19.1.519 · Genetic Technology

Genetic Engineering- Vectors — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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19.1.5 Genetic Engineering: Vectors — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe plasmids and other vectors and how recombinant plasmids are made and transferred into host cells.Vectors — carriers used to transfer desired genes into a foreign cell.Plasmi…

Transformation
Electroporation
Electrophoresis
Microarray
Nucleotide
Plasmids are the most commonly used vector

Part 1 of 8Vectors

Vectors

Vectors carriers used to transfer desired genes into a foreign cell.

Plasmids are the most commonly used vector.

Viruses and liposomes (small vesicles with a phospholipid layer) can also be used.

A good vector replicates in the host, has a restriction site for the gene and carries marker genes.

Part 2 of 8Plasmids

Plasmids

Plasmids small, circular rings of double-stranded DNA.
They occur naturally in bacteria, and also in archaea and eukaryotes such as yeast.
They can contain genes for antibiotic resistance.

Plasmids are used because they self-replicate, so the gene is copied at each cell division.

Scientists can modify bacterial plasmids or produce them artificially.

They can carry one or more marker genes so cells with recombinant plasmids can be identified.

Part 3 of 8Making a recombinant plasmid

Cut the plasmid open with the same restriction endonuclease used to isolate the gene.

This leaves the plasmid with sticky ends complementary to those on the gene fragment.

Mix the gene and plasmid so the sticky ends base-pair.

DNA ligase forms phosphodiester bonds between the sugar-phosphate backbones.

The result is a recombinant plasmid, a closed circle of double-stranded DNA containing the desired gene.

Part 4 of 8Transformation of host cells

Transformation of host cells

Plasmids are transferred into host cells, usually bacteria, by transformation.

Only a small proportion of bacteria become transformed, so markers are used to identify them.

Methods of transformation:

Bathe plasmids and bacteria in ice-cold calcium chloride solution, then incubate briefly at 40 °C, which makes the membrane permeable.
Electroporation — a small electrical shock makes the membranes very porous;
- it can also be used for eukaryotic cells.

Part 5 of 8Viruses and liposomes

Viruses naturally insert their nucleic acid into host cells,

so modified (disabled) viruses are used as vectors, especially in animal cells and gene therapy.
Retroviruses integrate DNA into the host genome but may disrupt genes.
Adenoviruses give temporary expression and may cause an immune response.
Bacteriophages are used as vectors for bacteria.

Liposomes

Liposomes lipid vesicles containing DNA that fuse with the cell surface membrane.
They avoid immune reactions and insertional mutations but are less efficient.

Part 6 of 8Delivery methods for plants and animals

Method	Host	How it works
Agrobacterium tumefaciens	Plants	The Ti plasmid T-DNA carries the gene and integrates into the plant genome
Gene gun	Plants	DNA-coated gold or tungsten particles are fired into cells
Electroporation	Bacteria, yeast, animal cells	An electric pulse opens pores in the membrane
Microinjection	Fertilised eggs	DNA injected into a nucleus with a fine needle

Part 7 of 8Using the vector in practice

The same restriction endonuclease must cut the plasmid and isolate the gene.

Two enzymes are used to prepare the vector: restriction endonuclease and DNA ligase.

The gene must be linked to a promoter that works in the host so it is expressed.

A high-copy plasmid with an origin of replication gives many copies of the gene per cell.

Part 8 of 8More from the specification

Reverse Transcriptase and cDNA Synthesis

The enzyme reverse transcriptase synthesises complementary single-stranded DNA (cDNA) from mature mRNA molecules, after which DNA polymerase forms the double-stranded functional gene sequence without introns.

Recombinant DNA and Genetically Modified Organisms

Restriction endonucleases cleave plasmid vectors at specific recognition sequences,
allowing DNA ligase to insert foreign DNA molecules to produce a genetically modified organism.

Electrophoresis and Microarray Analysis

Gel electrophoresis separates charged nucleic acid fragments by size,
while microarrays utilise fluorescent cDNA probes to simultaneously detect gene expression profiles across thousands of sequences.

Organism

A living thing, such as a plant or an animal.

Sequence

The order of bases in DNA or amino acids in a protein.

Electrophoresis

Separation of molecules by size in an electric field

Microarray

Chip with probes used to detect many genes at once.

Protein

A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

Nucleotide

The unit of DNA made of a phosphate, a deoxyribose sugar and a base.

Reverse transcriptase

Enzyme synthesising DNA from an RNA template

Genetically modified

Having had genes from another organism inserted.

Key terms

Term	Meaning
Vector	Carrier that transfers a gene into a cell.
Plasmid	Small circular double-stranded DNA molecule in bacteria that self-replicates.
Recombinant plasmid	Plasmid containing the desired gene.
Transformation	Uptake of a plasmid by a host cell.
Electroporation	Using an electrical shock to make membranes porous.
Liposome	Small vesicle with a phospholipid layer used to deliver DNA.

Avoid these mistakes

Saying the plasmid is cut by ligase — a restriction endonuclease cuts and ligase joins.

Using a different restriction enzyme for the plasmid and the gene — the sticky ends would not match.

Saying all bacteria take up the plasmid — only a small proportion are transformed.

Forgetting that a promoter is needed for expression.

Last updated: 18 October 2026