## DNA can make exact copies of itself

Cell division is the process by which genetic information is passed between generations. The sequence of the individual phases leading to cell division is referred to as the cell cycle. However, the actual division and creation of daughter cells is preceded by the activity of various preparatory mechanisms, which include DNA duplication, i.e. DNA replication. This process takes place during the S-phase of the cell cycle and is governed by the semi-conservative model of replication, which was introduced by the discoverers of the DNA structure, Watson and Crick, and later experimentally confirmed by Matthew Meselson and Franklin Stahl. The Meselson and Stahl model states that each newly formed DNA molecule contains one strand originating from the parent molecule, which serves as a template for the synthesis of the second strand (Figure 3.3). Synthesis of the new strand takes place on the basis of the principle of base pairing, which means that if the sequence of the template is known this can be used to derive the sequence of the new, just emerging strand.

Several different enzymes and proteins are involved in DNA replication. DNA helicase unravels the double stranded molecule into two separate strands, which can then be replicated. When they are unravelled, tension is created, which is reduced by the DNA-topoisomerase enzyme, which also prevents the separated strands from knotting. After successful separation, SSB-proteins (Single Strand Binding proteins) bind to prevent the stands from rejoining and reforming a double strand. DNA polymerases play the most important role in DNA replication. Polymerases are a group of enzymes with similar properties responsible for the synthesis of a new DNA strand (Figure 3.4). Some polymerases also show exonuclease activity, which means that after creating a short section of newly synthesied DNA, the enzyme can check whether the correct nucleotides have been included and possibly remove and repair their mistake. In this way, the cells eliminate a large number of potential mutations that could have harmful consequences. The synthesis of a new strand takes place in the direction from the 5'-end of DNA to its 3'-end (Figure 3.4). In a simplified way, it can be imagined that DNA polymerase always extends the 3'-end of the DNA strand. Cells cannot synthesise a new strand de novo, that is, from nothing. DNA polymerase always requires the presence of a template that serves as a model for the formation of a strand. At the same time, deoxynucleoside triphosphates, or dNTPs for short, are needed, which represent the basic building blocks from which DNA polymerases can assemble a new DNA strand. For their activity, DNA polymerases also need magnesium ions (Mg2+) and a short RNA-primer that borders the replicated section and at the same time provides the enzyme with a free 3'-OH group (at the 3'-end of DNA) to which nucleotides are added during DNA synthesis.

Replication always begins at an area known as the replication origin. While prokaryotic cells, e.g., bacteria have one origin of replication present in their circular DNA molecules, large linear chromosomes that are part of eukaryotic cells have many more origins of replication. At the origin of replication, the DNA helix is locally untangled, and a replication bubble is formed. The place where the double strand of DNA has split into two separate strands is referred to as a replication fork. Replication forks move along the DNA molecule and ensure its replication (Figure 3.4).

Replication occurs in three basic phases:

1. Initiation,
2. Elongation,
3. Termination.

After the double strand is unravelled, each strand replicates independently and at different rates. We distinguish the continuous synthesis of the leading strand and the synthesis of the lagging strand, which takes place in smaller sections and is slower.