The pyrimidine bases present in DNA are (a) Cytosine and adenine (b) Cytosine and guanine (c) Cytosine and thymine (d) Cytosine and uracil

Nucleic acids are fundamental components of all living cells, carrying the all-important blueprints for life in the form of DNA and RNA. These complex molecules store and transmit genetic information, ensuring that an organism's traits can be passed on from one generation to the next.

There are two primary types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the hereditary material in humans and almost all other organisms, while RNA plays a critical role in the encoding, transmission, and expression of this genetic information.

Each nucleic acid is made up of long chains of nucleotides, which have three components: a sugar molecule, a phosphate group, and a nitrogenous base. In the case of DNA, the sugar is deoxyribose; in RNA, it's ribose. The sequence of these nitrogenous bases along the nucleic acid strand encodes the biological information necessary for life functions.

The structure of DNA is described as a double helix, with two strands winding around each other like a twisted ladder. This remarkable structure was first identified by James Watson and Francis Crick in 1953, who postulated that DNA's configuration allows for replication and transmission of genetic information.

The sides of the ladder are formed by alternating sugar and phosphate groups, while the rungs consist of paired nitrogenous bases. These bases pair in a specific way: adenine (A) with thymine (T), and cytosine (C) with guanine (G). This pairing is known as base pairing, which is critical for the accuracy of DNA replication.

In terms of chemical stability, the DNA structure is further stabilized by hydrogen bonds between the nitrogenous bases and hydrophobic interactions among the stacked bases, playing a significant role in the three-dimensional shape of the molecule.

Nitrogenous bases are the component in nucleic acid structure that carry the genetic code. These molecules are classified into two groups based on their structures: pyrimidines and purines.

Pyrimidines, smaller in size, consist of a single six-membered ring and include cytosine (C), thymine (T), and uracil (U) - with the latter found only in RNA. In contrast, purines like adenine (A) and guanine (G) feature a fused double ring structure, with a six-membered ring joined to a five-membered ring.

Understanding these distinctions is crucial, particularly when addressing questions about DNA bases. As observed in the given exercise, adenine is not a pyrimidine but a purine, as is guanine. Similarly, uracil is a component of RNA, not DNA. Hence, the pyrimidine bases present in DNA are cytosine and thymine, highlighting the importance of accurately identifying base pairs to truly grasp DNA's structure and function.