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DNA, or deoxyribonucleic acid, is the fundamental molecule that holds the genetic instructions for the development, functioning, and reproduction of all living organisms. The structure of DNA is often described as a double helix, which looks like a twisted ladder.
At the core of this structure are repeating units called nucleotides. Each nucleotide consists of three parts:

• a phosphate group
• a five-carbon sugar called deoxyribose
• a nitrogenous base (adenine, thymine, cytosine, or guanine)

The sugar-phosphate backbone forms the sides of the ladder, while the nitrogenous bases form the rungs, pairing specifically (adenine with thymine and cytosine with guanine) through hydrogen bonds.
This complementary base pairing is crucial for the replication of DNA, allowing for the transmission of genetic information.

RNA, short for ribonucleic acid, is another type of nucleic acid, involved in various biological roles such as coding, decoding, regulation, and expression of genes. Unlike DNA, RNA is typically single-stranded, giving it a more flexible structure.
RNA is composed of nucleotides similar to DNA, but with a few key differences:

• Instead of deoxyribose, RNA nucleotides contain ribose sugar.
• RNA uses uracil in place of thymine (pairs with adenine).

The presence of ribose, which has an extra oxygen compared to deoxyribose, makes RNA less stable than DNA.
This structural difference results in RNA being more reactive and suited for its role in protein synthesis and regulation within the cell.

Pentose sugars, essential components in the structure of nucleic acids, are so named because they contain five carbon atoms. In nucleic acids, the two fundamental pentose sugars are ribose and deoxyribose.

• Ribose is found in RNA and has the chemical formula C\(_5\)H\(_{10}\)O\(_5\). Each carbon atom is bonded to a hydroxyl group (OH).
• Deoxyribose, the sugar in DNA, has the formula C\(_5\)H\(_{10}\)O\(_4\), notably missing an oxygen at the second carbon position compared to ribose, hence the term 'deoxy'.

This slight variance in the sugar’s structure plays a significant role in determining the different functions and stability of DNA and RNA.
Understanding these sugars' roles helps to illuminate why DNA is less reactive, making it ideal for long-term information storage, while RNA's structure allows it to participate actively in cell functions.