What is a primer in DNA replication

DNA (replication)

The replication or reduplication of the DNA naturally takes place in eukaryotes during cell division (mitosis), namely during the S phase (synthesis phase), shortly before the cell divides. Replication usually means an exact duplication of the DNA, i.e. the set of chromosomes, so that the new cell receives the complete genetic information.

In order to obtain a duplication, the DNA strand (double helix) is first split up so that the separate base pairs located in the 'interior' can be added (semiconservative principle, i.e. semi = half, conservare = preserved, half of the DNA is preserved, the other adds). The starting point (origin of replication or origin, in eukaryotes there are several due to the slowness of the DNA polymerase) is often in the middle area of ​​the DNA strand. From there, replication is usually carried out in both directions (bidirectional).

DNA replication is divided into four phases:

In the initiation phase, the DNA, which is wound up in many loops, is first "disentangled". Similar to an untied telephone cord, torsional stresses arise, which are eliminated by the so-called topoisomerase by causing targeted single or double strand breaks, smoothing the strand and then patching it again. The topoisomerase is followed by the helicase, which unwinds and splits the double helix structure. This opens the DNA strand like a zipper in two replication forks and can now be duplicated on the inside. In order to prepare the elongation phase, i.e. to pave the way for the main enzyme for elongation, DNA polymerase, primers are synthesized by primase, an RNA polymerase. They serve to provide the DNA polymerase with a suitable possibility of binding to the single strand to be doubled. These primosomes also serve as a regulatory mechanism, since no elongation can take place without a suitable primer.

In the elongation phase, the DNA polymerase docks to the primer of the respective strand and now moves along the DNA section in front of it (polymerisation). It is important that the DNA polymerase, like the RNA polymerase in protein biosynthesis, can only read from the 5'-direction to the 3'-direction. That works wonderfully with one strand (the guide strand), here it moves forward directly behind the helicase. In the case of the opposite strand (following strand), on the other hand, replication can only be carried out piece by piece. New primers are regularly applied here so that the DNA polymerase can continue to work. It then works its way ’backwards’ from the primer to the primer of the previous section. The resulting sections are called Okazaki fragments.

So that primer residues (RNA fragments) are not stuck everywhere on the following strand, the RNA fragments are dissolved again and the gaps are closed by a second DNA polymerase. The end of the process is the DNA ligase, which closes the connections between the old and new complementary single strands of the DNA. This part is called the interphase.

Unlike prokaryotes, eukaryotes do not have a termination sequence that announces the end of replication to the polymerase. The polymerase simply stops as soon as the DNA strand runs out. However, there is a small problem here: At the end of the following strand (the end of the chromosome or telomere), a primer can no longer be added, nor can DNA polymerase add to the last base pairs. As a result, the chromosome becomes shorter and shorter. This shortening is initially without consequence, since the ends contain non-coding elements. However, if these are used up, the cell can no longer divide, and cell death usually occurs. This effect is associated with the aging of organisms and with the development of cancer.