Which statement correctly describes the 5' end of a DNA strand?

• A. The 5' end has a phosphate group on the 5' carbon
• B. The 5' end has a phosphate on the 3' carbon
• C. The 5' end has a hydroxyl on the 5' carbon
• D. The 5' end has a sugar on the 1' carbon

Answer: (A)

DNA strand directionality is defined by where the terminal phosphate sits on the sugar backbone. The 5' end is the end that has a phosphate group attached to the 5' carbon of the terminal deoxyribose. This terminal phosphate marks the end of the strand, while the 3' end carries a free hydroxyl group on the 3' carbon, allowing nucleotides to be added during synthesis in the 5' to 3' direction. So the statement describing a phosphate on the 5' carbon at the end is the correct description. The other options describe features that do not define the 5' end: a phosphate on the 3' carbon wouldn’t designate the terminus, a hydroxyl on the 5' carbon isn’t how the end is characterized, and a free sugar on the 1' carbon isn’t how the strand terminates.