Ans. is 'b' i.e., Thymine o Adenine is always paired with thymine by the formation of two hydrogen bonds. Guanine is always paired with cytosine by the formation of three hydrogen bonds. Deoxyribonucleic acid (DNA) o DNA is the repository of genetic information. DNA is located in nucleus, DNA is also present in mitochondria (but less than 0.1% of the total DNA). Most accepted model for DNA structure is "Watson and Crick" model. Which was proposed by Watson and Crick in 1953. We will discuss that here, o DNA is a double-stranded molecule. Each strand is a polymer of nucleotides (deoxyTibonucleotides). Each strand possesses a palority. It has a 31 end and a 51 end. The two strands are antiparalleh i.e. they are parallel but run in opposite directions. o The sugar of DNA is deoxyribose. The purine bases in DNA are adenine (A) and Guanine (G), and pyrimidines are thymine (T) and cytosine (C). Uracil (U), a pyrimidine, is not found in DNA. o Bonds involved in DNA structure are - Within nucleotides: the Glycosidic bond between carbon-1 of the pentose sugar (deoxyribose) and nitrogen-9 of a purine base or nitrogen-1 of the pyrimidine base. Between two strands of DNA: The two chains (strands) are held together by hydrogen bonds between complementary pairs of bases. Adenine is always paired with thymine by the formation of two hydrogen bonds and Guanine is always paired with cytosine by the formation of three hydrogen bonds, Between successive nucleotides: Successive nucleotides in a strand of DNA are attached to each other by phosphodiester bonds. Carbon-31 and -51 of deoxyribose subunits are involved in ester linkages with an inorganic phosphate to form a 31, S' phosphodiester bond (a covalent bond). Thus alternating phosphate and deoxyribose units form the backbone structure of DNA while purine and pyrimidine bases are stacked inside. Base-staking interactions: The bases (purines and pyrimidines) lie flat in the interior, stacked on top of one another. They interact with one another through van der Waals and hydrophobic interactions. Collectively these two are known as base-stacking interactions.