12. Nitrogenous bases are organic bases that are heterocyclic in nature and contain nitrogen. There are two types of nitrogenous bases, purines and pyrimidines. Adenine and Guanine are purines and cytosine, uracil and thymine are pyrimidines. Out of these nitrogenous bases, adenine, guanine, cytosine and thymine are found in the DNA and the thymine is replaced by uracil in RNA. Adenine always pairs with thymine and cytosine always pairs with guanine.
13. DNA formed by predominantly by CG base pair will be harder to separate as cytosine is paired with guanine with three hydrogen bonds. In case of adenine and thymine, there are only two hydrogen bonds between them.
14. Mutation is any heritable change occuring in the genetic makeup of any organisms. It may lead to change the genotype and phenotype of an organism.
15. In a DNA, two polynucleotide chains are coiled to form a helix. Sugar-phosphate forms backbone of this helix while bases project inwards to each other. There are hydrogen bond present between two strands of DNA. These hydrogen bonds attaches two strand and makes the molecules more stable. Many nucleotides link together through 3′ – 5′ phosphodiester bonds (between phosphate component of 5′ carbon and -OH group of 3′ carbon atom) to form polynucleotide chain (as in DNA and RNA). In course of formation of polynucleotide chain, a phosphate moiety remains free at 5′ end of ribose sugar (5′ end of polymer chain) and one -OH group remains free at 3′ end of ribose (3′ end of polymer chain).
The deoxyribose sugar is linked to the nitrogenous base through N-glycosidic bond.