You are Dr. DNA, the brains behind an internet-based help desk for “molecular bi

Question

You are Dr. DNA, the brains behind an internet-based help desk for “molecular biologists who can't get their freaking experiments to work.” Yesterday, you received the following email:

Dear Dr. DNA,

Help! My advisor's chained me to the bench until I’ve completed this restriction digest. The DNA I’m working with is 1000 bp long and contains one recognition site for EcoRI and two sites for the enzyme TasI. When I cut the DNA with EcoRI, I get two fragments. When I cut the DNA with TasI, I get three fragments. That’s all well and good. But here’s the thing: when I cut with EcoRI and then cut with TasI, I still get three fragments. Why don’t I get four?

Sincerely,

Confused About Cutting

Google the recognition sites for TasI and EcoRI and then compose a reply that explains the data and liberates Confused from the lab.

Explanation / Answer

EcorI recognition site: G^AATTC

TasI recongintion site: ^AATT^

Consider this is a 1000bp sequence,

5'......................AATT....................GAATTC....................3'

Now when I digest this sequence with EcorI, I get the following two sequences:

5'......................AATT....................G 3'            +         5' AATTC....................3'

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Consider this is a 1000bp sequence,

5'......................AATT....................GAATTC....................3'

Now when I digest this sequence with TasI, I get the following three sequences:

5'......................3'     +       5' AATT....................G 3'     +    5' AATTC....................3'

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Consider this is a 1000bp sequence,

5'......................AATT....................GAATTC....................3'

Step 1: EcorI digestion

Now when I digest this sequence with EcorI, I get the following two sequences:

5'......................AATT....................G 3'           +          5' AATTC....................3'

Step 2: TasI digestion

Two sequence strand from the step 1 is taken, namely

5'......................AATT....................G 3'         +        5' AATTC....................3'

It is then digested with TasI to get the following three strand,

5'......................3'     +      5' AATT....................G 3'      +       5' AATTC....................3'

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