Skip to main content
SingaporeBiotechnologySyllabus dot point

Once you have a gene, how do you actually carry it into a cell so the cell will use it?

Describe plasmids and other vectors and explain how they are used to carry genes into host cells

A focused answer to the O-Level outcome on vectors. What plasmids are, why they make good vectors, other vectors such as viruses, and how a gene is carried into a host cell.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

This outcome asks you to describe plasmids and other vectors and to explain how they carry a gene into a host cell. The key idea is that a useful gene cannot just be dropped into a cell; it needs a carrier, a vector, that the cell will take up and copy.

The answer

What a vector is

A vector is something used to carry a gene into a host cell so that the cell takes it up and uses it. The most common vector is the plasmid.

Plasmids

A plasmid is a small, circular piece of DNA found in bacteria, separate from the main bacterial chromosome. Plasmids are ideal vectors because:

  • They are small and easy to handle in the laboratory.
  • They can be cut open with a restriction enzyme so a gene can be inserted.
  • They are taken up by bacteria (a process called transformation).
  • Once inside, they are copied as the bacteria divide, so the inserted gene is multiplied and expressed.
  • They often carry marker genes (such as antibiotic-resistance genes) that let scientists identify which bacteria took up the plasmid.

Other vectors

Plasmids are not the only vectors:

  • Viruses can be used as vectors, because they naturally insert their genetic material into host cells. Modified, harmless viruses are used to carry genes, for example in some gene therapy.
  • Other specialised carriers exist for moving genes into plant and animal cells.

Building a recombinant plasmid

To insert a gene, the plasmid and the gene are both cut with the same restriction enzyme so they have matching sticky ends. The sticky ends pair up, and DNA ligase seals them, forming a recombinant plasmid that carries the new gene.

Examples in context

Example 1. Insulin-producing bacteria. The human insulin gene is inserted into a plasmid, which is taken up by bacteria. As the bacteria multiply, they copy the plasmid and express the gene, producing insulin. The plasmid is the vehicle that makes this possible.

Example 2. Viral vectors in gene therapy. To deliver a healthy gene into human cells, a harmless modified virus can be used as a vector, because viruses naturally insert their genes into the cells they infect. This shows a vector chosen to suit a human host rather than a bacterium.

Try this

Q1. Define the term vector as used in genetic engineering. [1 mark]

  • Cue. Something used to carry a gene into a host cell so the cell takes it up and uses it.

Q2. State two reasons plasmids make good vectors. [2 marks]

  • Cue. They can be cut open to insert a gene, and they are taken up and copied by bacteria as the bacteria divide (carrying marker genes for selection is also acceptable).

Q3. Name one type of vector other than a plasmid and state why it can carry genes. [2 marks]

  • Cue. A (modified, harmless) virus, because viruses naturally insert their genetic material into the host cells they infect.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksExplain what a vector is and why plasmids are commonly used as vectors in genetic engineering.
Show worked answer →

Examiners want a definition of a vector and several reasons plasmids suit the role.

A vector is something used to carry a gene into a host cell. A plasmid is a small circular piece of DNA found in bacteria, separate from the main chromosome, and it is a common vector.

Plasmids make good vectors because they are small and easy to handle, they can be cut open with a restriction enzyme and have a gene inserted, and they are taken up by bacteria, which then copy the plasmid and express the inserted gene as the bacteria divide. Plasmids often also carry marker genes (such as for antibiotic resistance) that let scientists identify which bacteria took them up.

What markers reward: a vector as a carrier of a gene into a host, a plasmid as a small circular bacterial DNA, and reasons such as being easy to cut and insert into, being taken up and copied by bacteria, and carrying marker genes for selection.

Original4 marksDescribe how a gene is inserted into a plasmid to form recombinant DNA.
Show worked answer →

The answer should cover cutting with the same enzyme and joining with ligase.

The plasmid is cut open using a restriction enzyme, which cuts at a specific recognition sequence and often leaves short single-stranded sticky ends. The gene to be inserted is cut from its source using the same restriction enzyme, so it has complementary sticky ends.

The cut plasmid and the gene are mixed. The complementary sticky ends pair up by base pairing, and the enzyme DNA ligase joins them together by sealing the backbone, forming a recombinant plasmid that now contains the inserted gene.

What markers reward: cutting the plasmid and the gene with the same restriction enzyme to give matching sticky ends, base pairing of the sticky ends, and DNA ligase sealing them to form recombinant DNA.

Related dot points