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D3.6 · Describe plasmids, enzymes, and other biotechnology tools

Learn to describe plasmids, enzymes, and other biotechnology tools through clear examples and targeted practice.

Ontario Grade 12 Biology

Molecular Genetics

How scientists use biological molecules and laboratory tools to study DNA

In SBI3U, you learned that DNA carries inherited information and that genes are sections of DNA. Biotechnology uses living systems, biological molecules, or laboratory tools to investigate or work with that information. For example, a scientist may need to copy a particular DNA section, compare DNA fragments, or carry a selected DNA section in a cell. Different tools serve different purposes. Plasmids can carry DNA, enzymes can cut or join DNA, and other tools can copy or examine it. This lesson describes these tools at a Grade 12 level. A tool’s result is evidence to interpret, not a complete explanation of what happens in every organism.

What you will learn

Describe plasmids and explain why they are useful in biotechnology.
Explain how restriction enzymes, DNA ligase, and other enzymes can be used with DNA.
Describe how tools such as PCR, gel electrophoresis, and DNA probes help scientists study genetic material.
Choose appropriate tools for a simple biotechnology question and explain what the results can and cannot show.

1. From genes to plasmids

A quick review: DNA is a molecule that stores genetic information. A gene is a DNA section with information used by a cell. A chromosome is a larger DNA structure that contains many genes. These ideas help explain why biotechnology tools can work with small DNA sections as well as whole chromosomes.

A plasmid is a small, usually circular DNA molecule found in many bacteria. It is separate from the bacterium’s main chromosome. A plasmid can carry genes, and it can be copied inside a bacterial cell. Some plasmids occur naturally. In biotechnology, scientists can use plasmids as vectors. A vector is a carrier used to move a DNA section into a cell.

A plasmid is useful because it is a compact DNA carrier that can be handled as a separate molecule from the main chromosome. In a simplified model, a selected DNA section is placed in a plasmid, and the plasmid can then be introduced into a suitable cell. The cell may copy the plasmid as it grows. This model describes the general role of a plasmid; it does not mean that every introduced plasmid will work in every cell.

The DNA section carried by a plasmid may be called inserted DNA or foreign DNA, depending on its source. The plasmid carrying that section is often called recombinant DNA. Recombinant means that DNA from different sources has been combined. The phrase describes the combined DNA molecule, not a guarantee that a cell will use the inserted information.

A plasmid is small DNA separate from a bacterial chromosome.
A vector carries a DNA section into a cell.
Recombinant DNA combines DNA sections from different sources.

2. Enzymes as DNA tools

An enzyme is a biological molecule, usually a protein, that helps a chemical reaction happen. Enzymes are selective: their shape and chemical properties help determine which molecules they act on. In biotechnology, some enzymes are used to cut, join, or copy DNA. Their roles are different, so they are not interchangeable.

Restriction enzymes cut DNA at particular short sequences. They act like molecular scissors, but the comparison has a limit: each restriction enzyme recognizes particular DNA sequences rather than cutting anywhere. A cut may leave matching exposed ends on two DNA pieces. These are often called sticky ends. Other cuts leave blunt ends, which do not have the same short exposed pairing region.

DNA ligase is an enzyme that joins DNA pieces by sealing the DNA backbone. If a DNA section and a plasmid have compatible ends, ligase can help join them. This is one way to make a recombinant plasmid. The enzymes make the joining possible, but they do not by themselves ensure that the plasmid enters a cell or that the cell uses the inserted DNA.

DNA polymerase is an enzyme that builds a new DNA strand using an existing strand as a guide. In biotechnology, DNA polymerase is used in PCR, a method for making many copies of a selected DNA region. PCR stands for polymerase chain reaction. At this course level, remember its purpose: it amplifies, or increases the amount of, a chosen DNA region. It does not make a protein. Making a DNA copy is different from transcription, which makes RNA from DNA, and translation, which uses RNA information to build a protein.

Restriction enzymes cut DNA at particular sequences.
DNA ligase joins DNA pieces by sealing their backbones.
DNA polymerase builds DNA copies; PCR uses it to copy a selected region.

