Amplification of DNA using PCR and Gel Electrophoresis

1. Polymerase Chain Reaction (PCR)

1.1. Purpose of PCR

• PCR (Polymerase Chain Reaction): A technique used to amplify a specific DNA sequence, creating millions to billions of copies from a small initial sample.
• Increases the amount of DNA available for analysis.
• Essential for various applications like DNA profiling, genetic testing, and research.

KEY TAKEAWAY: PCR is like a molecular photocopier, allowing scientists to create many copies of a specific DNA sequence.

1.2. Process of PCR

PCR involves repeated cycles of three main steps:

1. Denaturation:
   • Heating the DNA sample to a high temperature (typically 94-96°C) to break the hydrogen bonds between complementary base pairs.
   • This separates the double-stranded DNA into two single strands.
2. Annealing:
   • Cooling the sample to a lower temperature (typically 50-65°C) to allow primers to bind to the single-stranded DNA.
   • Primers: Short, single-stranded DNA sequences (oligonucleotides) that are complementary to the regions flanking the target DNA sequence.

3. Extension/Elongation:

   • Increasing the temperature to an optimal temperature for the DNA polymerase enzyme (typically 72°C).
   • DNA polymerase: An enzyme (e.g., Taq polymerase) that synthesizes new DNA strands by adding nucleotides to the 3’ end of the primers, using the original DNA strands as templates.

4. These three steps are repeated for 20-40 cycles, resulting in exponential amplification of the target DNA sequence.

REMEMBER: “DAT” - Denaturation, Annealing, and Extension/Elongation are the three steps of PCR.

1.3. Key Components of PCR

• DNA template: The original DNA sample containing the target sequence to be amplified.
• Primers: Short DNA sequences that define the region to be amplified.
• DNA polymerase: An enzyme (e.g., Taq polymerase) that synthesizes new DNA strands. Taq polymerase is used because it is heat-stable, allowing it to withstand the high temperatures of the denaturation step without being denatured itself.
• Deoxynucleotide triphosphates (dNTPs): The building blocks of DNA (A, T, C, and G).
• Buffer solution: Provides the optimal chemical environment for the PCR reaction.

EXAM TIP: Be able to describe the role of each component of a PCR reaction in detail.

1.4. Applications of PCR

• DNA profiling: Amplifying specific DNA regions for identification purposes.
• Genetic testing: Detecting the presence of specific genes or mutations.
• Disease diagnosis: Identifying pathogens (e.g., viruses, bacteria) in a sample.
• Forensic science: Analyzing DNA samples from crime scenes.
• Research: Cloning genes, studying gene expression, and developing new biotechnologies.

APPLICATION: PCR is used to amplify ancient DNA from fossils, allowing scientists to study the genetic makeup of extinct organisms.

2. Gel Electrophoresis

2.1. Purpose of Gel Electrophoresis

• Gel electrophoresis: A technique used to separate DNA fragments based on their size and charge.
• DNA fragments are loaded into wells of a gel (typically agarose or polyacrylamide) and an electric field is applied.
• DNA is negatively charged due to the phosphate groups in its backbone, so it migrates towards the positive electrode (anode).
• Smaller DNA fragments move through the gel faster than larger fragments, resulting in separation by size.

KEY TAKEAWAY: Gel electrophoresis sorts DNA fragments by size, with smaller fragments traveling farther than larger ones.

2.2. Process of Gel Electrophoresis

1. Gel Preparation:
   • The gel (agarose or polyacrylamide) is prepared by dissolving it in a buffer solution and pouring it into a mold.
   • A comb is inserted into the gel to create wells for loading the DNA samples.
2. Sample Preparation:
   • DNA samples are mixed with a loading dye, which contains a dense substance (e.g., glycerol) to help the sample sink into the wells, and a tracking dye to monitor the progress of the electrophoresis.
3. Loading the Gel:
   • The DNA samples are loaded into the wells of the gel.
   • A DNA ladder (also known as a DNA marker) is also loaded into a separate well. The DNA ladder contains DNA fragments of known sizes, which are used to estimate the sizes of the unknown DNA fragments in the samples.
4. Electrophoresis:
   • The gel is placed in an electrophoresis chamber filled with a buffer solution.
   • An electric field is applied across the gel, with the negative electrode (cathode) near the wells and the positive electrode (anode) at the opposite end.
   • DNA fragments migrate through the gel towards the positive electrode.
5. Staining and Visualization:
   • After electrophoresis, the gel is stained with a DNA-binding dye (e.g., ethidium bromide or SYBR Green) to visualize the DNA fragments.
   • The stained gel is then placed under UV light, which causes the DNA bands to fluoresce.
   • The gel is photographed to record the results.

