Introduction:
Nucleotides are the building blocks of nucleic acids, such as DNA and RNA. They consist of three components: a sugar molecule, a nitrogenous base, and a phosphate group. The sugar molecule in nucleotides can be either ribose or deoxyribose.

Deoxyribose:
Deoxyribose is a 5-carbon sugar molecule that lacks an oxygen atom on the second carbon (C2). This absence of an oxygen atom gives deoxyribose its name. Deoxyribose is the sugar present in DNA (deoxyribonucleic acid).

Ribose:
Ribose is also a 5-carbon sugar molecule but contains an oxygen atom on the second carbon (C2). Ribose is the sugar present in RNA (ribonucleic acid). The presence of the oxygen atom on C2 differentiates ribose from deoxyribose.

Key Differences:

1. Presence or Absence of Oxygen:
- Ribose has an oxygen atom on the second carbon (C2).
- Deoxyribose lacks an oxygen atom on the second carbon (C2).

2. Functionality:
- Ribose is involved in the synthesis of RNA, which plays a crucial role in protein synthesis and various cellular processes.
- Deoxyribose is involved in the synthesis of DNA, which carries genetic information and is responsible for the transmission of hereditary traits.

3. Stability:
- The presence of an oxygen atom in ribose makes it less stable compared to deoxyribose. The absence of an oxygen atom in deoxyribose makes DNA more stable and less prone to degradation.

4. Evolutionary Significance:
- The presence of deoxyribose in DNA is believed to have evolved as a more stable form of ribose, allowing for the long-term storage and transmission of genetic information.

Conclusion:
In summary, the sugar present in nucleotides can be either ribose or deoxyribose. Ribose is found in RNA, while deoxyribose is found in DNA. The key difference between these two sugars lies in the presence or absence of an oxygen atom on the second carbon (C2). Ribose has an oxygen atom on C2, while deoxyribose lacks it. This structural difference has functional and evolutionary implications, making ribose suitable for RNA synthesis and deoxyribose essential for DNA synthesis.