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Understanding the structure of DNA is fundamental to comprehending the intricate mechanisms of genetic inheritance and expression. DNA, or Deoxyribonucleic Acid, features a double helix configuration, resembling a twisted ladder. The sides of this 'ladder', forming the backbone of the molecule, consist of alternating phosphate groups and sugar molecules (deoxyribose). Connecting the backbones are the ladder's 'rungs', made up of pairs of nitrogenous bases. These bases strictly pair up: adenine (A) with thymine (T), and cytosine (C) with guanine (G), following the principle of complementary base pairing.

The double helical structure of DNA allows for the storage of genetic information securely, with the two strands running antiparallel to each other – one in a 5' to 3' direction and the other in a 3' to 5'. This orientation is crucial during DNA replication and when it is being transcribed to RNA.

Unlike DNA, RNA, or Ribonucleic Acid, is primarily a single-stranded molecule and exhibits tremendous structural flexibility. The ribose sugar in RNA's backbone, distinct from DNA's deoxyribose, includes an additional hydroxyl (-OH) group. This subtle chemical difference renders RNA more reactive and liable to participate in various cellular processes.

RNA molecules can take on several shapes, from linear to complex three-dimensional structures, enabling them to fulfill diverse roles – from being a messenger that carries genetic code from DNA (as mRNA) to being an integral part of the cell's protein-synthesis machinery (as rRNA and tRNA). The single-stranded nature allows for more intricate within-molecule base pairing, causing RNA to fold into unique conformations necessary for its functions.

Nucleotides serve as the building blocks of both DNA and RNA. Each nucleotide consists of three components: a phosphate group, a sugar molecule, and a nitrogenous base. In DNA, the sugar is deoxyribose, while in RNA, it is ribose, which, as mentioned earlier, has a hydroxyl group at the 2' position.

These nucleotides link together through phosphodiester bonds, forming the long chains that make up the genetic material. Their sequence within a strand carries the genetic information, specifying the exact instructions required for creating proteins and managing the cell's various activities. Understanding how nucleotides form the backbone and encode genetic information provides a foundation for learning about gene expression and regulation.

The specific nitrogenous bases present in DNA and RNA are crucial for the proper transfer of genetic information. DNA includes adenine (A), cytosine (C), guanine (G), and thymine (T), while RNA contains adenine (A), cytosine (C), guanine (G), and replaces thymine with uracil (U).

These bases engage in hydrogen bonding with their complementary pairs: A with T (or U in RNA) and C with G. This specific pairing is essential for DNA replication and for ensuring RNA transcripts maintain the correct genetic code for synthesizing proteins. Alterations in these base pairs can lead to mutations, which can sometimes have profound effects on an organism's traits or health.