PCR

Synthetic DNA oligonucleotides (oligos) are short, chemically synthesised DNA fragments that play a central role in modern molecular biology. They are widely used as PCR primers, DNA probes, sequencing primers, and building blocks for gene synthesis and molecular diagnostics. Oligos are produced using automated phosphoramidite chemistry, which has dramatically reduced costs and increased accessibility. Researchers typically order oligos online, receive them as dried (lyophilised) samples, and rehydrate them before use. The widespread use of oligos has made them a major component of global biotechnology research.

The polymerase chain reaction (PCR), invented by Kary Mullis in 1986, is a powerful technique for amplifying specific DNA sequences in vitro. PCR requires two short primers that flank the target region, a heat-stable DNA polymerase, nucleotides, and a template DNA molecule. Through repeated cycles of denaturation, annealing, and extension, PCR achieves exponential amplification, allowing large amounts of DNA to be generated from a single starting molecule. The development of heat-stable polymerases such as Taq DNA polymerase was a key breakthrough that enabled automation and widespread adoption of PCR.

Modern PCR is performed in programmable thermocyclers and is widely used in research, clinical diagnostics, forensic science, archaeology, and infectious disease detection. Compared to traditional cloning, PCR is faster, more selective, and easily automated, making it essential for next-generation sequencing workflows. However, PCR has limitations, including the need for prior sequence knowledge for primer design, difficulty amplifying very long DNA fragments, and the risk of introducing errors or contamination due to its extreme sensitivity.

Quantitative PCR (qPCR) extends standard PCR by measuring DNA amplification in real time using fluorescence. Unlike conventional PCR, qPCR allows accurate quantification of starting DNA templates by monitoring signal accumulation during early cycles. Fluorescent dyes such as SYBR Green bind double-stranded DNA, while probe-based systems like TaqMan provide higher specificity by using fluorophore–quencher probes that emit fluorescence only after probe cleavage. The cycle threshold (Ct) value is inversely related to the amount of starting material.

Reverse transcriptase PCR (RT-PCR) is used to detect and analyse RNA molecules. In this method, RNA is first converted into complementary DNA (cDNA) using reverse transcriptase, and the cDNA is then amplified by PCR. RT-PCR is especially important for studying gene expression, as it reveals whether a gene is actively transcribed in a cell or tissue.

✅ VERY DETAILED BULLET-POINT SUMMARY

1. Oligonucleotide Synthesis

• Synthetic DNA oligonucleotides (“oligos”) are widely used in:

  • DNA probes

  • PCR primers

  • DNA sequencing

  • Gene synthesis

  • Molecular diagnostics

• Global research spending on oligos is estimated at $700 million per year.

• DNA oligo synthesis is fully automated using chemical phosphoramidite chemistry.

• Costs have decreased significantly due to automation and commercial production.

• Commercial ordering process:

  • Researcher fills online form with desired sequence.

  • Company synthesizes and ships lyophilized (dried) oligos.

  • Cost example: ~€10 for a 20-mer (varies with scale and purification).

• Oligos arrive dry and are rehydrated before use.

2. Polymerase Chain Reaction (PCR)

Overview

• Invented by Kary Mullis (1986).

• Purpose: Enzymatic amplification of a specific DNA sequence.

• Occurs entirely in vitro in a microfuge tube.

• Needs two primers (~20 nt each) that flank the region of interest.

• The reaction achieves exponential amplification of target DNA.

Basic Steps (Thermal Cycling)

1. Denaturation (≈95–100°C)

   • Melts double-stranded DNA into single strands.

2. Annealing (≈45–68°C)

   • Primers bind complementary sequences on template strands.

   • Temperature depends on:

     • Primer length

     • GC content

3. Extension (≈72°C)

   • Optimal temperature for heat-stable DNA polymerases (e.g., Taq).

Exponential Amplification

• Each cycle doubles the number of DNA molecules.

• From one molecule, PCR can generate hundreds of nanograms of product.

• Product is easily detected by agarose gel electrophoresis.

Historical PCR

• Early PCR used:

  • Three water baths for manual temperature changes.

