Question

Which of the following sequences can be recognised by a restriction enzyme?

• A. 5' GAATTC 3'
  3' CTTAAG 5'
• B. 5' GATC 3'
  5' CTAG 3'
• C. 5' GTTA 3'
  3' CAAT 5'
• D. 5' CATGG 3'
  5' GTACC 3'

Answer 1

The correct option is A 5' GAATTC 3'

3' CTTAAG 5'
The following sequence reads the same on the two strands in $5^{\prime }\to 3^{\prime }$ direction. This is also true if read in the $3^{\prime }\to 5^{\prime }$ direction. Such sequence is known as pallindromic sequence.
$5^{\prime }\to \text{GAATTC}\to 3^{\prime }$
$3^{\prime }\to \text{CTTAAG}\to 5^{\prime }$

Restriction enzymes cut the strand of DNA a little away from the centre of the palindrome sites, but between the same two bases on the opposite strands. This leaves single stranded portions at the ends.