AP Biology

## Learning Outcomes

- Explain why PCR is used to amplify small DNA samples.
- Describe the three main steps of a PCR cycle: denaturation, primer annealing, and extension.
- Explain how gel electrophoresis separates DNA fragments by size.
- Predict the direction DNA fragments move in an electric field and which fragments travel farthest.
- Interpret banding patterns from gels in contexts such as forensics, genetic testing, and DNA comparison.
- Distinguish PCR and gel electrophoresis from related biotechnology tools such as restriction enzymes, DNA ligase, and bacterial transformation.
- Identify common experimental controls and sources of error in PCR and gel electrophoresis investigations.

## AP Biology Syllabus

PCR and gel electrophoresis fit within Unit 6, Gene Expression and Regulation, under biotechnology. Unit 6 is commonly assessed through questions that ask you to apply DNA structure, base pairing, mutations, and gene expression concepts to experimental data. You are not usually expected to memorize every procedural detail of a lab; instead, you must be able to interpret results, identify variables and controls, make predictions, and justify conclusions using evidence.

- PCR is used to make many copies of a specific DNA region from a small starting sample.
- Primers determine which DNA sequence is amplified during PCR.
- Gel electrophoresis separates DNA fragments based mainly on size.
- DNA and RNA are negatively charged and move toward the positive electrode in a gel.
- Smaller DNA fragments move farther through the gel than larger fragments.
- Banding patterns can be used to compare DNA samples, detect alleles, or analyze restriction fragments.
- Restriction fragment length polymorphisms, or RFLPs, can produce different gel patterns among individuals.
- AP questions may ask you to choose the correct biotechnology method for a task, such as PCR for amplification or gel electrophoresis for separation.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. Why are primers required in PCR, and how do they help determine which DNA sequence is copied?
2. In gel electrophoresis, why does DNA move toward the positive electrode?
3. If two DNA fragments are placed in the same gel, one 200 base pairs long and one 1,000 base pairs long, which will travel farther?
4. A forensic scientist has only a tiny amount of DNA from a crime scene. Which technique should be used before comparing the sample on a gel?
5. How can restriction enzymes and gel electrophoresis work together to create a DNA fingerprint?

## Introduction

Biotechnology uses biological molecules, cells, and laboratory techniques to analyze or manipulate genetic material. In AP Biology, two of the most testable biotechnology methods are polymerase chain reaction, usually called PCR, and gel electrophoresis. PCR answers the question: “How can scientists make enough DNA to study?” Gel electrophoresis answers the question: “How can scientists separate and compare DNA fragments?”

These methods often appear together. For example, a scientist may use PCR to amplify a region of DNA linked to a genetic condition, then use gel electrophoresis to determine whether the amplified fragment has the expected size. In forensics, PCR can amplify small amounts of DNA from a crime scene, and gel electrophoresis can help compare DNA fragment patterns from different samples.

> **Key Term: Biotechnology**
> The use of organisms, cells, biological molecules, or DNA-based methods to study, modify, copy, or analyze biological material.

### PCR amplifies a target DNA sequence

Polymerase chain reaction is a laboratory method used to make many copies of a specific DNA sequence. This is useful because many biological samples contain very little DNA. A tiny sample from a crime scene, a small amount of ancient DNA, or a few cells from a medical test may not contain enough DNA for direct analysis. PCR increases the number of copies so the DNA can be detected, sequenced, or compared.

> **Key Term: Polymerase chain reaction (PCR)**
> A biotechnology technique that amplifies, or makes many copies of, a specific DNA region.

PCR depends on the same basic principles as DNA replication: complementary base pairing and DNA polymerase activity. However, instead of copying an entire genome, PCR copies only the DNA region between two short DNA primers.

