The oxygens
on the sugar that makes up DNA
are counted in which direction?
counter clockwise
clockwise
The Correct Answer and Explanation is:
Correct Answer: Clockwise
Explanation:
The oxygen atoms in the sugar of DNA, specifically deoxyribose, are numbered in a clockwise direction starting from the oxygen atom in the ring structure. Deoxyribose is a five-carbon sugar, and its structure is central to the DNA backbone.
Understanding Carbon Numbering in Deoxyribose
- Five-membered Ring Structure: Deoxyribose forms a cyclic (ring) structure with one oxygen atom and four carbon atoms.
- Clockwise Numbering: The numbering of carbons in deoxyribose begins with the oxygen atom in the ring as the reference point:
- Carbon 1′ (1-prime): The first carbon adjacent to the oxygen atom in the ring is numbered as 1′.
- Carbon 2′: Moving clockwise, the next carbon is numbered as 2′. This carbon is significant in DNA because it lacks a hydroxyl group (-OH), distinguishing deoxyribose from ribose in RNA.
- Carbon 3′: Continuing clockwise, the next carbon is numbered as 3′. This carbon is critical for forming phosphodiester bonds between nucleotides.
- Carbon 4′: The fourth carbon in the ring is labeled as 4′.
- Carbon 5′: The fifth carbon is not part of the ring; it is attached to the 4′ carbon as part of the sugar’s structure. It is important for attaching the phosphate group.
Significance of Clockwise Numbering
The standardized clockwise numbering system ensures consistency in describing DNA’s structure and function. For example:
- The 3′ carbon is where new nucleotides are added during DNA synthesis.
- The 5′ carbon is where the phosphate group attaches, creating the 5′-end of the DNA strand.
Thus, the clockwise numbering of oxygen and carbons in deoxyribose provides a universal framework for understanding DNA’s chemical structure and its biological processes.