5' phosphate group

The 5' phosphate group refers to the end of a DNA strand which has a phosphate group attached to its carbon. The 3' end has a hydroxyl group attached to it; this difference allows for the replication of the DNA strand as DNA polymerase for example can only add nucleotides in a 5' to 3' direction. It is vital for a primer to have a 3' hydroxyl group in a DNA sequencing reaction but it is not vital for a primer to have a 5' phosphate group.