The theta mode of replication is a type of replication that is seen in circular DNA molecules. A circular chromosome is a form of circular DNA found in bacteria and archaea that has no free ends, unlike the linear DNA strands seen in most eukaryotes.
There are three types of replication modes in circular DNA: rolling-circle, strand displacement, and theta. Let us discuss the theta mode of replication briefly.
The theta mode of replication is similar to the linear mode of chromosome replication, where leading strands are replicated continuously, and lagging strands are replicated discontinuously. No DNA breaks are required in this mode of replication.
The theta mode of replication is commonly found in bacteria, such as Escherichia coli and Bacillus subtilis, as well as in chloroplast and mitochondria. It is referred to as the theta mode of replication because of an intermediate form that resembles the Greek letter θ.
Procedure of Replication
Replication of circular DNA takes place in 3 stages: initiation, elongation, and termination.
- Initiation: The process of replication begins at a conserved sequence site known as the origin of replication or oriC. At the oriC, initiator proteins begin to assemble. The assembly of initiator proteins is a regulated process to make sure that replication takes place only once per cell cycle.
- Elongation: At the time of elongation, the proteins at the oriC begin to move around each arm of the chromosome in a direction opposite to oriC. This creates two identical copies of the chromosome, and the process is known as bidirectional replication. Each arm of the chromosome is known as a replichore, and the whole assembly with proteins is known as a replisome.
In the front of the replisome is an enzyme called DNA helicase that unwinds the two strands of the circular DNA and creates a replication fork. The unwound strands then serve as a template for the DNA polymerase and other assembled proteins for the synthesis of the complementary strand.
At the time of replication, when the replication fork moves around the circular chromosome, a theta-shaped structure is formed. To demonstrate this, John Cairns, a British physician, radioactively labelled the chromosomes of E.coli with 3H-thymidine and observed it under X-ray on an electron micrograph grid.
- Termination: Opposite the oriC on the circular chromosomes is the terminus region, which is composed of several “Ter” sites. The two moving replication forks get arrested at the Ter site by the replication terminator protein and meet at a common point in the chromosome. The enzyme machinery is disassembled, and the daughter chromosomes are separated.
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Also Read:
- DNA Replication and Meselson and Stahl’s Experiment
- Difference between Prokaryotic and Eukaryotic Replication
- MCQs on DNA Replication for NEET 2022
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