How do you find the sequence of bases on a DNA strand?

First of all, make sure that you indicate on the question itself the directionality of the sequence: which makes this (by convention) 5′ AGCTCAG 3′. We assume that any sequence written without the 5′ and 3′ designations is from the sense strand, and reads from 5′ on the left to 3′ on the right. Therefore, the complementary sequence, which is being asked for, must go from 3′ to 5′ as matched up (because it runs in the opposite direction). So, this is 3′ TCGAGTC 5′. It is true that we often write sequences without the formal directionality indicators, and if so, we do it from 5′ to 3′. However, if you put “CTGAGCT” as your answer, experience tells me that you have a very good chance of getting it marked wrong (unless your instructor is testing you all with a little tricky questioning).

Explanation:

Hope this helps

From,

1kvibing