DNA sequence orientation is written in which direction?

• A. 3' to 5'
• B. 5' to 3'
• C. 1' to 2'
• D. 2' to 3'

Answer: (B)

DNA sequences are written in the 5' to 3' direction because the backbone ends are defined by a 5' phosphate and a 3' hydroxyl, and DNA is synthesized by adding nucleotides to the 3' end. Writing from 5' toward 3' provides a consistent way to describe the order of bases and the direction of synthesis and reading. If you were to write from 3' to 5', it would describe the opposite orientation, which isn’t the standard way we record sequence information. The other directional options refer to the sugar carbons in the backbone (like 1' to 2' or 2' to 3'), which aren’t used to denote sequence direction.