Restriction enzymes cut DNA at specific sequences; ligase joins pieces, so a gene can be put into a plasmid and taken up by bacteria, which then make the protein, such as human insulin.
People with diabetes once used insulin taken from the pancreas of cattle and pigs. Since the 1980s, human insulin has been made by microbes, such as bacteria, that carry the human insulin gene. That is recombinant DNA: DNA from two sources joined together. It uses gels and the idea of gene expression.
🔑 Key idea
Restriction enzymes cut DNA at specific short sequences. DNA ligase joins pieces together. A gene can be cut out, joined into a plasmid (a small ring of bacterial DNA) and put into bacteria. The bacteria copy the plasmid as they divide and express the gene, making its protein.
💡 Why it works
It works because the genetic code is shared by nearly all life. A bacterium reads the codons of a human gene the same way a human cell does, so it builds the same chain of amino acids. A restriction enzyme cuts both the gene's DNA and the plasmid at the same sequence, leaving matching "sticky ends" that pair up so ligase can seal them.
The steps in order.
✏️ Worked example: A restriction site
The enzyme EcoRI cuts at the sequence GAATTC. How many times does it cut the strand GGAATTCATTGAATTCAA?
Search for GAATTC: it starts at the 2nd base.
Search again after it: it starts at the 11th base.
No others.
Answer: 2 times.
✏️ Worked example: Counting pieces
A plasmid is a closed ring. If an enzyme cuts it at 3 places, how many pieces form? What if the DNA were a straight line cut at 3 places?
A ring cut once opens into one piece; each later cut adds a piece. So 3 cuts make 3 pieces.
A line cut once makes 2 pieces; 3 cuts make 4.
Answer: Ring: 3 pieces. Line: 4 pieces.
EcoRI reads GAATTC on both strands (the sequence is the same read 5′→3′ on either strand) and cuts each strand between the G and the first A, leaving four unpaired bases, AATT, on each end. Human insulin made by bacteria was approved for use in the United States in 1982, the first medicine made with recombinant DNA. Crops can be engineered in similar ways; people weigh the benefits and concerns of those crops differently, while the laboratory methods themselves are well established.
⚠️ Common mistake
It is easy to assume bacteria cannot read a human gene, since bacteria and people are so different. But the genetic code is nearly universal: the same codons mean the same amino acids in both. (Human genes do contain introns, which bacteria cannot remove, so scientists insert a version of the gene with the introns already taken out.)
Practice
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