recap

18.1 recap

DNA fragments from different sources can be linked together to make recombinant DNA. DNA can be cut with a restriction enzyme and then rejoined with DNA ligase. There are many different restriction enzymes, each with its own recognition sequence.

learning outcomes

You should be able to:

  • Show how staggered cuts in DNA made by restriction enzymes lead to the creation of sticky ends.

  • Summarize the Cohen and Boyer experiments on creating recombinant DNA.

Question 1

How does a staggered cut in DNA create a sticky end?

The sequences for recognition by a restriction enzyme often occur near one another on the opposite strands of DNA. For example,

5'….GAATTC….3'

3'….CTTAAG….5'

The top and bottom strands have the same 5'-to 3'-sequence. A restriction enzyme recognizes this and cuts between the G and A:

5'….GAATTC….3'

3'….CTTAAG….5'

Note that the cuts are staggered, not directly opposite each other.

Question 2

How did Cohen and Boyer make the first recombinant DNA?

DNA from two genetically different strains of bacteria, each carrying a different antibiotic resistance gene, was cut with the restriction enzyme EcoRI and then spliced together using DNA ligase.

Recombinant DNA has no biological significance until it is inserted inside a living cell, which can replicate and transcribe the transplanted genetic information. How can recombinant DNA made in the laboratory be inserted and expressed in living cells?