Why Do Parent DNA Strands Not Recombine During PCR Primer Annealing?
During the polymerase chain reaction, the two DNA strand are separated at 95 °C after which the reaction mixture is cooled to 54 °C to allow the primers to hybridize to the DNA strands. Why do parent DNA duplexes not form instead?
a. The primers are present in such a large excess that they “out compete” the parent strands coming back together
b. The 54° C is not below sufficiently below the Tm for the strands to recombine
c. The DNA polymerase binds to the single strands and prevents them from coming back together to form a double helix.
d. Parental DNA are degraded
Correct Answer:
The correct answer is (a) The primers are present in such a large excess that they “outcompete” the parent strands coming back together.
Why Do Parent DNA Strands Not Recombine During PCR Primer Annealing?
Polymerase Chain Reaction (PCR) is a widely used molecular biology technique for amplifying specific DNA sequences. The process involves three key steps: denaturation, annealing, and extension. During the annealing phase, primers bind to the single-stranded DNA templates, initiating the amplification process. A common question arises: why don’t the original parent DNA strands recombine instead of allowing primers to bind?
Understanding the PCR Process
PCR follows a cyclic process of three steps:
- Denaturation (94–98°C): The double-stranded DNA (dsDNA) is heated to separate it into two single strands.
- Annealing (50–65°C): The temperature is lowered to allow short single-stranded DNA primers to bind to complementary sequences on the single-stranded DNA template.
- Extension (72°C): Taq polymerase synthesizes a new DNA strand by adding nucleotides complementary to the template strand.
Why Parent DNA Strands Do Not Recombine:
-
Excess of Primers:
- During the PCR reaction, primers are added in a large molar excess compared to the template DNA.
- When the reaction mixture cools down to the annealing temperature (~54°C), the primers are more likely to hybridize with the single-stranded DNA than the original complementary strands.
- This prevents the parent strands from reannealing.
-
Annealing Temperature and Thermodynamics:
- The annealing temperature is carefully selected to be high enough to prevent non-specific binding but low enough to allow primer hybridization.
- The temperature is typically optimized to be slightly lower than the melting temperature (Tm) of the primers, promoting primer binding over the reformation of the parent duplex.
-
Kinetic Favorability:
- The primer binding is kinetically favored over the reformation of the parent strands due to the high primer concentration and rapid cooling.
- The primers are short and can bind more quickly compared to the longer parent strands.
-
Role of DNA Polymerase:
- Once the primers bind, DNA polymerase rapidly extends the new strand.
- This further reduces the chance of parental DNA strands reannealing because the template is already occupied by the growing complementary strand.
Why Other Options Are Incorrect:
- (b) The 54°C is not sufficiently below the Tm for the strands to recombine – This is incorrect because the temperature is chosen to be optimal for primer binding and is below the Tm.
- (c) The DNA polymerase binds to the single strands and prevents them from coming back together to form a double helix – DNA polymerase only binds after primer hybridization and does not prevent reannealing directly.
- (d) Parental DNA are degraded – Parent DNA strands remain intact throughout the reaction.
Significance of Primer Annealing in PCR:
The ability of primers to outcompete the parent strands and bind specifically to the template DNA is critical for:
✅ High amplification efficiency
✅ Specificity of target sequence amplification
✅ Avoiding non-specific binding and primer-dimer formation
Conclusion:
During PCR, parent DNA strands do not recombine because the primers are present in a large excess and bind rapidly to the single-stranded template DNA at the annealing temperature. This ensures efficient and specific amplification of the target sequence, making PCR a powerful tool in molecular biology and genetic research.
73 Comments
Ujjwal
March 17, 2025Done sir
Suman bhakar
March 17, 2025Ok sir
Parul
March 22, 2025Easily done .
Abhilasha
March 25, 2025Done ✅
Nisha
March 27, 2025Done ✅
Khushi Pareek
August 24, 2025Primers concentration very high
Answer: a
Mohd juber Ali
August 24, 2025Option a
Annealing step me single strand dna pe large excess of primer bind hote so parents strand do not recombine
Meera Gurjar
August 24, 2025The primers are present in such a large excess that they “outcompete” the parent strands coming back together.
Sakshi yadav
August 24, 2025The primers are present in such a large excess that they “outcompete” the parent strands coming back together.
karishma don
August 24, 2025the primers in number are more and they out compete the parent dna strands coming back together
Arushi Saini
August 24, 2025Primers are present in large number so they will out compete and prevent to form double helix
Avni
August 24, 2025The correct answer is (a) The primers are present in such a large excess that they “outcompete” the parent strands coming back together
Neha Yadav
August 24, 2025The primer are present in such a large excess that they out compete the parent strands coming back together
roopal sharma
August 24, 2025primers are present in large excess.
roopal sharma
August 24, 2025primers are present in large excess. option a is correct
Roopal sharma
August 24, 2025primers are present in large molar excess than template dna. option a is correct
Anurag Giri
August 24, 2025Ans a
The ability of primers to outcompete the parent strands and bind specifically to the template DNA is critical for
High amplification efficiency
Specificity of target sequence amplification
Avoiding non-specific binding and primer-dimer formation
Khushi Agarwal
August 24, 2025Option a is correct answer
The primers are present in such a large excess that they “out compete” the parent strands coming back together
Priyanka Choudhary
August 24, 2025Correct answer is option A,
The primers are present in such a large excess that they “outcompete” the parent strands coming back together
Dharmpal Swami
August 24, 2025Primer are present in large concentration during anneling process these primers are bind with parental strand
Aakanksha Sharma
August 24, 2025Bcz primer is present in excess
Aafreen Khan
August 24, 2025The primers are present in such a large excess that they outcompete the parent strands coming back together
Karishma
August 24, 2025Anwer is a
Karishma
August 24, 2025Primer is not present in large excess
Dipti Sharma
August 24, 2025The primers are present in such a large excess that they “outcompete” the parent strands coming back together.
anjani sharma
August 24, 2025Answer a
The primers are present in large quantity so they stop stands from coming towards each other
Tanvi Panwar
August 24, 2025The primers are present in large extent that the outcompete the parental strands from coming together.
