Self Assessment Answer # 7 for Lesson 19

by Dr Jamie Love © 2002 - 2005
Genetics Course

The complement of GATACAA is TTGTATC.

It is not CTATGTT because that would not be the correct direction.

Have a closer look at the original sequence GATACAA. Remember that it is written from 5' to 3' so we could think of it as 5'GATACAA3'. A common mistake is to repeat the right to left pattern as you line up the correct base due to the proper pairing like this
GATACAA
CTATGTT

That is the correct pairing but NOT the correct sequence because, when written in that direction it means you have
5'GATACAA3'
5'CTATGTT3'
but that is NOT antiparallel! (Right?) Those bases might want to match up but they cannot because the sugars won't allow it.

It is a big difference between 5'CTATGTT3' and 5'TTGTATC3'. They are not the same molecule or sequence. (Before you ask - no, they are not complementary. Take a look at them. 5'CTATGTT3' will not pair with 5'TTGTATC3' even if you didn't have to worry about the sugar orientation.)
Writing the complementary strand's sequence requires you to keep track of the pairing AND 5' to 3' orientation - both at the same time or sequentially, whatever works best for you.

Sequentially, it works like this. The original sequence is GATACAA so the pairing (ignoring for a moment the direction) would be CTATGTT but this answer is wrong or at least incomplete because now we have to switch it around. So CTATGTT becomes TTGTATC. That is the correct answer. We have not included (written) the directions but if we did we MUST write them 5' to 3' (left to right) so the original sequence is 5'GATACAA3' and its compliment is 5' TTGTATC3'.
(Check it to see if that is right by using the 5' to 3' directions to make antiparallel pairings.)

Some people are more comfortable doing the pairing and writing at the same time and maybe you will like it this way too. Start with the original sequence GATACAA write the paired complement one base at a time but as you WRITE them from left to right, READ them from right to left. That is, start at the right side of the original sequence GATACAA - start with the A. What is its complement? T. (Right?) Then write that down, "T". Now move to the second base from the right - another A so you write another T to give you the sequence "TT". Continue to the third base from the right, a "C" and write its complement "G" so your new sequence is now "TTG". Continue the process until you are done.

Which method should you use? Whatever one you like just as long as you are comfortable with it. The sequential method takes more time and you might make an error as you rewrite it in the opposite direction. However, some people easily miss a bass when trying to do the right-to-left reading method (but with practice they usually become proficient at it).

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