AP Biology

## Learning Outcomes

- Explain how restriction enzymes cut DNA at specific recognition sequences.
- Distinguish sticky ends from blunt ends and predict which DNA fragments can join.
- Describe how plasmids act as vectors in gene cloning.
- Outline the main steps used to make recombinant DNA and transform bacteria.
- Interpret cloning diagrams involving restriction sites, selectable markers, and gel patterns.
- Identify common AP Biology mistakes involving DNA ligase, antibiotic selection, and controls.

## AP Biology Syllabus

In AP Biology Unit 6, biotechnology is assessed as part of gene expression and regulation, which connects to the broader idea that living systems store, retrieve, transmit, and respond to genetic information. Unit 6 is commonly weighted at about 12–16% of the multiple-choice section, and biotechnology questions often test your ability to apply molecular tools to unfamiliar data rather than memorize lab procedures.

- Restriction enzymes cut DNA at specific nucleotide sequences.
- DNA fragments with compatible ends can be joined to form recombinant DNA.
- Plasmids can be used as vectors to carry foreign DNA into bacterial cells.
- DNA ligase forms covalent bonds in the sugar-phosphate backbone.
- Bacterial transformation introduces plasmids into bacteria.
- Selectable markers, such as antibiotic resistance genes, help identify transformed cells.
- Gel electrophoresis can be used to analyze restriction fragments and cloning results.
- Questions may ask you to predict effects of mutations in restriction sites or plasmid genes.

## 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 must a plasmid and a gene of interest often be cut with the same restriction enzyme before cloning?
2. What is the difference between a sticky end and a blunt end?
3. What role does DNA ligase play in producing recombinant DNA?
4. Why are antibiotic resistance genes useful in bacterial transformation experiments?
5. If a mutation removes a restriction enzyme recognition site, how would the sizes of DNA fragments change after digestion?

## Introduction

Biotechnology uses biological molecules and cells to analyze or manipulate genetic information. In AP Biology, the most tested ideas are not long technical protocols; they are the logic behind the tools. You should be able to explain why an enzyme cuts DNA, why a plasmid can carry a gene, why transformed bacteria can be selected, and how DNA fragment patterns provide evidence.

Restriction enzymes and cloning are closely linked. Restriction enzymes can cut DNA into predictable fragments. Those fragments can then be inserted into plasmids, forming recombinant DNA. When bacteria take up recombinant plasmids, they can copy the plasmid as they divide, producing many copies of the inserted gene. In some cases, the gene can also be expressed to make a protein, such as human insulin.

> **Key Term: biotechnology**
> The use of organisms, cells, or biological molecules to analyze, copy, modify, or manipulate genetic material.

### How restriction enzymes cut DNA at specific sequences

Restriction enzymes are proteins that cut DNA at particular nucleotide sequences called recognition sequences. Many recognition sequences are palindromic, meaning the sequence reads the same in opposite directions on complementary strands when each strand is read 5′ to 3′.

For example, the restriction enzyme EcoRI recognizes this sequence:

```text
5′ GAATTC 3′
3′ CTTAAG 5′
```

EcoRI cuts between G and A on both strands, producing overhanging single-stranded ends:

```text
5′ G     AATTC 3′
3′ CTTAA     G 5′
```

These overhangs are called sticky ends because they can form hydrogen bonds with complementary sticky ends produced by the same enzyme. If two DNA molecules are cut with EcoRI, their sticky ends can base-pair with each other. This makes it easier for the fragments to be joined permanently by DNA ligase.

> **Key Term: restriction enzyme**
> An enzyme that cuts DNA at a specific recognition sequence.
>
> **Key Term: recognition sequence**
> A specific DNA sequence recognized and cut by a restriction enzyme.
>
> **Key Term: sticky end**
> A single-stranded DNA overhang produced when a restriction enzyme cuts DNA in a staggered pattern.

Not all restriction enzymes make sticky ends. Some cut straight across both DNA strands and create blunt ends. Blunt ends do not have overhangs, so they can be joined to other blunt-ended DNA fragments, but there is no complementary overhang to help align the pieces.