Heena Mahlawat
August 24, 2025Primer concentration are much higher
Khushi Singh
August 24, 2025primers are present in large excess. option a is correct
MOHIT AKHAND
August 24, 2025Done sir ✅
Sneha Kumawat
August 24, 2025During PCR parent DNA stand do not recombine because the primers are present in a large access and bind rapidly to the single standard template DNA at the annealing temp.
Surbhi Rajawat
August 24, 2025Primers are present in large amounts and they bind to DNA strands prevent their reformation
Neelam Sharma
August 24, 2025The primers are present in such a large excess that they “outcompete” the parent strands coming back together.
Bharti Yadav
August 24, 2025The primers are present in such a large excess that they “outcompete” the parent strands coming back together.
Priyanka Verma
August 24, 2025Parent DNA strands do not recombine because the primers are present in a large excess and bind rapidly to the single-stranded template DNA at the annealing temperature
Ayush Dubey
August 25, 2025Option A
Aman Choudhary
August 25, 2025Option A is correct
The primers are present in such a large excess that they “out compete” the parent strands coming back together
Divya rani
August 25, 2025Excess of primers prevents the parent DNA strands to be recombine together.
shruti sharma
August 25, 2025Primers are present in large amounts and they bind to DNA strands prevent their reformation
Payal Gaur
August 25, 2025PCR complete in three step
Denaturation (high temp DNA single strand )
Annealing (low temp primer add)
Extension(2 DNA form)
In annealing large molar excess primer add with ssDNA and prevent the parent strand from reannealing
Bhavana kankhedia
August 26, 2025The primers are present in a large amount and they bind to dna strands and prevent their reformation
Vanshika Sharma
August 26, 2025Primers are present in large amount and they bind to dna strand prevent their reformation
Minal Sethi
August 26, 2025Concentration of primers are kept high so that parent strands do not combine with each other
Shivani
August 26, 2025The primers are present in such a large excess that they “outcompete” the parent strands coming back together.
Rishita
August 26, 2025Done sir 👍🏻
Surendra Doodi
August 26, 2025The primers are present in such a large excess that they “outcompete” the parent strands coming back together. Option A is correct
Pallavi Ghangas
August 26, 2025Primers are present in such high concentration that they out compete the formation of parent duplex
Devika
August 26, 2025Option A.
Simran Saini
August 26, 2025The primers are present in such a large excess that they “out complete” the parent strands coming back together.
Muskan singodiya
August 27, 2025Option a
Seema
August 27, 2025A) is right
Dna strands don’t recombine becouse primers are present in excess which help in target dna amplification.
Rakesh Dhaka
August 27, 2025Option a is correct
Sakshi Kanwar
August 27, 2025As primers in large quantity wants to bind with single stranded DNA then the two original parent strands that’s why the primers outcompete the parent strands coming back together.
Mitali saini
August 28, 2025The correct answer is (a) The primers are present in such a large excess that they “outcompete” the parent strands coming back together.
Nilofar Khan
August 28, 2025Correct answer is a
Kanica Sunwalka
August 28, 2025dono parental strand bind isliye nahi krenge kyuki primer ki appropriate conc. h toh vo fatak se bind ho jaega
dna pol only binds when primer a jata h
Deepika Sheoran
August 28, 2025Option A is correct
Primers are present in large molar excess than template DNA.Concentration during Annealing process these primers are bind with parental strand..
Mohini
August 28, 2025The primers are present in such a large excess that they outcompete the parent strands coming back together.
Kajal
August 28, 2025Due to the large excess of primer the parent dna strands does not recombine option a is correct
Anisha Beniwal
August 28, 2025Option A is correct
Neeraj Sharma
August 28, 2025Due to excess primers the primers binds with the target site and the parent DNA cannot form the duplex again
Khushi Vaishnav
August 28, 2025The primers are present in such a large excess that they outcompete the parent strands coming back together.
Parul Yadav
August 29, 2025Parental DNA duplex doesn’t form during annealing step as the number of primer subdue the parental DNA strands.
Priya dhakad
August 29, 2025The primers are present in such a large excess that they “outcompete” the parent strands coming back together.
Asha Gurzzar
August 30, 2025Answer a is correct, primers are present in a large excess that they outcomplete the parent strand coming back together
Khushi Singh
August 30, 2025A is correct
Sonam Saini
August 30, 2025Option is right primary is present in such large quantity that it is unable to bring the original stand back together
Kirti Agarwal
August 31, 2025Primers are present in large concentration during anneling process these primers are bind with parental strand
Kajal
August 31, 2025correct answer is (a) The primers are present in such a large excess that they out compete the parent strands coming back together
Konika Naval
August 31, 2025primers are present in such a large excess that they “outcompete” the parent strands coming back together.
Meenakshi Choudhary
September 3, 2025DNA strands do not recombine because concentration of primer is more then sample
Muskan Yadav
September 8, 2025The primers are present in such a large excess that they outcompete the parent strands coming back together.
Lokesh Kumawat
September 11, 2025The primers are present in such a large excess that they outcompete the parent strands coming back together.