A typical PCR reaction includes:

- Template DNA: the original DNA containing the target sequence.
- Primers: short single-stranded DNA sequences that bind to opposite ends of the target region.
- DNA polymerase: an enzyme that builds new DNA strands.
- Free nucleotides: A, T, C, and G building blocks.
- Buffer and ions: chemical conditions that allow the enzyme to work.
- Repeated temperature changes: used to separate DNA strands and control primer binding.

> **Key Term: Primer**
> A short nucleic acid sequence that binds to a complementary DNA region and provides a starting point for DNA polymerase.

PCR occurs in repeated cycles. Each cycle has three main steps.

First, denaturation occurs. The DNA sample is heated, usually to a temperature high enough to break hydrogen bonds between complementary bases. This separates the double-stranded DNA into single strands. The covalent bonds in the sugar-phosphate backbone are not broken; only the hydrogen bonds between paired bases are disrupted.

Second, primer annealing occurs. The reaction is cooled so the primers can bind to complementary sequences on the template DNA. The primers mark the boundaries of the DNA segment that will be amplified. If the primers do not match the target DNA, amplification will fail or may amplify the wrong sequence.

Third, extension occurs. DNA polymerase adds nucleotides to the primers, building new DNA strands in the 5' to 3' direction. A heat-stable DNA polymerase is commonly used because the reaction repeatedly reaches high temperatures during denaturation.

In an ideal PCR reaction, the number of target DNA copies approximately doubles each cycle. This means amplification is exponential. After one cycle, one double-stranded target region can become two. After two cycles, it can become four. After three cycles, it can become eight. A simplified model is:

$$
\text{Number of copies after } n \text{ cycles} \approx 2^n
$$

Real PCR reactions may not be perfectly efficient, but AP Biology questions often expect you to recognize that PCR can quickly produce many copies of a DNA region.

> **Test Tip:**
> If an exam question says a scientist has a very small amount of DNA and needs more copies for analysis, the answer is PCR. If the question says the scientist needs to separate DNA fragments by size, the answer is gel electrophoresis.

PCR can be used in many AP-relevant contexts. In genetic testing, primers can target a region containing a mutation. In pathogen detection, primers can target DNA from a virus or bacterium. In evolutionary studies, PCR can amplify DNA regions from different organisms so their sequences can be compared. In forensics, PCR allows scientists to analyze DNA even when only a small sample is available.

PCR does not, by itself, show the size of DNA fragments or compare banding patterns. After amplification, scientists often need another method, such as gel electrophoresis, to analyze the PCR products.

### Gel electrophoresis separates DNA fragments by size

Gel electrophoresis is used to separate DNA fragments according to size. The DNA samples are placed into small wells in a gel, and an electric field is applied. Because DNA has a negatively charged sugar-phosphate backbone, it migrates toward the positive electrode.

> **Key Term: Gel electrophoresis**
> A laboratory technique that separates DNA, RNA, or proteins as they move through a gel in response to an electric field.

For DNA analysis, the gel acts like a molecular sieve. Smaller fragments can move through the gel matrix more easily, so they travel faster and farther. Larger fragments have more difficulty passing through the gel and remain closer to the wells.

This produces a banding pattern. A band represents many DNA fragments of the same or very similar length. Since individual DNA molecules are too small to see directly, a stain or fluorescent dye is used to visualize the bands.

A DNA ladder is often loaded into one lane of the gel. The ladder contains DNA fragments of known sizes. Scientists compare sample bands with the ladder to estimate fragment length.

A simple gel interpretation might look like this:

- A band close to the wells represents a larger DNA fragment.
- A band far from the wells represents a smaller DNA fragment.
- Two samples with bands at the same position likely contain DNA fragments of the same length.
- A sample with multiple bands contains multiple fragment sizes.

> **Key Term: DNA ladder**
> A mixture of DNA fragments of known lengths used as a reference to estimate the sizes of sample fragments on a gel.

For example, imagine a gel with a DNA ladder and three unknown samples. If sample A has a band at the same position as the 500-base-pair ladder band, the fragment is about 500 base pairs long. If sample B’s band is farther from the wells than sample A’s band, sample B’s fragment is smaller than 500 base pairs. If sample C’s band remains closer to the wells, sample C’s fragment is larger.

