h but primer A 3′-5′ h new produce nahi hogi (same primer D)
But primer B and primer C new strand produce kar sakte h forward (5′ end)and reverse primer 3′ end
Khushi Pareek
August 24, 2025B&C
Sakshi yadav
August 24, 2025The correct one is 1 .at B,A site the forward primer is 3′ and backward is 5′ so at this site correct orientation for primer binding and amplification
Avni
August 24, 2025Correct answer is (A) B and C
Correct orientation for primer binding and amplification.
Neha Yadav
August 24, 2025B and C primer are correctly oriented
Arushi Saini
August 24, 2025b and c
Mansukh Kapoor
August 24, 2025The correct answer is option 1st B and C
Primer B acts as forward primer
Primer C acts as reverse primer
karishma don
August 24, 2025primer b as forward and primer c as reverse
Mahima Sharma
August 24, 2025a forward primer and a reverse primer — that bind to opposite strands of the target DNA in a specific orientation
Mahima Sharma
August 24, 2025Answer a (b and c)
Anurag Giri
August 24, 2025Ans 1
Bcoz primer B act as forward primer
And primer c act as reverse primer
Aafreen Khan
August 24, 2025B and C is correct answer
Primar B is forward primer
Primer C is reverse primer
Dharmpal Swami
August 24, 2025Forword primer = B
Backward primer= c
Ans 1 is write
Aakanksha Sharma
August 24, 2025B&C is right
Dipti Sharma
August 24, 2025(A) B and C
B is forward and C is reverse primer.
Karishma
August 24, 2025B and c is right answer
Karishma
August 24, 2025B and c is correct
Priyanka Choudhary
August 24, 2025Option A (b and c) will be Right answer, forword and reverse primer
Karishma
August 24, 2025B and c is correct option A is correct
HIMANI FAUJDAR
August 24, 2025Ans correct answer is B and C
Priya dhakad
August 24, 2025The option is B and c correct orientation for primer binding and amplification
Sneha Kumawat
August 24, 2025Answer is B and C
Forward primer binds to the 5′ end of the template strand
Reverse primer binds to the 3 prime end of complimentary strand
Sheikh Aleen
August 24, 2025B is forward and C is reverse primer.
Divya rani
August 24, 2025B and C primers are the right oriented because B is the forward primer and C is the reverse primer.
AKANKSHA RAJPUT
August 24, 2025B and C primer are correctly oriented
Santosh Saini
August 24, 2025Option B and C are correct because 2 primer are used in PCR one is forward primer ( 5′ to 3′) and another is reverse primer (complementary)
Payal Gaur
August 24, 2025B and C is right two primer are form starting from 5′ end
anjani sharma
August 24, 2025answer (A) B and C
Correct orientation for primer binding and amplification.
Heena Mahlawat
August 24, 2025Forward and reverse primers used i.e B and C
Tanvi Panwar
August 24, 2025Correct answer is B and C .
MOHIT AKHAND
August 24, 2025Done sir ✅
Surbhi Rajawat
August 24, 2025Primers are designed for 3 prime end of each strand because replication occurs from 5 primer to 3 prime. Hence B and C is the correct answer
Ayush Dubey
August 24, 2025B and C
Neelam Sharma
August 24, 2025Primer B binds to the 5′ end of the template strand, and Primer C binds to the 3′ end of the complementary strand
B and c correct answer
Samiksha bajiya
August 24, 2025Option b and c is correct
Aman Choudhary
August 24, 2025The forward primer is bind with 5′ end and the reverse primer is bind with the opposite direction
Bhawna Choudhary
August 24, 2025Correct answer is B and C
Priyanka Verma
August 24, 2025option A is correct answer because the forward primer binds to 5′ end and reverse primer binds to 3′ end.
Monika jangid
August 24, 2025Ans a correct orientation 5′ end nd 3′ end
Bharti Yadav
August 24, 2025Forward primer: B
Reverse primer: C
Falguni Prakash
August 24, 2025B c
Varsha Tatla
August 24, 2025B forward primer n c reverse primer bcz polymerase act in 5′-3′ direction
shruti sharma
August 25, 2025B is forward
C is reverse
Vanshika Sharma
August 25, 2025Correct answer is A
Khushi Singh
August 30, 2025A is correct
Bhavana kankhedia
August 26, 2025B and C
Surendra Doodi
August 26, 2025B is forward and C is reverse primer
Manisha
August 26, 2025Done ✅
Shivani
August 26, 2025(A) B and C
Rishita
August 26, 2025Opt a
Devika
August 26, 2025B and C
Pallavi Ghangas
August 26, 2025Band C
Simran Saini
August 26, 2025B abd C.
Primer B is forward primer.
Primer C is reverse primer.
Sakshi Kanwar
August 27, 2025As forward primer binds to 5′ end whereas reverse primer to 3′ end so answer will be B and C
Nilofar Khan
August 28, 2025Correct answer is (A) Band C
Primer B is forward primer
Primer C is reverse primer
Kanica Sunwalka
August 28, 2025taq only have 5′-3’exoNu activity
Muskan singodiya
August 28, 2025Option a
B and c, forward 5′ and reverse 3′
Kajal
August 28, 2025Option a which includes b and c is the correct answer
Deepika sheoran
August 28, 2025Option 1st is correct
B & C
Forward primer & Reverse primer
Neeraj Sharma
August 28, 2025Option A is correct
Mitali saini
August 29, 2025the Answer is (A) B and C
Parul Yadav
August 29, 2025Forward Primer: Binds to the 5’end of the sense strand.
Reverse Primer: Binds ate 3’end of the antisense strand.
Asha Gurzzar
August 30, 2025Option a is correct,
Kajal
August 31, 2025Correct Answer is (A) B and C
Aartii sharma
August 31, 2025B&C
Konika Naval
August 31, 2025B and C
Meenakshi Choudhary
September 3, 2025Correct answer option A
Muskan Yadav
September 8, 2025B and C