The function of DNA polymerase is to continuously add deoxynucleotides to DNA chains. However, we know that DNA is polar, with exposed hydroxyl groups at one end of 3'-OH and exposed phosphate groups at the other end of 5'-P.. At present, all DNA polymerases can only add deoxynucleotides at the 3'-OH end.

Then, the contradiction came out. Where did the 3'-OH end come from when DNA was first synthesized?

In order to solve this contradiction, we choose to synthesize a short RNA chain first, and then add deoxynucleotides to the 3'-OH end of RNA.

RNA is chosen because RNA polymerase does not need to add nucleotides at the 3'-OH end.

Supplement: As we know above, the initial synthesis is to synthesize a section of RNA first, so the DNA behind this RNA can only be synthesized later, and this DNA occupied by RNA cannot be copied, which means that this DNA will be "lost" when copied, and this process is irreversible. Once lost, it's gone.

Fortunately, however, there is often a long piece of DNA at both ends of DNA that is meaningless, called telomere. Its function is to "be lost" and prevent useful DNA from being "lost".

At present, it is found that a few cells have an enzyme related to telomere repair, called telomere synthase, but generally somatic cells do not, only blastocyst cells, germ cells and some microorganisms do.

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