Lagging strands and Okazaki fragments
DNA can only be replicated from the 5' end of a new strand to the 3' end. New nucleotides must be added to the 3' end of a strand.
The two anti-parallel parent strands have different directions.
The leading strand is exposed in the 3' to 5' direction. The complementary strand can form continuously along the leading strand as the DNA 'unzips', since nucleotides are free to join the 3' end of the strand that is forming.
The lagging strand is exposed in the 5' to 3' direction. The new strand cannot form in the 3' to 5' direction so instead it is made in short sections.
This process is complex and so production of this new strand lags behind.
The segments formed by this discontinuous process are called Okazaki fragments.
They are joined together by an enzyme called DNA ligase.