Question

The nucleotide sequence of one DNA strand of a DNA double helix is 5-ʹGGATTATTGTGCACTTTCA-3ʹ. What is the sequence of the complementary strand and briefly explain the answer?

Answer 1

`3'-"CCTAATAACACGTGAAAGT"-5'`

Explanation:

We have directionality involved in this `"DNA"` sequence. If it starts with a `5'` and ends in a `3'`, then the complementary strand will start with `3'` and end with `5'`.

So, the first step is to write:

`3'-5'`

Now, we need to note the complementary values. Adenine `("A")` only bonds with thymine `("T")` because both of them have place
only to form two hydrogen bonds, while cytosine `("C")` only bonds with guanine `("G")`, as they have three places for hydrogen bonds to form. Note that this can also happen vice-versa.

So, we just need to change letters in the complementary strands to their "partners", i.e. `"A"rightleftharpoons"T" \ "and" \ "C"rightleftharpoons"G"`.

Our complementary `"DNA"` strand to this would be:

`3'-"CCTAATAACACGTGAAAGT"-5'`

Hope this helps!