> **Key Term: blunt end**
> A DNA end with no single-stranded overhang, usually produced by a restriction enzyme that cuts straight across both strands.

Restriction enzymes occur naturally in bacteria, where they help defend against foreign DNA, such as viral DNA. Bacteria protect their own DNA by chemically modifying recognition sites, often by methylation, so their own chromosomes are not cut. In biotechnology, scientists use purified restriction enzymes as precise DNA-cutting tools.

AP Biology questions often connect restriction enzymes to fragment analysis. If a restriction enzyme cuts a DNA molecule at two sites, it produces multiple fragments. If a mutation removes one recognition site, the enzyme cuts fewer times, and two smaller fragments may become one larger fragment. This is the basis of restriction fragment length polymorphisms, or RFLPs, where DNA sequence differences cause different restriction fragment patterns.

> **Key Term: restriction fragment length polymorphism**
> A difference in DNA fragment lengths among individuals caused by sequence variation that affects restriction enzyme cutting sites.
>
> **Test Tip:**
> When predicting restriction fragment sizes, draw the DNA as a line or circle and mark every restriction site. Count the distances between cuts rather than trying to reason from memory.

### Recombinant DNA and the role of DNA ligase

After DNA is cut, fragments can be joined to make recombinant DNA. Recombinant DNA contains DNA from two or more sources. For example, a bacterial plasmid can be combined with a human gene. The resulting recombinant plasmid contains both bacterial plasmid DNA and inserted human DNA.

> **Key Term: recombinant DNA**
> A DNA molecule made by joining genetic material from two or more sources.

The formation of recombinant DNA usually depends on two major molecular events:

1. Complementary base pairing between compatible DNA ends.
2. Formation of covalent bonds in the DNA backbone by DNA ligase.

Sticky ends can temporarily pair through hydrogen bonds, but hydrogen bonds alone do not make a stable DNA molecule. DNA ligase seals nicks in the sugar-phosphate backbone by forming phosphodiester bonds. This produces a continuous DNA molecule that can be copied by cells.

> **Key Term: DNA ligase**
> An enzyme that joins DNA fragments by forming covalent bonds in the sugar-phosphate backbone.

A common exam example involves a gene of interest and a plasmid both cut with the same restriction enzyme. Cutting both DNA molecules with the same enzyme produces compatible ends. The gene can base-pair with the opened plasmid, and DNA ligase can seal the fragments together.

For example:

- A plasmid is cut once with EcoRI, opening the circular DNA.
- A gene of interest is cut out of another DNA molecule using EcoRI.
- The gene and plasmid have matching sticky ends.
- DNA ligase joins the gene into the plasmid.
- The recombinant plasmid is introduced into bacteria.

If the plasmid and insert are cut with different enzymes that produce incompatible sticky ends, they will not base-pair correctly. If two different enzymes produce compatible overhangs, joining may still be possible, but AP questions usually provide the sequences needed to decide.

> **Exam Warning:**
> Do not say DNA ligase “cuts” DNA. Restriction enzymes cut DNA; DNA ligase joins DNA fragments.

Sticky-end cloning also has a limitation: if both ends of the insert are the same, the gene may insert in either orientation. Orientation matters if the inserted gene must be expressed from a promoter in the plasmid. If a gene inserts backward relative to the promoter, RNA polymerase may not transcribe the correct coding sequence. To control orientation, scientists may cut the plasmid and insert with two different restriction enzymes, creating two different sticky ends. This forces the insert to enter in only one direction because each end can pair only with its matching plasmid end.

This connects directly to gene expression. A cloned gene is not automatically expressed. For expression, the gene must be placed near appropriate regulatory sequences, such as a promoter, that allow RNA polymerase to bind and begin transcription. In bacteria, a plasmid designed for protein production must include bacterial regulatory sequences, because bacterial cells need to recognize the promoter.

