Question

What does it mean when we say the DNA is two complimentary, and is composed of antiparallel chains of nucleotides?

Answer 1

DNA (Deoxyribo Nucleic Acid) is an antiparallel double helix molecule which serves as heriditary material. It is composed of two polynucleotide chains that are bonded to each other resulting in double helix structure. Each polynucleotide chain is composed of nucleotides of four types -adenine (A), cytosine (C), guanine (G), and thymine (T) - arranged in a way that gives rise to a sugar-phosphate backbone on the outside with nitrogenous bases stacked towards inside. The pair of bases, one each from opposing strand,  are bound to each other by hydrogen bonds on the inside with the purine adenine (A) always pairing with the pyrimidine thymine (T) and  the pyrimidine cytosine (C) always pairs with the purine guanine (G). This results in base compliments A = T, G ≡ C. This base complementary dictates the antiparallel orientation of polynucleotide chains with nitrogenous bases arrayed in a 5' to 3' orientation on one chain hydrogen bonded to complementary bases in the 3' to 5' orientation of the opposite strand. The complementary and antiparallel structure of DNA is key in DNA functions like replication and other nucleic acid interactions.

DNA is composed of two polynucleotide strands linked by complementary hydrogen bonding. The two strands run in opposite direction. That is the reason we say DNA is a double stranded molecule with complimentary and antiparallel chains.