  • Klenow fragment (DNA polymerase I) which was heat-labile → had to be added every cycle.

• The breakthrough: heat-stable polymerases (e.g., Taq) from thermophilic bacteria.

Modern PCR

• Uses automated thermocyclers programmed for repeated cycles.

• Typical program:

  • Denaturation: 95–100°C

  • Annealing: 45–68°C

  • Extension: 72°C

Applications

• Molecular biology research

• Clinical diagnostics

• Forensic science

• Archaeology (ancient DNA)

• Genetic engineering

• Infectious disease detection

3. PCR vs. Traditional Cloning

Advantages of PCR

• Much faster

• Easily automated

• Allows selective amplification from complex genomic DNA

• Widely used in next-generation sequencing workflows

Limitations of PCR

1. Need to know sequences at both ends for primer design.

2. Difficult to amplify very long sequences (>15 kb), though long-range PCR is improving.

3. Many thermophilic polymerases (e.g., Taq):

   • Lack proofreading

   • Introduce errors

   • High-fidelity enzymes are available.

4. Extremely sensitive → contamination can cause false positives:

   • Problem in forensics

   • Ancient DNA studies

Current trends

• Many genomes are sequenced first; then genes are amplified by PCR.

• Cosmid/BAC/YAC libraries remain useful for very large DNA fragments.

4. Quantitative PCR (qPCR)

Need for qPCR

• Standard PCR reaches a plateau phase → final yield does not reflect initial quantity.

• qPCR measures product formation during early cycles.

Principle

• Fluorescent dyes (e.g., SYBR Green):

  • Bind double-stranded DNA

  • Fluoresce upon binding

• Signal increases as more product accumulates.

• The cycle threshold (Ct) indicates how many starting copies were present.

TaqMan Probes

• A specialized qPCR method using:

  • Fluorophore at one end

  • Quencher at the other end

• Probe hybridizes to target DNA between primers.

• During extension, the 5′→3′ exonuclease activity of polymerase:

  • Degrades the probe

  • Releases the fluorophore from quencher

  • Fluorescence increases proportionally to product accumulation

Interpretation

• Fluorescence intensity correlates with amount of target DNA present.

• Lower Ct = more starting material.

5. Reverse Transcriptase PCR (RT-PCR)

Purpose

• Detects and amplifies RNA sequences.

• Commonly used to determine whether a gene is being actively transcribed.

