DNA replication: semiconservative and its enzymes

How one DNA molecule becomes two identical ones. The semiconservative model, the key enzymes and why one strand is made 'in pieces'.

Direct answer: DNA replication is semiconservative: the two strands separate and each serves as a template, so each daughter molecule has one old strand and one new strand. DNA polymerase always synthesizes the new strands in the 5'→3' direction.

Main enzymes

Enzyme	Function
Helicase	Unwinds the double helix (breaks the hydrogen bonds).
Primase	Lays down an RNA primer to start.
DNA polymerase	Adds nucleotides in the 5'→3' direction.
Ligase	Joins the fragments of the lagging strand.

Leading and lagging strand

At the replication fork, the leading strand is synthesized continuously. The lagging strand is made in short pieces called Okazaki fragments, which ligase joins afterward.

Exam trap: DNA polymerase can only add nucleotides in the 5'→3' direction. That is why one strand (the lagging one) cannot be made in one go and is synthesized as Okazaki fragments.

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Frequently asked questions

Why is replication semiconservative?

Because each daughter molecule keeps one original (template) strand and one new strand.

What are Okazaki fragments?

The short pieces in which the lagging strand is synthesized, later joined by ligase.

In which direction does DNA polymerase work?

Always in the 5'→3' direction.