### Plasmids as cloning vectors

A plasmid is a small, usually circular DNA molecule separate from the bacterial chromosome. Plasmids can replicate inside bacterial cells, making them useful vectors. A vector is a DNA molecule used to carry foreign DNA into a cell.

> **Key Term: plasmid**
> A small, usually circular DNA molecule separate from the main chromosome, often found in bacteria.
>
> **Key Term: vector**
> A DNA molecule used to carry foreign genetic material into a cell.

A useful cloning plasmid usually has several features:

- An origin of replication, so the plasmid can be copied inside the host cell.
- One or more restriction sites, where restriction enzymes can cut the plasmid.
- A selectable marker, often an antibiotic resistance gene.
- Sometimes a reporter gene or expression control sequence.

The origin of replication allows bacterial enzymes to copy the plasmid. Without an origin, the plasmid would not be maintained as bacteria divide. The restriction site, or group of restriction sites, allows the plasmid to be opened so a DNA fragment can be inserted.

Selectable markers help scientists identify cells that successfully took up a plasmid. A common selectable marker is an antibiotic resistance gene. Suppose a plasmid carries a gene for ampicillin resistance. After transformation, bacteria are spread on agar containing ampicillin. Cells that did not take up the plasmid are killed or fail to grow. Cells that contain the plasmid survive and form colonies.

> **Key Term: selectable marker**
> A gene that allows cells with a desired DNA molecule to survive or be identified under specific conditions.

However, antibiotic selection alone may not prove that the plasmid contains the gene of interest. It may only show that bacteria took up a plasmid. Some plasmids may close without an insert, especially if the cut ends rejoin. To distinguish recombinant plasmids from nonrecombinant plasmids, scientists may use screening methods.

One common screening strategy disrupts a reporter gene. For example, if the insertion site lies inside a reporter gene, insertion of foreign DNA prevents the reporter from functioning. Colonies with plasmids lacking the insert may show one color, while colonies with recombinant plasmids show a different color or no color. AP Biology questions may describe this pattern and ask which colonies contain recombinant DNA.

> **Exam Warning:**
> Antibiotic resistance usually selects for bacteria that received a plasmid, not necessarily bacteria with the correct insert. Look for a second marker or screening step if the question asks which cells contain recombinant plasmids.

Plasmids also connect to bacterial transformation, a major AP Biology biotechnology lab theme. In transformation, bacteria take up foreign DNA from their environment. In classroom and research settings, bacteria can be treated to make their membranes more likely to take up plasmids. After transformation, selection identifies cells that contain plasmids.

> **Key Term: transformation**
> The uptake of foreign DNA by a cell, commonly used to introduce plasmids into bacteria.

### Gene cloning workflow and AP-style data interpretation

Gene cloning means making many copies of a gene or DNA fragment. In AP Biology, the word “cloning” in this context usually refers to cloning DNA, not cloning an entire organism. Bacteria are useful because they reproduce quickly. If a bacterial cell contains a recombinant plasmid, each cell division copies the plasmid and passes it to daughter cells. A colony derived from one transformed bacterium can contain many copies of the inserted gene.

> **Key Term: gene cloning**
> The production of many copies of a specific gene or DNA fragment, often using plasmids and bacterial cells.

A typical gene cloning process can be summarized as follows:

1. Identify and isolate the gene of interest.
2. Cut the gene and plasmid with restriction enzyme(s).
3. Mix the DNA fragments so compatible ends can pair.
4. Add DNA ligase to form recombinant plasmids.
5. Transform bacteria with the plasmids.
6. Grow bacteria on selective medium.
7. Screen colonies to find those with the correct recombinant plasmid.
8. Culture selected bacteria to copy the gene or express the protein.

A classic example is production of human insulin. The human insulin gene can be inserted into a plasmid designed for bacterial expression. Bacteria transformed with the recombinant plasmid can be grown in large numbers, and the insulin protein can be harvested and purified. For AP Biology, you do not need to memorize industrial details, but you should understand that gene cloning can connect DNA sequence information to protein production through transcription and translation.

