A segment of DNA has the following sequence 5TTGGAT3 3AACCTA

A segment of DNA has the following sequence: 5\'-TTGGAT-3\' 3\'-AACCTA-5\' What would be the sequences immediately after reaction with nitrous acid? Let the letters H represent hypoxanthine and U represent uracil. (Remember to label the 5\' and 3\' ends of BOTH strands) Let\'s suppose this DNA was reacted with nitrous acid. The nitrous acid was then removed, and the DNA was replicated for two generations. What would be the sequences of the DNA products after the DNA had replicated two times? Your answer should contain the sequences of four double helices. Indicate 5\' and 3\' ends, put strands of a helix on successive lines, separate helices with \"AND\" on a line in between helices.

Solution

5\'-TTGGAT-3\'

3\'-AACCTA-5\'

Since nitrous acid is a mutagen, adenine will become hypoxanthine and cytosine will become uracil along with a deamination reaction resulting in removal of an NH2 (amine group), thus it will alter the base pairing. On treatment with nitrous acid (HNO2) , the resulting new sequence will be

Step 1:

5\'-TTXXHT-3\'

3\'-HHUUTH-5\',

Uracil would pair with adenine, xanthine would pair with cytosine . All the A-T will be now transited to G-C.

STEP 2:

5\'-CCAAAC-3\'

3\'-GGTTTG-5\'

THIS WILL ALTER THE 3\' -->5\' strand completely as all of the adenine will be replaced by hypoxanthine (H) and all of the cytosine will be replaced by Uracil (U), thus resulting a completely new strand with altered base pairing. Also, guanine is replaced by Xanthine (X), and hypoxanthine would pair with cytosine.