In this article we will discuss about the theta structure of DNA, explained with the help of suitable diagrams.
It is a structure formed in replication of circular DNA. Since this structure resembles the Greek letter θ it is known as the theta structure. Semiconservative mode of DNA replication in bacterial chromosome begins at the origin of replication called ori C, a region of DNA where specific initiation protein gets attached. The ori C in Escherichia coli cell-chromosome consists of about 245 bp and contains three 13-bp repeat sequences and four 9-bp repeat sequences where DNA protein initially binds.
Therefore, DNA polymerases move bio directionally from origin of termination of DNA replication abbreviated as ter (termination of replication) resulting two replication forks that move in opposite direction around circular chromosome. The bidirectional replication forks move at identical speed after initiation, therefore, both replication forks meet at the termination site. The terminus in E. coli and in some other bacteria is not exactly opposite to the origin in the circular chromosome occurring in bacterial cells.
The terminus possesses four ter sequences which are are:
1. The ter A and the ter D which replicate termination of one strand of DNA.
2. The ter B and the ter C terminate the second strand of the DNA.
All the ter sequences contain a 23-bp consensus sequence which is easily recognized by a turs protein which prevents the replication of fork further.
In this way a theta structure is formed in replication of circular DNA in the bacteria.