Question

If a sequence of one strand of DNA is ATTGCTCG, what is the complementary sequence?

Answer 1

Ans.

There are four nitrogen base that a DNA nucleotides can contain.They are Adenine (A),thymine (T),guanine (G) and cytosine (C). When these nucleotides form a long chain by coming together, they are known as a DNA strand. It is the hydrogen bonds that forms between these bases that holds two strands of DNA together. In general, Adenine bonds with thymine whereas cytosine bonds with guanine.A can pair with either T or C but do not pair with G.

Now, In genetics the term complementary refers to the double strands where one polynucleotide strand that is paired with the reverse complement of the other strand.

According to Chargaff's rule of complementary base-paring, Adenine (A) only bonds with Thymine (T) and Cytosine (C) only bonds with Guanine (G)  in a strand of DNA. This is because adenine and guanine are purines whereas thymine and guanine are pyrimidines.Two purines if bond together will take a lot of space between the strands,which will make the bond weaker and thus won't allow the strands to be held together.Same is the case for thymine and guanine both of which are pyrimidines, but instead of too much space,they will take too little space thus making the bond unstable. Apart from this, the hydrogen bonding between the bases A & T and C & G is what actually holds the DNA strands.

Hence by the above rule, for the DNA strand ATTGCTCG, following will be its complementary strand:

TAACGAGC