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Wednesday, January 09, 2008

Complementary sequence of 5' TTAAGCTAC 3’ is 5’ GTACGCTTAA 3’

033. Which one of the following is the complementary sequence of 5' TTAAGCTAC 3’?

1. 5’ GTACGCTTAA 3’

2. 5’ AATTCGCATG 3’

3. 5’ CATGCGAATT 3’

4. 5’ TTAAGCGTAC 3’

Answer

1. 5’ GTACGCTTAA 3’

Reference:

Harper 26th Edition Page 330

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Discussion

A double-stranded DNA or RNA strand consists of two complementary strands of base pairs, which are non-covalently connected via two or three hydrogen bonds. Since there is only one complementary base for any of the bases found in DNA/RNA, one can reconstruct a complementary strand for any single strand. Complementary sequence is a sequence of bases that can form a double-stranded structure by matching base pairs. The complementary sequence to G-T-A-C, for instance, is C-A-T-G.

Explanation

The complementary sequence of TTAAGCTAC is AATTCGATG and thus you may immediately jump to conclusion that Choice 2 is the answer.

But if you see carefully the question gives the DNA as not just TTAAGCTAC, but as 5' TTAAGCTAC 3’. When a DNA strand is noted as 5’xxxxxxxxxx3’ the complementary strand will be of 3’xxxxxxxxxx5’ So the complementary sequence of 5' TTAAGCTAC 3’ is 3’AATTCGATG5’ But if you see the choices, all the choices start with 5’ Now again you may jump into a conclusion that this is a “dummy question” or that the question setter has made a mistake J

Now if you analyze carefully 3’ATGC5’ can also be written as 5’CGTA3’ So the complementary sequence 3’AATTCGATG5’ can also be written as 5’GTAGCTTAA3’ Thus we choose choice 1 as the answer

Comments

Never underestimate the questions in AIPG, especially the questions that are small and that appear simple at first sight.

Tips

Genetics is the most important topic in Biochemistry in AIPG

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