Restriction enzymes are also used to verify cloning results. Suppose a plasmid is 4,000 base pairs and an insert is 1,000 base pairs. If the recombinant plasmid is cut with enzymes that flank the insert, gel electrophoresis should show two bands: one at 4,000 base pairs and one at 1,000 base pairs. If the plasmid lacks the insert, only the 4,000-base-pair band may appear, depending on the restriction sites.

Gel electrophoresis separates DNA fragments by size. DNA is negatively charged, so it moves toward the positive electrode. Smaller fragments move farther through the gel than larger fragments. After restriction digestion, the band pattern can show whether DNA contains a particular restriction site or insert.

> **Key Term: gel electrophoresis**
> A technique that separates DNA fragments by size as they move through a gel in an electric field.
>
> **Test Tip:**
> On a gel, smaller DNA fragments travel farther from the wells. If a band is lower on the gel, it usually represents a shorter fragment.

Many AP questions ask you to interpret an experimental design. Focus on variables and controls:

- The independent variable might be whether plasmid DNA was added.
- The dependent variable might be bacterial growth, colony color, or band pattern.
- A negative control might contain bacteria without plasmid DNA.
- A positive control might contain a plasmid known to transform successfully.
- A selective plate contains an antibiotic or other condition that only certain cells can tolerate.

For example, imagine bacteria are plated on nutrient agar and nutrient agar plus ampicillin:

- Bacteria without plasmid grow on nutrient agar but not on ampicillin.
- Bacteria transformed with an ampicillin-resistance plasmid grow on both.
- If no colonies appear on the nonselective nutrient plate, the bacteria may have died or the procedure failed.
- If many colonies grow on the ampicillin plate without plasmid, the antibiotic may be ineffective or the bacteria may already be resistant.

Free-response questions may ask you to justify a prediction. A strong answer links the molecular event to the observed result. For instance: “A mutation in the restriction site prevents the enzyme from cutting at that location, so two fragments that would normally be separated remain joined. Therefore, the gel will show one larger band instead of two smaller bands.”

Restriction enzymes can also be used in DNA fingerprinting. Different individuals often have small DNA sequence differences. If these differences change restriction sites, digestion with the same enzyme produces fragments of different lengths. These RFLP patterns can be compared by gel electrophoresis. In an AP question, a child’s bands should match bands inherited from biological parents, or DNA from a crime scene may be compared with suspect DNA. The reasoning is based on fragment sizes, not on the idea that one person’s entire genome is being sequenced.

A final point: restriction enzymes and cloning are tools, but they rely on core Unit 6 concepts. DNA has complementary base pairing. Enzymes have specific shapes and bind specific substrates. Genes must be transcribed and translated to affect protein production. Mutations can alter DNA sequences, including restriction sites and coding regions. Biotechnology questions often combine these ideas in one diagram or data set.

## Key Point Checklist

This article has covered the following key knowledge points:

- Restriction enzymes cut DNA at specific recognition sequences.
- Many restriction enzyme recognition sequences are palindromic.
- Staggered cuts create sticky ends; straight cuts create blunt ends.
- DNA fragments with compatible sticky ends can base-pair with each other.
- DNA ligase joins DNA fragments by forming covalent bonds in the sugar-phosphate backbone.
- Recombinant DNA contains genetic material from two or more sources.
- Plasmids are small circular DNA molecules that can act as cloning vectors.
- A plasmid used for cloning usually needs an origin of replication, restriction site, and selectable marker.
- Transformation introduces plasmid DNA into bacterial cells.
- Antibiotic resistance can select for transformed bacteria but may not prove that an insert is present.
- Gel electrophoresis separates smaller DNA fragments farther than larger fragments.
- Mutations in restriction sites can change restriction fragment lengths and gel band patterns.

## Key Terms and Concepts

- biotechnology
- restriction enzyme
- recognition sequence
- sticky end
- blunt end
- restriction fragment length polymorphism
- recombinant DNA
- DNA ligase
- plasmid
- vector
- selectable marker
- transformation
- gene cloning
- gel electrophoresis


Biotechnology - Restriction enzymes and cloning