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

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.

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.