IGCSE Biology / Genetic modification
Cutting and pasting genes revision notes
Edexcel International GCSE Biology (4BI1), chapter 22: Genetic modification, specification point 5.12, 5.13. Free lesson.
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Genetic modification moves a gene from one organism into another so the new organism can make the protein that gene codes for. Restriction enzymes cut DNA, while DNA ligase joins DNA together.
How the gene is moved
The process has six main steps. The gene and plasmid are cut with the same restriction enzyme, so their ends can pair. DNA ligase then joins the DNA, forming a recombinant plasmid.
How sticky ends help DNA join
A restriction site is the specific base sequence where a restriction enzyme cuts. The enzyme cuts the two DNA strands a few bases apart, rather than straight across. This staggered cut leaves short single-stranded sections with exposed bases. These are sticky ends, which can pair with complementary sticky ends on the cut plasmid. Use the same restriction enzyme to cut the gene and the plasmid. This produces matching, complementary sticky ends. Once the ends have paired, DNA ligase binds the sugar-phosphate backbones together.
Why plasmids are used as vectors
A plasmid is a small circular loop of bacterial DNA, separate from the bacterium's chromosome. A vector is something that carries a gene into another organism. Plasmids are useful because bacteria naturally take them up and copy them. A virus can also be used as a vector. Viruses are usually used to get a gene into an animal cell, where a plasmid would not work.
Which enzyme cuts and which joins
The enzyme names are easy to mix up, but their jobs are different. A restriction enzyme cuts DNA at a specific sequence. DNA ligase joins DNA together. Ligate means to tie or bind, which helps you remember that ligase joins. A restriction enzyme cuts the gene out of the chromosome and cuts open the plasmid. DNA ligase joins the gene into the plasmid.