Question

Identify the palindromic sequence among the following.

• A. 3' GAATTC 5'
  3' GAATTC 5'
• B. 5' GAATTC 3'
  3' CTTAAG 5'
• C. 5' GAATTC 3'
  3' GAATTC 5'
• D. 5' GAATTC 3'
  5' GAATTC 3'

Answer 1

The correct option is B 5' GAATTC 3'

3' CTTAAG 5'
Palindromic sequences are the DNA sequences that are read the same way forward or backward when the polarity is kept the same in both the strands.
Restriction enzymes are a type of endonucleases that cleave phosphodiester bonds between consecutive nucleotides within or near to specific DNA sequences which they identify. These identifiable sequences are known as recognition sequences that are generally palindromic in nature.

In this question, option ‘d’ is the only sequence that is read the same way forward or backward, with the same polarity of DNA strands in both directions. It is the recognition site of the EcoR I.

The correct way to represent a double stranded DNA is in anti-parallel fashion. Therefore, in option b and c, they are represented wrongly. The sequence in option ‘a’ is not read the same keeping the polarity same for both the strands. In addition to this, all the options except option ‘d’ do not have complementary base pairing and thus their existence is quite impossible.