Steps

1. Extract total RNA.

2. Treat with RNase-free DNase to remove contaminating genomic DNA.

3. Use reverse transcriptase + specific primer to synthesize complementary DNA (cDNA).

4. Amplify cDNA using standard PCR.

40 Flashcards (Question → Answer)

1. What are synthetic DNA oligonucleotides?
   Short, chemically synthesized DNA fragments.

2. Name one common use of oligonucleotides.
   PCR primers.

3. What chemistry is used to synthesize DNA oligos?
   Phosphoramidite chemistry.

4. Why have oligo costs decreased?
   Automation and large-scale commercial production.

5. In what form do oligos typically arrive?
   Lyophilized (dried).

6. Who invented PCR?
   Kary Mullis.

7. What is the main purpose of PCR?
   Amplify a specific DNA sequence.

8. Where does PCR take place?
   In vitro in a microfuge tube.

9. How many primers are required for PCR?
   Two.

10. What determines the annealing temperature in PCR?
    Primer length and GC content.

11. What happens during denaturation?
    Double-stranded DNA separates into single strands.

12. What temperature is used for DNA extension?
    Approximately 72°C.

13. Which enzyme is commonly used in PCR?
    Taq DNA polymerase.

14. Why is Taq polymerase important?
    It is heat-stable.

15. What type of amplification does PCR produce?
    Exponential.

16. How is PCR product commonly detected?
    Agarose gel electrophoresis.

17. What was a limitation of early PCR methods?
    Heat-labile polymerases.

18. What device automates PCR cycles today?
    A thermocycler.

19. Name one application of PCR in medicine.
    Infectious disease detection.

20. Why is PCR faster than traditional cloning?
    It does not require vectors or host cells.

21. What is a major limitation of PCR primer design?
    Target sequence must be known.

22. Why can PCR introduce errors?
    Some polymerases lack proofreading.

23. Why is PCR prone to contamination?
    It is extremely sensitive.

24. What problem does qPCR solve?
    Quantifies DNA during early amplification cycles.

25. What does Ct stand for in qPCR?
    Cycle threshold.

26. What does a low Ct value indicate?
    High starting DNA quantity.

27. What dye is commonly used in qPCR?
    SYBR Green.

28. How does SYBR Green work?
    Fluoresces when bound to double-stranded DNA.

29. What is a TaqMan probe?
    A fluorescent probe used in qPCR.

30. What enzyme activity degrades the TaqMan probe?
    5′→3′ exonuclease activity of DNA polymerase.

31. Why does fluorescence increase in TaqMan qPCR?
    Fluorophore is separated from quencher.

32. What is the purpose of RT-PCR?
    Detect and amplify RNA sequences.

33. What enzyme is used in RT-PCR first?
    Reverse transcriptase.

34. What is cDNA?
    DNA is synthesised from an RNA template.

35. Why is DNase treatment used in RT-PCR?
    To remove genomic DNA contamination.

36. What does RT-PCR reveal about a gene?
    Whether it is being expressed.

37. What field commonly uses PCR in legal contexts?
    Forensic science.

38. Why is PCR important for next-generation sequencing?
    It amplifies target DNA fragments.

39. What is a key advantage of qPCR over standard PCR?
    Quantitative measurement.

40. What remains useful for very large DNA fragments?
    Cosmid, BAC, or YAC libraries.

📘 40 MULTIPLE-CHOICE QUESTIONS (MCQs)

1–10: Oligonucleotide Synthesis

1. Oligonucleotides are commonly used as:
   A. Restriction enzymes
   B. DNA probes
   C. Growth factors
   D. RNA polymerases

2. Chemical DNA synthesis today is:
   A. Manual
   B. Mostly automated
   C. Impossible for sequences >10 bases
   D. No longer used

3. A typical 20-mer oligo costs approximately:
   A. €1
   B. €10
   C. €100
   D. €1000

4. Synthetic oligos are typically delivered:
   A. Frozen in liquid nitrogen
   B. In lyophilized form
   C. As a gas
   D. In living cells

5. DNA synthesis used for oligos operates using:
   A. RNA polymerase
   B. Phosphoramidite chemistry
   C. Protein synthesis machinery
   D. DNA ligase

6. Worldwide spending on oligos is approximately:
   A. $7 million
   B. $70 million
   C. $700 million
   D. $7 billion

7. Oligos are essential for:
   A. PCR
   B. Photosynthesis
   C. Electron microscopy
   D. Cell culture medium

8. Oligos used in PCR are usually ≈:
   A. 5 nucleotides
   B. 20 nucleotides
   C. 100 nucleotides
   D. 1000 nucleotides

9. Oligos are synthesized:
   A. 5′→3′ direction
   B. 3′→5′ direction (chemically)
   C. Randomly
   D. Using ribosomes

10. Lyophilization means:
    A. Heating
    B. Freeze-drying
    C. Autoclaving
    D. UV-sterilizing

11–20: PCR Principles

11. PCR was invented by:
    A. Watson
    B. Crick
    C. Kary Mullis
    D. Sanger

12. PCR amplifies DNA:
    A. Linearly
    B. Exponentially
    C. Randomly
    D. Not at all

13. PCR requires:
    A. Two primers
    B. One primer
    C. Reverse transcriptase only
    D. Helicase

14. Early PCR used which polymerase?
    A. Taq
    B. Pol III
    C. Klenow fragment
    D. Ligase

15. Heat-stable polymerases are used because:
    A. They move faster
    B. They remain active after denaturation steps
    C. They bind RNA
    D. They eliminate primer dimers

16. Denaturation temperature is usually:
    A. 30°C
    B. 50°C
    C. 72°C
    D. ~95–100°C

17. Annealing temperature depends primarily on primer:
    A. Color
    B. Length and GC content
    C. Price
    D. Shape

