What is Complementary Bases in DNA ?

What is Complementary Bases in DNA ? - DNA is made up of two chains of molecules called nucleotides; thing of it as two bead necklaces, each bead being a nucleotide. Complementary base pairing ensures that the daughter DNA molecule obtained during replication is exactly like that of the original strand. DNA replication is semiconservative and at such, complementary base pairing ensure that if adeinine is found on one strand, thymine is found on the other complementary base pairing also help to hold the double helix together by the formation of hydrogen bond.

The two chains (or strands) are joined by each bead (nucleotide) so that it looks like a ladder (a helix). The bonds between the nucleotides on the two strands are hydrogen bonds, and these are formed between a special part of the nucleotide called a base (or nitrogenous base).

DNA double helix is formed Video

Scheme of unusual pairing of complementary bases in DNA :

Besides the normal pairing of Dna bases, for which quantum-mechanical calculations have been carried out by B. and A. Pullman (Biochim Biophys Acta 36:34 1959), other, unusual types of pairing are possible, in which H - bonds are formed between the 6 and 7-positions of purine rings and 6 and 1-positions of pyrimidine rings.

Calculations of the energy of delocalization De of electrons for the unusual pairing of adenine - thymine (At) and guanine -cytosine (Gc) yielded values of 6.110 and 6.048 for At and Gc, respectively. The contributions of H-bonds to De in this type of pairing are 0.166 and 0.064 for At and Gc, respectively. The results of the calculations indicated that from the energy standpoint unusual pairing of At is as stable as normal pairing, while unusual pairing of Gc is less advantageous than normal pairing.

There are four types of bases in DNA, adenine, thiamine, guanine and cytosine, so you could think of each bead necklace as a chain of four different coloured beads, each DNA molecule having its own unique sequence.

When the bases of the two strands join by the hydrogen bonds, only specific bases will join together; adenine to thiamine and cytosine to guanine. We call these complementary bases.

So complementary bases are specific pairs that join up the two strands of double-stranded DNA via hydrogen bonds.

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