A Theta structure is an intermediate structure formed during the replication

DNA replication

DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...

 of a circular DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

 (prokaryote DNA), two replication fork

Replication fork

The replication fork is a structure that forms within the nucleus during DNA replication. It is created by helicases, which break the hydrogen bonds holding the two DNA strands together. The resulting structure has two branching "prongs", each one made up of a single strand of DNA...

s can proceed independently around the DNA ring and when viewed from above it resembles the Greek letter "theta

Theta

Theta is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth...

" (θ). Originally discovered by John Cairns

John Cairns (biochemist)

John Forster Cairns FRS is a British physician and molecular biologist who made significant contributions to molecular genetics, cancer research, and public health....

, it led to the understanding that (in this case) bidirectional DNA replication could take place. Proof of the bidirectional nature came from providing replicating cells with a pulse of tritiated thymidine

Thymidine

Thymidine is a chemical compound, more precisely a pyrimidine deoxynucleoside. Deoxythymidine is the DNA nucleoside T, which pairs with deoxyadenosine in double-stranded DNA...

, quenching rapidly and then autoradiograph

Autoradiograph

An autoradiograph is an image on an x-ray film or nuclear emulsion produced by the pattern of decay emissions from a distribution of a radioactive substance...

ing. Results showed that the radioactive thymidine was incorporated into both forks of the theta structure, not just one, indicating synthesis at both forks, in opposite directions around the loop.