STUDY HINT: Draw a diagram of a gel electrophoresis setup to help you visualize the process.

2.3. Factors Affecting Migration Rate

• Size of DNA fragment: Smaller fragments migrate faster.
• Agarose concentration: Higher concentrations of agarose create smaller pores, which slow down the migration of larger fragments.
• Voltage: Higher voltage increases the migration rate, but can also cause the gel to overheat.
• Buffer: The buffer solution affects the electrical conductivity and pH of the gel, which can influence the migration rate.

COMMON MISTAKE: Forgetting that DNA migrates towards the positive electrode because it is negatively charged.

2.4. Applications of Gel Electrophoresis

• DNA profiling: Separating DNA fragments generated by PCR or restriction enzyme digestion to create a unique DNA fingerprint.
• Determining DNA fragment size: Estimating the size of DNA fragments by comparing their migration distance to that of known DNA markers.
• Analyzing PCR products: Confirming the presence and size of the amplified DNA fragment.
• Separating RNA and proteins: Gel electrophoresis can also be used to separate RNA and proteins, although different types of gels and buffers are used.

VCAA FOCUS: Be prepared to interpret gel electrophoresis results and draw conclusions about the size and quantity of DNA fragments.

3. DNA Profiling

3.1. Process of DNA Profiling

1. DNA extraction: Isolating DNA from a sample (e.g., blood, saliva, hair).
2. PCR amplification: Amplifying specific regions of the DNA that are highly variable between individuals (e.g., short tandem repeats or STRs).
3. Gel electrophoresis: Separating the amplified DNA fragments by size.
4. Interpretation: Analyzing the pattern of DNA bands to create a DNA profile.

3.2. Short Tandem Repeats (STRs)

• STRs: Short, repetitive DNA sequences that vary in length between individuals.
• Located at specific loci (locations) on chromosomes.
• The number of repeats at each locus is highly variable, making STRs useful for DNA profiling.
• Multiple STR loci are analyzed simultaneously to create a unique DNA profile.

3.3. Interpreting Gel Runs for DNA Profiling

• Each band on the gel represents a DNA fragment of a specific size.
• The position of the bands corresponds to the size of the DNA fragments.
• By comparing the DNA profiles of different individuals, it is possible to determine whether they match.
• A match indicates that the individuals are likely to be related or that the DNA sample came from the same source.

3.4. Applications of DNA Profiling

• Forensic science: Identifying suspects in criminal investigations.
• Paternity testing: Determining the biological father of a child.
• Identifying human remains: Matching DNA from unidentified remains to DNA from family members.
• Diagnosing inherited disorders: Identifying individuals who carry specific genetic mutations.

VCAA FOCUS: Be able to explain how DNA profiling is used in forensic science and paternity testing.

4. Example Gel Interpretation

Consider a gel electrophoresis result with the following:

• Lane 1: DNA ladder (marker) with bands at 100 bp, 200 bp, 300 bp, 400 bp, and 500 bp.
• Lane 2: Sample from a crime scene showing bands at approximately 250 bp and 400 bp.
• Lane 3: Sample from Suspect A showing bands at approximately 250 bp and 300 bp.
• Lane 4: Sample from Suspect B showing bands at approximately 250 bp and 400 bp.

Interpretation:

• The DNA ladder in Lane 1 allows us to estimate the sizes of the DNA fragments in the other lanes.
• The DNA profile of the crime scene sample (Lane 2) matches the DNA profile of Suspect B (Lane 4).
• Suspect A (Lane 3) can be excluded as a suspect because their DNA profile does not match the crime scene sample.

EXAM TIP: When interpreting gel electrophoresis results, always compare the samples to the DNA ladder to estimate the size of the DNA fragments.

Summary Table

KEY TAKEAWAY: Understanding the purpose, process, and applications of PCR and gel electrophoresis is crucial for success in VCE Biology.