Answer:
The nitrogenous bases present in DNA are:
- Adenine: It pairs with thymine (T) in DNA. Adenine is a purine base, meaning it has a double-ring structure.
- Guanine: It pairs with cytosine (C) in DNA. Guanine is also a purine base.
- Cytosine: It pairs with guanine (G) in DNA. Cytosine is a pyrimidine base, meaning it has a single-ring structure.
- Thymine: It pairs with adenine (A) in DNA. Thymine is a pyrimidine base.
The correct option is A, which includes all four nitrogenous bases found in DNA: adenine, guanine, cytosine, and thymine.
Explanation:
DNA (deoxyribonucleic acid) is a molecule that carries the genetic information in living organisms. It is composed of two strands that are twisted together to form a double helix structure. Each strand is made up of a series of nucleotides, which are the building blocks of DNA.
A nucleotide consists of three components: a nitrogenous base, a sugar molecule (deoxyribose), and a phosphate group. There are four different nitrogenous bases found in DNA, and they are responsible for carrying the genetic code. The arrangement of these bases along the DNA molecule determines the instructions for the formation and functioning of an organism.
Adenine, guanine, cytosine, and thymine are the four nitrogenous bases found in DNA. Adenine always pairs with thymine, and guanine always pairs with cytosine. These base pairs are held together by hydrogen bonds, forming the rungs of the DNA ladder.
Uracil (U) is not present in DNA; it is found in RNA (ribonucleic acid) instead. RNA is a single-stranded molecule that plays a role in protein synthesis.
In summary, the correct answer is A: Adenine, guanine, cytosine, and thymine are the nitrogenous bases present in DNA.

Four different types of nitrogenous bases are found in DNA: two purines adenine (A) and guanine (G) & two pyrimidines cytosine (C) and thymine (T). In RNA, the thymine is replaced by uracil (U).