Question

What is an Okazaki fragment? In which strand of replicating DNA are Okazaki fragments found? Based on the properties of DNA polymerase, why is it necessary to make these fragments?

Short answer

Okazaki fragments are short DNA sequences formed on the lagging strand during DNA replication. They are necessary because DNA polymerase, the enzyme that catalyzes DNA synthesis, can only add nucleotides in the 5' to 3' direction, and thus, while the leading strand can be synthesized continuously, the lagging strand has to be synthesized discontinuously in the form of Okazaki fragments.

Step-by-step solution

Defining Okazaki Fragments

Okazaki fragments are short sequences of DNA nucleotides (approximately 150 to 200 base pairs long in eukaryotes), which are synthesized discontinuously and later linked together by the enzyme DNA ligase to create the lagging strand during DNA replication.

Locating Okazaki Fragments in DNA strand

Okazaki fragments are found in the lagging strand of the replicating DNA. So during DNA replication, one strand (leading strand) is replicated continuously in the 3' to 5' direction, while the other strand, known as the lagging strand, is replicated discontinuously in fragments known as Okazaki fragments, in the 5' to 3' direction.

Linking Okazaki fragments to the Properties of DNA Polymerase

The reason why DNA polymerase makes these fragments lies in its properties. DNA polymerase can only add nucleotides in the 5' to 3' direction. On the leading strand, replication can proceed continuously, but on the lagging strand, replication is discontinuous, with DNA synthesized in short stretches (Okazaki fragments) moving away from the replication fork. Thus, these fragments are necessary due to the unidirectional nature of DNA polymerase, making it unable to synthesize both strands continuously.