> **Exam Warning:**
> Do not say that larger DNA fragments move farther because they are “heavier.” In gel electrophoresis, smaller DNA fragments travel farther because they pass more easily through the gel.

Gel electrophoresis can separate DNA fragments produced in several ways. The fragments may come from PCR amplification, restriction enzyme digestion, or other DNA-processing methods. AP Biology questions may combine these methods in a single scenario.

For instance, DNA samples can be cut with restriction enzymes before gel electrophoresis. Restriction enzymes cut DNA at specific base-pair sequences. If two individuals have slight differences in DNA sequence, a restriction enzyme may cut one individual’s DNA at a site that is missing in another person’s DNA. This leads to different fragment lengths, which can be detected on a gel.

> **Key Term: Restriction enzyme**
> An enzyme that cuts DNA at specific nucleotide sequences.

### Banding patterns reveal DNA differences

Gel electrophoresis is often used to compare DNA samples. The comparison depends on band positions, not on the darkness of the bands unless the question provides information about DNA quantity. In most AP Biology interpretation questions, the key point is whether bands line up across lanes.

A major use of gel electrophoresis is DNA fingerprinting. In DNA fingerprinting, DNA fragments from different samples are compared. If a crime scene sample and a suspect sample have the same banding pattern at the tested regions, that provides evidence that the samples may come from the same individual. If the banding patterns differ, the samples likely come from different individuals.

> **Key Term: DNA fingerprinting**
> The use of DNA fragment patterns to compare DNA samples, often in forensics, paternity testing, or identification.

One type of DNA variation used in DNA fingerprinting is a restriction fragment length polymorphism, or RFLP. A polymorphism is a difference in DNA sequence among individuals. If a polymorphism changes the position of a restriction enzyme cut site, the lengths of the DNA fragments will differ. When separated by gel electrophoresis, the fragments form different banding patterns.

> **Key Term: Restriction fragment length polymorphism (RFLP)**
> A DNA sequence variation that produces different restriction fragment lengths among individuals or alleles.

Suppose a restriction enzyme cuts a DNA region at two sites in one person, producing three fragments. In another person, a mutation removes one of those cut sites, producing fewer but larger fragments. After gel electrophoresis, the first person may show more bands, including smaller fragments farther down the gel. The second person may show fewer bands, with a larger fragment closer to the wells.

PCR and gel electrophoresis can also detect differences in repeated DNA sequences. Huntington’s disease is associated with expansion of a CAG repeat in the HTT gene. Individuals with fewer than 35 CAG repeats do not develop Huntington’s disease, while individuals with 40 or more repeats will develop it. A PCR-based test could amplify the repeat-containing region, and gel electrophoresis could help compare fragment sizes. More CAG repeats produce a longer DNA fragment, which would travel a shorter distance through the gel.

> **Test Tip:**
> When interpreting a gel, first identify the wells. DNA starts there. Then find the direction of movement. The bands farthest from the wells are the smallest fragments.

Gel data can also be used to determine genotype. For example, a recessive disease allele may create a shorter PCR product than the normal allele. A homozygous normal individual might show only the larger band. A homozygous affected individual might show only the smaller band. A heterozygote might show both bands. However, always use the information in the question; different tests can be designed in different ways.

### Choosing the correct biotechnology tool on exam questions

Many AP Biology questions test whether you can match a technique to its purpose. PCR and gel electrophoresis are often presented alongside other biotechnology tools, so you need to keep their roles separate.

PCR amplifies DNA. If a sample is too small to analyze, PCR is the method used to make more copies. PCR does not cut DNA, insert DNA into cells, or directly separate fragments.

Gel electrophoresis separates DNA fragments by size. If the question asks how to compare fragment lengths, read a banding pattern, or separate DNA pieces after restriction enzyme digestion, gel electrophoresis is the method.

