The DNA synthesis takes place as part of the replication of DNA. The DNA is the carrier of genetic information and controls all life processes. In humans, as in all other living things, it is located in the nucleus of the cell. DNA is double-stranded, similar to a twisted rope ladder called a helix. This double helix consists of two DNA molecules. Each of the two complementary single strands is made up of a backbone of sugar molecules (deoxyribose) and phosphate residues, to which the four organic, nitrogenous bases guanine, adenine, cytosine and thymine are bound. The two strands are bound to one another via hydrogen bonds between opposing, so-called complementary, bases. According to the principle of complementary base pairing, only linkages between guanine and cytosine on the one hand and adenine and thymine on the other hand are possible.
DNA synthesis takes place as part of the replication of DNA. The DNA is the carrier of genetic information and controls all life processes.
In order for DNA to replicate, the process of DNA synthesis is necessary. It describes the structure of deoxyribonucleic acid (abbreviated as DNA or DNA). The key enzyme here is DNA polymerase. This is the only way cell division is possible.
For replication, the twisted DNA double strand is first untied by enzymes, so-called helicases and topoisomerases, and the two single strands are separated from one another. This preparation for the actual replication is called initiation. Now a piece of RNA is synthesized, which the DNA polymerase needs as a starting point for its enzymatic activity.
During the subsequent elongation (strand extension), each single strand can be used by the DNA polymerase as a template to synthesize the complementary counterpart DNA. Since one of the bases can only bind to one other base, it is possible to use a single strand to reconstruct the other, associated strand. This assignment of the complementary bases is the task of DNA polymerase.
The sugar-phosphate backbone of the new DNA strand is then linked by a ligase. This creates two new DNA double strands, each containing a strand from the old DNA helix. The new double helix is therefore called semiconservative.
Both strands of the double helix have a polarity that indicates the orientation of the molecules. The direction of the two DNA molecules in a helix is opposite. Since the DNA polymerase only works in one direction, only the strand that is in the appropriate orientation can be built up continuously. The other strand is synthesized piece by piece. The resulting DNA segments, also known as Okazaki fragments, are then joined by the ligase.The termination of DNA synthesis with the help of various cofactors is known as termination.
Since most cells only have a limited lifespan, new cells must constantly be formed in the body through cell division to replace those that are dying. The red blood cells in the human body, for example, have an average lifespan of 120 days, whereas some intestinal cells have to be replaced by new cells after one or two days. This requires mitotic cell division, in which two new, identical daughter cells are created from a mother cell. Both cells need the complete set of genes, which means that, unlike other cell components, it cannot simply be split up. So that no genetic information is lost during division, the DNA must be doubled (“replicated”) before division.
Cell divisions also take place during the maturation of male and female germ cells (egg and sperm cells). In the meiotic divisions that take place, the DNA is not doubled because a reduction by half of the DNA is desired. When the egg and sperm cells fuse, the complete number of chromosomes, the packaging state of the DNA, is reached again.
DNA is essential for the functioning of the human body and all other organisms as it is the basis for the synthesis of proteins. A combination of three consecutive bases stands for one amino acid, which is why we speak of a triplet code. Each base triplet is “translated” into an amino acid via messenger RNA (mRNA); these amino acids are then linked to proteins in the cell plasma. The mRNA differs from the DNA only in one atom in the sugar residue of the backbone and in a few bases. MRNA mainly serves as an information carrier for the transport of information stored in the DNA from the cell nucleus into the cell plasma.
An organism that is not capable of DNA synthesis would not be viable because new cells have to be formed by cell division during the embryonic development. Errors in DNA synthesis, that is, individual incorrectly incorporated bases that do not follow the principle of complementary base pairing, occur relatively often. Because of this, human cells have repair systems. These are based on enzymes that control the DNA double strand and correct incorrectly inserted bases using various mechanisms.
For this purpose, for example, the area around the wrong base can be cut out and rebuilt according to the synthesis principle explained. However, if the cell's DNA repair systems are defective or overloaded, base mismatches, so-called mutations, can accumulate. These mutations destabilize the genome and thus increase the likelihood of new errors in the course of DNA synthesis. An accumulation of such mutations can lead to cancer. The mutation gives some genes a cancer-promoting effect (gain of function), whereas other genes lose their protective effect (loss of function).
In some cells, however, an increased error rate is even desirable in order to make them more adaptable, for example in certain cells of the human immune system.