A theta structure is an intermediate structure formed during the replication of a circular

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

. Two

replication fork In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms, acting as the most essential part of biological inheritanc ...

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

theta Theta (, ) uppercase Θ or ; lowercase θ or ; ''thē̂ta'' ; Modern: ''thī́ta'' ) is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth 𐤈. In the system of Greek numerals, it has a value of 9. Gree ...

" (θ). Originally discovered by John Cairns, 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 (nucleoside#List of nucleosides and corresponding nucleobases, symbol dT or dThd), also known as deoxythymidine, deoxyribosylthymine, or thymine deoxyriboside, is a pyrimidine nucleoside, deoxynucleoside. Deoxythymidine is the DNA nuc ...

, quenching rapidly and then

autoradiograph An autoradiograph is an image on an X-ray film or nuclear emulsion produced by the pattern of decay emissions (e.g., beta particles or gamma rays) from a distribution of a radioactive substance. Alternatively, the autoradiograph is also availab ...

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.

References

DNA replication {{Molecular-biology-stub