Final answer:
The 5' end of a DNA sequence is characterized by a phosphate group, which distinguishes it from the 3' end that has a hydroxyl group.
Step-by-step explanation:
The 5' end of a DNA sequence is characterized by the presence of a phosphate group. DNA molecules have a backbone made of sugar and phosphate groups, where the nucleotides are linked together by phosphodiester bonds between the 5' phosphate group of one nucleotide and the 3' hydroxyl group of another.
At the very end of the chain, the 5' end of a nucleic acid strand has a free phosphate group, distinguishing it from the 3' end, which has a free hydroxyl group.