DNA replication: mechanism, components, and biological importance

Replication is the duplication of nucleic acid molecules. In cells, the nucleic acid molecules present in the form of a double helix as a DNA double strand are the carriers of the genetic information. In (eukaryotic) cells with a cell nucleus, they are located in the nucleus and are duplicated here for a nuclear division before a cell division. During re(du)plication, two identical DNA double-stranded molecules are created from one. This means that the same genetic information can be assigned to the nuclei of two daughter cells.

In molecular biology, the process of replication consists of a series of steps that are controlled by various enzymes. The construction of new DNA strands is carried out by an enzyme called DNA polymerase, which requires a single strand as a template. This is because a complementary DNA strand can only be synthesized on the basis of this template by linking the appropriate nucleotide with the previous one. Therefore, the rotation of the helix must first be unwound at the site of the replication origin by the enzyme topoisomerase. Then the double strand can be separated by the enzyme helicase in this region - by loosening the hydrogen bonds between the base pairs - into two single-stranded sections. With their spreading, a so-called replication fork is formed, which is kept stable by single-strand-binding proteins (SSB proteins). A primase is now able to attach short RNA segments to the two single strands, so-called primers, which are used to initiate the actual strand synthesis. The DNA polymerase can then attach to these primers and - using the existing single strand as a template by means of base pairing - continuously link together matching deoxyribonucleotides to form the new complementary strand. The DNA polymerase attaches a new building block of the polynucleotide to its 3′-end, thus always synthesizing the new strand complementary in the 5′→3′-direction, while moving accordingly along the antiparallel template strand in the 3′→5′-direction.

Since the two individual strands of the former double strand, each serving as a template, are also complementary to each other and run antiparallel to each other, a synthesizing DNA polymerase moves on one template strand in the direction of the migrating replication fork and on the other in the opposite direction. Thus, the so-called leading strand can be continuously synthesized on one strand of the template. On the other matrix strand, on the other hand, the replication process is discontinuous. The opposite new strand, called lagging strand, has to be synthesized piece by piece by a DNA polymerase. This process produces the so-called Okazaki fragments. Their RNA primers are then replaced by DNA by another DNA polymerase. The fragments can then be linked by a DNA ligase to form a complete strand. The result of replication is two (almost) identical DNA double strands, one half of which is new. Replication is thus semi-conservative, because each double strand is composed of a pre-existing and a newly synthesized single strand.

Watson and Crick already recognized that the bases paired in the double strand are a prerequisite for the formation of new DNA: "It has not escaped our attention that the specific pair formation which we presuppose here immediately suggests a possible copying mechanism for the genetic material." Three possibilities for replication were conceivable: dispersive or totally conservative or semi-conservative. The latter model was proved by the Meselson-Stahl experiment. According to this, the original double strand is opened, then both single strands serve as a template (as a conservative matrix) in the replication. The new strand is formed according to the rules of Watson-Crick base pairing. Replication according to the semiconservative principle represents the generally accepted mechanism. All other principles are special cases, each of which has only been partially proved.

Since all bacteria and the cell nuclei of all eukaryotes contain double-stranded DNA (dsDNA), this replication mechanism occurs most frequently in nature. Exceptions are some mitochondria, where a different mechanism takes place, and plasmid and viral genomes, where the genetic information may be present as single-stranded DNA (ssDNA). Here, a completely different mechanism had to be found: the rolling circle. In retroviruses, whose genetic information is always present in the form of an RNA double or single strand, replication is taken over by the host cell in that the RNA is transcribed into DNA by a reverse transcriptase and incorporated into the host genome.