Why is phosphate important at restriction-cut ends?

Correct answer: For phosphodiester bond formation by DNA ligase

DNA ligase joins cut DNA by forming a phosphodiester bond between the 3′-OH end of one fragment and the 5′-phosphate end of another fragment. Thus, a 5′-phosphate at a restriction-cut end is required for ligation. DNA in gels is stained by dyes such as ethidium bromide, not by phosphate groups. Exam tip: remember the ligase-end pair—3′-OH and 5′-phosphate.