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DNA, or deoxyribonucleic acid, is the molecule responsible for storing and transmitting genetic information in living organisms. It is a complex structure composed of repeating units called nucleotides. Each nucleotide consists of a sugar molecule, a phosphate group, and a nitrogenous base. The DNA structure is famously recognized as a double helix, which resembles a twisted ladder.

The backbone of this ladder is formed by the sugar and phosphate molecules, creating a stable framework. Meanwhile, the rungs are made of paired nitrogenous bases, which face each other, forming hydrogen bonds. The specific pairing of these bases is crucial for maintaining the overall DNA structure and ensuring accurate DNA replication, which is essential for cell division and the transmission of genetic information.

In DNA, nitrogenous bases are organic molecules that form the genetic code within living organisms. These bases are classified into two main groups: purines and pyrimidines.

**Purines** include adenine (A) and guanine (G). They are larger molecules with a two-ring structure. **Pyrimidines** consist of cytosine (C) and thymine (T), which are smaller and have a single-ring structure. One key aspect of DNA is the principle of complementary base pairing. In the DNA double helix, adenine forms hydrogen bonds exclusively with thymine, and cytosine pairs only with guanine.

The order of these nitrogenous bases along the DNA strand determines an organism's genetic instructions. Variations in the sequence lead to the diversity of life observed on Earth.

The four main nitrogenous bases in DNA: adenine, thymine, cytosine, and guanine, are fundamental to the structure and function of DNA.

**Adenine (A)** and **thymine (T)** always pair together, because they form two hydrogen bonds, creating a reliable and stable pairing that contributes to the DNA's helical structure.

**Cytosine (C)** and **guanine (G)** pair up as well, but unlike A-T, they form three hydrogen bonds. This makes the C-G pairing slightly stronger compared to A-T.

These base pairs are not just structural components. They also carry the genetic instructions used in the growth, development, functioning, and reproduction of all living things. Understanding how these bases pair and function is key to understanding genetics as a whole. Importantly, all these components must be correctly identified and arranged for the genetic code to be accurately expressed and transferred.