## Discontinuous synthesis of the lagging strand

The synthesis of the second, oppositely oriented strand differs from the replication described so far. While the leading strand replicates continuously, the opposite strand is generated discontinuously through Okazaki fragments and is therefore called the lagging strand. Okazaki fragments were named after their discoverers, husband and wife team Reiji and Tsuneko Okazaki, who described the formation of the fragments in 1968 during their experiments focused on DNA replication .

After the formation of the replication fork, the lagging strand not provide a free 3'-end in the direction of DNA replication (because it is turned in the opposite direction). However, cells can deal with this problem with the presence of RNA primers. Several RNA primers are attached to the lagging strand, creating sections with a free 5'-end which is recognizable by DNA polymerase. After the synthesis of short DNA sequences, a hybrid DNA strand is formed, where the DNA is interrupted by several RNA (from the RNA primers). However, the presence of the RNA primers is not desirable, and so they must be removed. This is done by another enzyme, RNase, which can cleave RNA sequences. In this way, larger gaps will again appear on the synthesised strand, which can be filled by DNA polymerase. The result is that the emerging strand is formed only from DNA. Since the synthesis did not proceed smoothly, but in fragments, it is necessary that the individual sections of DNA are joined into a single, uninterrupted DNA strand. The joining of DNA sections across these small gaps is achived by DNA ligase, which terminates replication and forms a complete strand (Figure 3.5).

The chromosome will get shorter by 50-250bp each replication cycle, which can be detrimental. To ensure this does not happen, important and specific structures are present at the ends of linear chromosomes, which are called telomeres. Their replication is ensured by the enzyme telomerase, whose activity is influenced by several factors. Telomerase is highly active in healthy human cells during embryonic development, when intensive cell division occurs, and in sex or stem cells. In other cells, telomerase is inactive and so the telomeres are not restored, which is manifested by their gradual shortening and the associated aging of the cells. Cells that have undergone repeated replication and division may harbour accumulated mutations with a potentially deleterious effect. Therefore, it is necessary for such cells to be recognised and safely eliminated. On the contrary, one of the characteristics of tumour cells is the presence of active telomerase, which ensures the continuous replication of telomeres. Thus, they do not shorten and the cells do not age, therefore cancer cells become "immortal" (you can read more about cancer in chapter 9 - When cells go crazy).