18. Extension temperature is optimal for Taq polymerase at:
    A. 15°C
    B. 45°C
    C. 72°C
    D. 100°C

19. PCR product is typically analyzed using:
    A. Western blotting
    B. PFGE
    C. Agarose gel electrophoresis
    D. Mass spectrometry

20. PCR can amplify:
    A. Only plasmids
    B. Only RNA
    C. Any specific DNA region with known flanking sequences
    D. Proteins

21–30: PCR vs. Cloning & Limitations

21. A requirement for PCR is knowing:
    A. Exact middle sequence
    B. Sequence at both ends
    C. Protein sequence
    D. Restriction map

22. Standard PCR struggles to amplify fragments longer than:
    A. 500 bp
    B. 2 kb
    C. 15 kb
    D. 1 Mb

23. A limitation of Taq polymerase is:
    A. Too high fidelity
    B. Lack of proofreading
    C. Low processivity
    D. Inability to bind primers

24. A major contamination risk in PCR results from:
    A. Buffers
    B. Air
    C. Tiny amounts of DNA
    D. Temperature

25. PCR is faster and more automatable than:
    A. Restriction digestion
    B. Traditional cloning
    C. Gel electrophoresis
    D. Protein purification

26. High-fidelity polymerases help reduce:
    A. Contamination
    B. Primer availability
    C. Misincorporation errors
    D. Mg²⁺ concentration

27. Standard PCR yields plateau at the end because:
    A. Primers increase
    B. Polymerase becomes more active
    C. Reagents become limiting
    D. DNA melts

28. Long DNA fragments (>150 kb) are best obtained from:
    A. PCR
    B. Cosmid/BAC libraries
    C. SDS-PAGE
    D. qPCR

29. PCR product amount depends on:
    A. Number of cycles
    B. Template amount (early cycles)
    C. Enzyme activity
    D. All of the above

30. Standard PCR cannot give quantitative information because:
    A. Primers degrade
    B. Plateau phase eliminates correlation with starting amount
    C. Template is destroyed
    D. Polymerase cannot extend fully

31–40: qPCR & RT-PCR

31. qPCR detects amplification using:
    A. Radioactive isotopes
    B. Heat
    C. Fluorescence
    D. Sound waves

32. A lower Ct value indicates:
    A. More initial template
    B. Less initial template
    C. No template
    D. Primer dimer formation

33. SYBR Green binds to:
    A. Single-stranded RNA
    B. Proteins
    C. Double-stranded DNA
    D. Lipids

34. In a TaqMan probe, when quencher is close to fluorophore, fluorescence is:
    A. Increased
    B. Decreased
    C. Random
    D. Unaffected

35. Fluorescence increases in TaqMan qPCR because:
    A. DNA melts
    B. Probe is degraded by 5′→3′ exonuclease
    C. Mg²⁺ is consumed
    D. Temperature increases

36. RT-PCR is used to detect:
    A. DNA only
    B. RNA
    C. Lipids
    D. Proteins

37. Reverse transcriptase converts:
    A. DNA → RNA
    B. RNA → DNA
    C. Protein → DNA
    D. DNA → protein

38. Before RT-PCR, samples must be treated with DNase to remove:
    A. rRNA
    B. Genomic DNA
    C. Proteins
    D. Lipids

39. The product of reverse transcriptase is called:
    A. cDNA
    B. mRNA
    C. tRNA
    D. siRNA

40. RT-PCR is commonly used to measure:
    A. DNA methylation
    B. Gene transcription levels
    C. Protein structure
    D. Membrane fluidity

✅ ANSWER KEY (MCQs)

1-B
2-B
3-B
4-B
5-B
6-C
7-A
8-B
9-B
10-B
11-C
12-B
13-A
14-C
15-B
16-D
17-B
18-C
19-C
20-C
21-B
22-C
23-B
24-C
25-B
26-C
27-C
28-B
29-D
30-B
31-C
32-A
33-C
34-B
35-B
36-B
37-B
38-B
39-A
40-B