In eukaryotes, the DNA strands are linear, and DNA polymerase can't replicate the very ends of the DNA strands! These ends are "protected" by repeated sequences called "telomeres." Either the chromosome gets shorter with each replication, or else a special enzyme-nucleic acid complex called telomerase adds new telomeres to the ends. A prokaryotic chromosome is circular and thus does not have the problem of having ends.
Eukaryotic DNA is wound around histones, coiled, and supercoiled -- to replicate it, there has to be unwinding mechanisms, and mechanisms to reduce the degree of coiling. In prokaryotes the winding problem is much less, and there aren't any histones