The eukaryotic chromosomal DNA is divided into hundreds to thousands of independent replication segments called replicons. Each replicon is replicated from one replication origin. In the S phase of the cell cycle, individual replicons are gradually activated and subsequently replicated (Edenberg & Huberman, 1975; Hand, 1978). The size of particular replicons varies and is usually within the range of 30–450 kbp. On the other hand, much smaller (shorter than 10 kbp) or much longer (longer than 1 Mbp) replicons have also been observed (Berezney et al., 2000; Edenberg & Huberman, 1975; Hand, 1978; Hyrien & Mechali, 1993; Jackson & Pombo, 1998; Yurov & Liapunova, 1977). It is supposed that several adjacent replicons are synchronously activated in the S phase (Edenberg & Huberman, 1975; Hand, 1978), whereas the number of replicons in one such group is lower than ten (Jackson & Pombo, 1998; Ma et al., 1998). The evidence of such replicon organisation comes mainly from studies mapping the newly-synthesised DNA on stretched DNA fibres (Edenberg & Huberman, 1975; Hand, 1978; Jackson & Pombo, 1998). The replication of replicons proceeds bi-directionally by means of two replication forks and is terminated when the replication forks of two adjacent replicons meet (Blow & Dutta, 2005; Heintz, 1996). The so-called “licensing” of replication origins is performed before the actual initiation of DNA synthesis. First, many different proteins such as the ORC complex, Cdc6 protein, Cdt1 protein, MCM 2-7 protein complex bind in that exact order at the sites of replication origins (Bell & Dutta, 2002; Blow & Dutta, 2005; DePamphilis, 2003; Diffley, 2004; Chesnokov, 2007; Lei & Tye, 2001; Sasaki & Gilbert, 2007; Stillman, 2005; Takahashi et al., 2005). Later, due to the regulation mechanisms, some of the proteins are removed and other new proteins are bound to DNA instead of them. Examples include the Cdc45 protein, MCM10 protein or GINS protein complex (Bauerschmidt et al., 2007; Diffley & Labib, 2002). Cyclin-dependent kinases and Dbf4-dependent kinase are important for the changes in the protein-DNA interactions (Bauerschmidt et al., 2007; Diffley & Labib, 2002). All of these processes result in the formation of two replication complexes, also referred to as replisomes, at the site of the active replication origin that ensure the synthesis of DNA in mutually opposite directions (Baker & Bell, 1998; Johnson & O'Donnell, 2005; Waga & Stillman, 1998). The main components of replisomes are a helicase complex enabling the unwinding of the parental DNA strands, DNA polymerases responsible for the duplication of DNA, and a complex of polymerase and primase (Langston et al., 2009). It is supposed that the MCM2-7 protein complex, which is necessary for the “licensing” of replication origins, plays also the role of a helicase in the common complex with Cdc45 protein and GINS complex (Aparicio et al., 2006).