If one of the strands of DNA has the following sequence of bases running in the 5'-G-G-A-C-A-A-T-C-T-G-C-3'

a. what is the sequence of bases in the complementary strand?

b. what base is closest to the 5′-end in the complementary strand?

The two antiparallel DNA strands are not linear but are twisted into a helix around a common axis. The base pairs are planar and parallel to each other on the inside of the helix. The secondary structure is, therefore, known as a double helix.

The complementary means that if you know the sequence of bases in one strand, you will know the sequence of bases in the other strand. For example, if the base sequence of bases in one DNA strand is A-C-T, the base sequence in the complementary strand will be T-G-A.

5'-G-G-A-C-A-A-T-C-T-G-C-3'

3'-C-C-T-G-T-T-A-G-A-C-G-5'

Complementary strand of a given sequence

b)

In the complementary strand, the base closed to 3' is cytosine and the base closest to 5' is guanine which is represented as follows: