In genetic engineering, genes from the chromosomes of humans and other…

The idea

Genetic engineering is the deliberate transfer of a gene from one organism to another. A gene is a length of DNA on a chromosome that codes for a protein. Enzymes cut the gene out of the donor chromosome, leaving short single-stranded ends called sticky ends. The same enzyme cuts open a vector, often a bacterial plasmid, producing matching sticky ends. The gene is inserted so that complementary bases pair, and ligase joins the DNA. The recombinant plasmid is put into a host cell, such as a bacterium, which then reads the new gene and makes the protein.

For example: The human insulin gene can be cut out and transferred into bacteria, which then produce insulin.

What earns the marks

• A gene is a section of DNA on a chromosome that codes for a particular protein.
• Restriction enzymes cut the DNA at specific base sequences, producing fragments with sticky ends.
• The same restriction enzyme is used to cut open a vector, such as a plasmid, so the sticky ends are complementary.
• The donor gene and vector are joined by ligase to form recombinant DNA.
• The vector carries the gene into a host cell, which can then express the gene and make the protein.
• The process is described as genetic engineering because the gene is transferred artificially between organisms.

Common mistakes and how to put them right

• Thinking the whole chromosome is transferred.

  Put it right: Only the specific gene is cut out and moved.

• Believing any enzyme can cut anywhere.

  Put it right: Restriction enzymes recognise specific base sequences, so the same enzyme must be used for donor DNA and vector.

• Confusing the roles of the enzymes.

  Put it right: Restriction enzymes cut, while ligase joins DNA fragments together.

Exam tips

• Name the vector and the host cell in your answer, for example a plasmid placed into a bacterium.
• Use the terms sticky ends and complementary bases to explain why the gene and vector join.
• Link the transfer to a named product, such as human insulin made by bacteria, to show the purpose of the process.