3. Other tools: copying, separating, and recognizing DNA

PCR is useful when a sample contains too little of a target DNA region for a planned analysis. A target is the DNA region of interest. PCR makes many copies of that region, so it can be examined more readily. The result depends on selecting a suitable target and having DNA that can be copied. A PCR result alone does not show the complete function of a gene or what a trait will be in every context.

Gel electrophoresis is a method for separating DNA fragments by size. DNA samples are placed in a gel, and an electric current moves the DNA through it. Smaller fragments generally move farther through the gel than larger fragments. The resulting bands show groups of fragments of similar size. A band is not a picture of a gene’s function, and fragments of the same size are not necessarily the same DNA sequence.

A DNA probe is a short, labelled DNA strand designed to bind to a matching DNA sequence. A label makes the binding detectable. A probe can help indicate whether a particular sequence is present in a sample, provided the probe and test conditions are suitable. A probe is not the same as a primer: a primer is a short DNA strand that provides a starting point for DNA polymerase during DNA copying.

These tools answer different questions. PCR increases the amount of a target region. Gel electrophoresis separates fragments by size. A probe detects a matching sequence. Restriction enzymes cut at recognition sequences, while ligase joins DNA pieces. Plasmids can carry DNA. Choosing a tool starts with asking what information is needed.

PCR copies a selected DNA region.
Gel electrophoresis separates DNA fragments mainly by size.
A probe binds to a matching DNA sequence so it can be detected.
A tool’s output must be interpreted within its limits.

4. Choosing tools and interpreting evidence

Imagine that a research question asks whether a sample contains a particular DNA sequence. A probe designed for that sequence could provide evidence of a match. If the question instead asks how DNA fragments differ in size, gel electrophoresis is more directly useful. If there is too little of a selected DNA region to examine, PCR can increase its amount before further analysis.

A simplified sequence of actions can be represented as

\text{DNA section} \xrightarrow{\text{restriction enzyme}} \text{DNA pieces} \xrightarrow{\text{DNA ligase}} \text{joined DNA}

. This is a model of the roles of two enzymes. It does not show every condition needed for a successful laboratory procedure. It also does not show whether a resulting plasmid will be taken up or used by a cell.

Scientific evidence needs careful interpretation. A band at a particular position supports the presence of DNA fragments of a certain size, but size alone does not identify their exact sequence. A detected probe signal supports the presence of a matching sequence under the conditions used, but it does not establish what that sequence does in an organism. The method, sample quality, and controls affect how confidently a result can be interpreted.

Match the tool to the question being asked.
A measured size, a matching sequence, and a DNA copy are different kinds of information.
Evidence supports a conclusion only within the method’s limits.

Biotechnology tools and their roles

Tool	Main role	What it does not show by itself
Plasmid	Carries a DNA section as a vector	That a cell will use the carried DNA
Restriction enzyme	Cuts DNA at particular sequences	That the cut DNA will be joined or work in a cell
DNA ligase	Joins DNA pieces by sealing their backbones	That a joined plasmid will enter or function in a cell
DNA polymerase and PCR	Copy a selected DNA region	The region’s function or effect on a trait
Gel electrophoresis	Separates DNA fragments mainly by size	The exact sequence or function of a fragment
DNA probe	Binds to a matching DNA sequence for detection	The biological effect of the sequence

Worked example

Selecting a carrier

A researcher wants a small DNA molecule that can carry a selected DNA section into a bacterial cell. Which tool is most appropriate, and what other tools may help prepare the joined DNA?

Identify the carrier
The requested molecule must carry DNA into a cell. A plasmid can serve as a vector, so it fits the carrier role.
Identify the DNA tools
A restriction enzyme can cut DNA at a particular sequence. DNA ligase can join compatible DNA pieces. These tools can help make a plasmid carrying the selected DNA section.
State the limit
The plasmid and enzymes can make a suitable DNA construct, but this alone does not prove that the plasmid will enter a cell or that the cell will use the inserted section.

Answer: Use a plasmid as the carrier. Restriction enzymes can cut DNA, and DNA ligase can join the selected section to the plasmid.

