Question

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

Short answer

The pyrimidine bases present in DNA are cytosine and thymine (option c).

Step-by-step solution

Understanding Pyrimidine Bases

In DNA, the nucleobases are divided into two categories: pyrimidines and purines. Pyrimidines are single-ring structures, while purines have double-ring structures. The pyrimidine bases in DNA are cytosine and thymine.

Reviewing Possible Options

Let's review the given options: (a) cytosine and adenine, (b) cytosine and guanine, (c) cytosine and thymine, (d) cytosine and uracil. We know that thymine is a pyrimidine base in DNA.

Eliminate Incorrect Options

Eliminate options based on what you know: (a) is incorrect because adenine is a purine, (b) is incorrect because guanine is a purine, and (d) is incorrect because uracil is found in RNA, not DNA.

Select the Correct Option

The only remaining option, which correctly lists pyrimidine bases in DNA, is (c) cytosine and thymine.

Key concepts

Pyrimidine Bases

The structure of DNA is an intricate and beautiful design, mainly comprised of sequences called nucleotides. These nucleotides have four bases that can be categorized into two groups: pyrimidines and purines. Pyrimidine bases in DNA are characterized by their single-ring structure, making them smaller in size compared to purines, which have double-ring structures. This single ring consists of six atoms, and it forms the backbone for nucleobases like cytosine and thymine. These pyrimidines are crucial for the hydrogen bonding that holds the DNA strands together. In essence, pyrimidines play a key role in the integrity and transmission of genetic information.

Cytosine

Cytosine, a key component of DNA, is one of the pyrimidine bases. It is distinct because of its ability to form stable hydrogen bonds with guanine, its complementary base that belongs to the purine category. This characteristic pairing, cytosine with guanine, is fundamental for the proper coding and decoding of genetic instructions.
Cytosine contains a single ring and includes a nitrogenous base with the chemical formula C4H5N3O. This structure facilitates the formation of three hydrogen bonds with guanine, which ensures the stability and consistency of DNA's double helix structure.
Moreover, the alteration of cytosine to uracil can result in genetic mutations, highlighting its importance in maintaining genetic fidelity.

Thymine

Thymine is another crucial pyrimidine base found in DNA. It pairs specifically with adenine, a purine base, through two hydrogen bonds. This pairing is critical for the replication process and ensures the accurate passage of genetic information from cell to cell. Thymine’s single-ring structure, like that of cytosine, helps create the DNA’s stable double-helix architecture.
The presence of thymine, as opposed to uracil which is found in RNA, provides DNA with greater stability. Thymine contains a methyl group on its ring, making it more stable over time compared to uracil. This stability is particularly crucial because it helps prevent mutations, supporting thymine's role in genetic integrity and cellular functions.

Purine Bases

Purine bases contrast with pyrimidine bases because of their larger size due to their double-ring structure, which consists of a fused pair of rings containing nine atoms. In DNA, the purine bases include adenine and guanine. These bases pair with thymine and cytosine, respectively.
Adenine and guanine are essential for pairing with the pyrimidine bases, enabling the ladder-like structure of the DNA molecule. These pairings are essential in the encoding of the genetic information within DNA. The purine-pyrimidine pairing rule ensures the DNA strands are of uniform width, enabling the stability and proper functioning of genetic material. Understanding these interactions is vital for many fields, including genetics and molecular biology, as they are the foundational mechanics of genetic reproduction and information transmission.