Question

Describe the discontinuous synthesis of the lagging strand in DNA replication.

Short answer

The lagging strand is synthesized discontinuously in short segments called Okazaki fragments, which are later joined together.

Step-by-step solution

Initial Unwinding

The enzyme helicase unwinds the double-stranded DNA, creating a replication fork with leading and lagging strands.

Primer Synthesis

Primase synthesizes short RNA primers that provide starting points for DNA synthesis on the lagging strand.

Okazaki Fragment Synthesis

DNA polymerase III adds nucleotides to the RNA primer, extending the new DNA strand in short segments called Okazaki fragments.

Primer Removal

The enzyme DNA polymerase I removes the RNA primers and fills in the gaps with DNA nucleotides.

Fragment Joining

DNA ligase seals the nicks between Okazaki fragments, creating a continuous DNA strand.

Key concepts

DNA replication

DNA replication is the biological process of creating two identical replicas from one original DNA molecule. This occurs in all living organisms as a way to pass genetic information from one generation to the next. The process is semi-conservative, which means each of the two resulting DNA molecules has one old and one new strand. The replication process begins at specific locations in the DNA called origins and involves several important enzymes, such as helicase, primase, DNA polymerases, and DNA ligase.

helicase

Helicase is a crucial enzyme in DNA replication. Its main function is to unwind the double-stranded DNA, creating a forked structure known as the replication fork. This action is essential because it exposes the single-stranded DNA, allowing other enzymes to access the genetic information.
Helicase works by breaking the hydrogen bonds between the complementary bases, effectively separating the two strands. This separation is the first step in both the leading and lagging strand synthesis.

primase

Primase is an enzyme that synthesizes short RNA primers on the DNA template. These primers are essential because DNA polymerases cannot initiate DNA synthesis on their own; they can only add nucleotides to an existing strand. The primers provide a starting point for DNA polymerase to begin adding DNA nucleotides.
In the context of the lagging strand, primase must repeatedly provide new primers as replication progresses, enabling the synthesis of multiple short DNA fragments called Okazaki fragments.

Okazaki fragments

Okazaki fragments are short sequences of DNA nucleotides (approximately 100 to 200 nucleotides in eukaryotes) synthesized discontinuously and later linked together on the lagging strand during DNA replication. Because the lagging strand is oriented in the 3' to 5' direction, DNA synthesis cannot occur continuously. Instead, it proceeds in short bursts as nucleotides are added, creating these fragments.
The synthesis of Okazaki fragments starts with an RNA primer synthesized by primase, after which DNA polymerase III extends the fragment. These fragments are eventually connected, forming a continuous DNA strand.

DNA polymerase III

DNA polymerase III is the primary enzyme responsible for DNA synthesis in Escherichia coli and many other bacteria. It elongates new DNA strands by adding nucleotides to an RNA primer or pre-existing DNA strand in a 5' to 3' direction.
On the lagging strand, DNA polymerase III extends RNA primers to form Okazaki fragments. This enzyme ensures high fidelity in DNA replication due to its proofreading activity, which corrects errors during DNA synthesis.

DNA polymerase I

DNA polymerase I plays a critical role in DNA replication by removing RNA primers used during initiation and replacing them with DNA nucleotides. This enzyme has both 5' to 3' exonuclease activity to remove RNA primers and 5' to 3' polymerase activity to fill in the resulting gaps.
After DNA polymerase III synthesizes Okazaki fragments, DNA polymerase I ensures that the RNA primers are completely removed and replaced with DNA. This step is crucial for creating a continuous DNA strand without leaving any RNA segments.

DNA ligase

DNA ligase is the enzyme responsible for sealing the nicks or breaks in the sugar-phosphate backbone of the DNA molecule. During lagging strand synthesis, gaps remain between Okazaki fragments after DNA polymerase I replaces RNA primers with DNA.
DNA ligase catalyzes the formation of phosphodiester bonds, effectively linking these fragments together into a continuous strand. Without DNA ligase, the newly synthesized DNA would remain fragmented and non-functional.