Check: The answer assigns distinct roles: plasmid carries, restriction enzyme cuts, and ligase joins.

Worked example

Choosing a tool for scarce DNA

A sample contains a very small amount of a DNA region that a scientist wants to examine. Which biotechnology tool can increase the amount of that region, and what enzyme does it use?

Match the need to a tool
The problem is that there is too little of a selected DNA region. PCR is used to make many copies of a chosen DNA region.
Name the enzyme
PCR uses DNA polymerase to build new DNA strands using existing DNA strands as guides.
Avoid overclaiming
PCR increases the amount of the target region. It does not, by itself, explain what the region does or show that it produces a particular trait.

Answer: Use PCR, which uses DNA polymerase to copy the selected region.

Check: PCR copies DNA. It does not perform transcription or translation.

Worked example

Interpreting a gel result

Two DNA samples produce bands at the same position after gel electrophoresis. What can the result support, and what does it not establish?

Read the observation
Bands at the same position indicate that the fragments have similar sizes in this separation method.
Separate size from sequence
Gel electrophoresis separates fragments mainly by size. Fragments with the same size can still have different DNA sequences.
State the conclusion carefully
The result supports a comparison of fragment sizes, but it does not establish that the fragments are identical or that they have the same function.

Answer: The bands support that the samples contain fragments of similar size. The result alone does not prove that the fragments have the same sequence or function.

Check: The interpretation is limited to what the tool measures: fragment separation by size.

Common mistakes and how to avoid them

Treating a plasmid as part of a bacterium’s main chromosome.

Correction: A plasmid is a separate, usually circular DNA molecule. It can carry genes and can serve as a vector.

Saying restriction enzymes join DNA or ligase cuts it.

Correction: Restriction enzymes cut DNA at particular sequences. DNA ligase joins DNA pieces.

Claiming that a gel band identifies a gene’s function.

Correction: Gel electrophoresis separates DNA fragments mainly by size. A band’s position does not reveal the fragment’s function.

Confusing PCR with transcription or translation.

Correction: PCR copies DNA. Transcription makes RNA from DNA, and translation uses RNA information to build a protein.

Lesson summary

Plasmids are small DNA molecules that can act as vectors.
Restriction enzymes cut DNA at particular sequences, and DNA ligase joins DNA pieces.
DNA polymerase helps copy DNA; PCR uses it to make many copies of a selected region.
Gel electrophoresis separates DNA fragments mainly by size, while probes detect matching DNA sequences.
Interpret each result according to what the tool measures. Do not claim more than the evidence supports.

Check your understanding

Question 1

Which tool is used to join DNA pieces by sealing their backbones?

DNA ligase
A DNA probe
Gel electrophoresis
A plasmid

Show answer and explanation

DNA ligase

DNA ligase joins DNA pieces. A probe detects a matching sequence, gel electrophoresis separates fragments, and a plasmid can carry DNA.

Question 2

A scientist wants to separate DNA fragments mainly by size. Which tool best matches that purpose?

DNA polymerase
Gel electrophoresis
DNA ligase
A plasmid vector

Show answer and explanation

Gel electrophoresis

Gel electrophoresis separates DNA fragments mainly by size. The other choices copy, join, or carry DNA.

Question 3

What conclusion is supported when two DNA fragments form bands at the same position on a gel?

They must have identical sequences.
They must have the same function.
They have similar sizes in this separation.
They must produce the same protein.

Show answer and explanation

They have similar sizes in this separation.

A gel separates fragments mainly by size. Matching band positions do not establish identical sequences or functions.

Key terms

DNA: The molecule that stores genetic information.
Gene: A section of DNA with information used by a cell.
Plasmid: A small, usually circular DNA molecule separate from a bacterial chromosome.
Vector: A carrier used to move a DNA section into a cell.
Enzyme: A biological molecule, usually a protein, that helps a chemical reaction happen.
Restriction enzyme: An enzyme that cuts DNA at particular sequences.
DNA ligase: An enzyme that joins DNA pieces by sealing their backbones.
DNA polymerase: An enzyme that builds a new DNA strand using an existing strand as a guide.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Biology (SBI4U), expectation D3.6. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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