Final answer:
The 5' end of a nucleic acid strand has a free phosphate group and the 3' end has a free hydroxyl (OH) group. Option number 3 is correct.
Step-by-step explanation:
The 5' end of a nucleic acid strand has a free phosphate group and the 3' end has a free hydroxyl (OH) group. This means that the 5' end of the DNA strand has a phosphate attached to the 5' carbon of the sugar molecule, while the 3' end has a hydroxyl group attached to the 3' carbon of the sugar molecule.
The phosphodiester bonds that hold the nucleotides together in the DNA strand are formed between the 5' phosphate group of one nucleotide and the 3' hydroxyl group of the next nucleotide.