The Autocatalytic Function of DNA Replication

Enzymes have been isolated from the most diverse sources that can support DNA synthesis in a cell-free system. They are called DNA polymerases or DNA replicases. They need, in addition to other cofactors, DNA as primer and deoxyribonucleoside-5 -triphosphates as DNA building blocks. The triphosphates align themselves on the DNA single strand introduced as primer according to the rules of base pairing an 

According to the findings in a cell-free system it seems that Kornberg s DNA polymerase forms only quite short pieces of DNA. A 

It seems likely that the DNA polymerase first investigated by Kornberg is important in vivo primarily for the repair of damaged DNA strands. Normal replication is carried out by other DNA polymerases which, in principle, work quite similarly. 

We have just mentioned a cell-free system derived from viruses and microorganisms. However, evidence for the semiconservative nature of DNA reduplication has also been obtained in higher plants. One piece of evidence consisted in repeating an experiment which had previously been carried out by Meselson and Stahl on bacteria and which is named after them the Meselson-Stahl experiment. 

Cells from tobacco callus tissue can be grown in liquid nutrient media as well as on solid. In a liquid medium the number of the cells doubles in about two days. Thus, in about two days the cells pass through one round of DNA synthesis. Now the cells were first maintained for several rounds of DNA synthesis in a medium with compounds containing heavy nitrogen, N. This leads to N being incorporated in the purine and pyrimidine bases of the DNA. Finally, all of the DNA is present as heavy DNA with N bases. This heavy DNA can be separated from normal DNA with N bases in a cesium chloride gradient in the ultracentrifuge. 

---