The replication fork is formed by helicase. It forms when the DNA strands have been separated to initiate DNA replication.
The replication bubble is formed from bidirectional DNA replication. If viewed via a microscope, it appears like a bubble in the DNA strand.

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- Lagging Strand
- Leading Strand
- Okazaki Fragment
- Replication Fork
- Replication Bubble

Key Components of DNA Replication

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This is the strand of DNA where DNA polymerase can add DNA nucleotides towards the replication fork continuously. DNA polymerase adds nucleotides in the 5' to 3' direction so DNA polymerase can add nucleotides to make the leading strand while moving towards the replication machinery. The parental DNA template for the leading strand is 3' to 5' to be complementary to the leading strand. 

Leading strand

Okazaki fragments are replicated DNA strands formed on the strand being replicated away from the replication fork (template is 3' to 5'). These fragments are separated by RNA primers. Because the DNA polymerase is moving away from the replication fork it must form replicated strands in Okazaki fragments. 
These fragments are named after the Japanese scientists Reiji and Tsuneko Okazaki who discovered them. 

Okazaki Fragments

Replication Fork vs. Replication Bubble

The lagging strand is replicated discontinuously because its synthesis proceeds opposite of the replication fork. It is synthesized in fragments that are divided by RNA primers known as Okazaki fragments. 

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