Why is replication on one strand of DNA continuous while on the other strand the replication must be discontinuous?

New strands of DNA can only be created in one direction - 5' to 3'. This is because only the 3' end of the DNA is able to join to a new nucleotide.

The two strands of DNA are antiparallel - meaning they run in different directions. Therefore only one strand (called the leading strand) is running in the correct direction for continuous replication. The other strand (called the lagging strand) must first be looped around so that small sections can be replicated in the correct direction.