Question

Theta replication of DNA not occur in
A. E. coli
B. Mitochondrial
C. Chloroplast
D. Nucleus

Answer 1

We are familiar with the word theta replication of DNA i.e. replication occurs in circular DNA molecules. Theta structure is an intermediate formed during replication of circular DNA and it does not occur in eukaryotes. In theta replication ds (double stranded) DNA unwinds and produces ss (single stranded) nucleotides on which new DNA can be synthesized.

Complete answer:
Theta replication takes place in circular DNA such as in E -coli and in many other bacteria because it forms a structure similar to the Greek letter theta that is why this replication is called as theta replication.
In theta replication, at the origin point ds DNA unwinds and produces ss nucleotide on which new DNA can be synthesized. The unwinding of double helix forms a loop which is known as a replication bubble and the unwinding may occur at one end or at both the ends of the bubble.
The point of unwinding where two single stranded nucleotides separate from double stranded DNA helix is called a replication fork. These replication forks proceed around the circle and two circular DNA molecules are produced.
The enzymes needed for theta replication are Gyrase type of topoisomerase, primase- synthesize a short segment which allow DNA elongation, ligase, and Topoisomerase II- break some DNA bonds so that the daughter chromosomes do not remain interlocked.
Theta replication is found in prokaryotes, mitochondria and chloroplast as they have circular DNA but it does not occur in eukaryotic chromosomes present in nucleus because they have linear double stranded DNA.

Hence, the correct answer is option (D) i.e. Nucleus as theta replication does not occur in it.

Note: In theta model two replication forks can independently proceed around the DNA ring and structure similar to Greek word theta is formed. It only takes place where there is circular DNA like bacteria, mitochondria, etc. If there are two replication forks and both of them proceed in both the directions then it is known as bidirectional replication.