Restriction enzymes cut DNA at specific sequences. If the question asks which tool cuts DNA at specific base-pair sequences, the answer is restriction enzymes, not PCR or gel electrophoresis.

DNA ligase joins DNA fragments. If the question asks how to attach a gene to a plasmid, DNA ligase is often involved because it forms bonds in the DNA backbone.

Bacterial transformation introduces DNA, often a plasmid, into bacterial cells. If the question asks how to insert a plasmid into a bacterium, the answer is transformation.

> **Key Term: Bacterial transformation**
> The uptake of external DNA, such as a plasmid, by a bacterial cell.

These distinctions are common in multiple-choice questions. For example:

> **Question:**
> A scientist recovers a small amount of DNA from a crime scene and wants to increase the number of copies of a target region before analysis. Which technique should be used?

The correct answer is PCR because the purpose is amplification.

> **Question:**
> A scientist cuts DNA from two individuals with the same restriction enzyme and wants to compare the sizes of the resulting fragments. Which technique should be used?

The correct answer is gel electrophoresis because the purpose is separation by size.

> **Question:**
> A scientist wants to cut DNA at a specific nucleotide sequence. Which tool is needed?

The correct answer is a restriction enzyme.

Experimental design may also appear in free-response questions. A PCR investigation should include controls. A positive control contains DNA known to amplify with the primers; it confirms that the PCR reagents and conditions work. A negative control contains no template DNA; it checks for contamination. If the negative control produces a band, contamination is likely.

Gel electrophoresis also requires careful interpretation. A DNA ladder allows size estimation. Replicate samples increase confidence. Loading equal volumes helps make band intensity more meaningful, although AP questions usually focus on band position unless the prompt discusses DNA quantity.

> **Exam Warning:**
> A band at the same height in two gel lanes means the fragments are similar in length, not necessarily that the entire DNA sequences are identical. Different sequences can sometimes produce fragments of the same size.

Another common exam skill is predicting the effect of changes to a procedure. If primers are not complementary to the target DNA, PCR amplification will be reduced or absent. If the annealing temperature is too high, primers may not bind well. If the annealing temperature is too low, primers may bind nonspecifically, producing extra bands. If a gel is run too briefly, fragments may not separate enough. If it is run too long, small fragments may run off the gel.

You may also be asked to connect biotechnology to gene expression and mutations. PCR and gel electrophoresis do not change gene expression by themselves; they are analytical tools. However, they can be used to study genes, alleles, mutations, and inherited conditions. For example, if a mutation changes the length of a DNA fragment or creates/removes a restriction site, gel electrophoresis can detect the resulting difference.

## Key Point Checklist

This article has covered the following key knowledge points:

- PCR is used to amplify a specific DNA sequence from a small starting sample.
- PCR requires template DNA, primers, DNA polymerase, nucleotides, and repeated temperature cycles.
- PCR cycles include denaturation, primer annealing, and extension.
- Primers determine which DNA region is amplified.
- Gel electrophoresis separates DNA fragments mainly by size.
- DNA is negatively charged and moves toward the positive electrode in an electric field.
- Smaller DNA fragments travel farther through a gel than larger fragments.
- A DNA ladder provides known fragment sizes for comparison.
- Restriction enzymes cut DNA at specific nucleotide sequences.
- RFLPs can create different restriction fragment lengths and different gel banding patterns.
- DNA fingerprinting compares banding patterns between DNA samples.
- On AP Biology questions, use PCR for amplification and gel electrophoresis for separation by size.

## Key Terms and Concepts

- Biotechnology
- Polymerase chain reaction (PCR)
- Primer
- Gel electrophoresis
- DNA ladder
- Restriction enzyme
- DNA fingerprinting
- Restriction fragment length polymorphism (RFLP)
- Bacterial transformation


Biotechnology - PCR and gel electrophoresis