DNA synthesis occurs as part of the replication of DNA. DNA is the carrier of genetic information and controls all life processes. It is located in the nucleus of the cell in humans as in all other living organisms. DNA has the form of a double strand, similar to a winding rope ladder, which is 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 attached. The two strands are bonded to each other via hydrogen bonds between opposite, so-called complementary, bases. Here, according to the principle of complementary base pairing, linkages are only possible between guanine and cytosine on the one hand, and adenine and thymine on the other.
What is DNA synthesis?
DNA synthesis occurs as part of the replication of DNA. 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 building of deoxyribonucleic acid (abbreviated as DNA or also DNA). The decisive enzyme in this process is DNA polymerase. Only in this way is cell division possible. For replication, the coiled DNA double strand is first unwound by enzymes known as helicases and topoisomerases, and the two single strands are separated from each other. 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 following 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 ever form bonds with another base, it is possible to use a single strand to reconstruct the other corresponding 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 one 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. However, since DNA polymerase only works in one direction, only the strand that is in the corresponding 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 together by the ligase. The termination of DNA synthesis with the help of various cofactors is called termination.
Function and task
Since most cells have a limited lifespan, new cells must constantly be formed in the body through cell division to replace those that die. For example, red blood cells in the human body have an average lifespan of 120 days, whereas some intestinal cells need to be replaced by new cells after only one or two days. This requires mitotic cell division, in which two new, identical daughter cells are created from a mother cell. Both cells require the complete set of genes, so unlike other cell components, this cannot simply be divided. To ensure that no genetic information is lost during division, the DNA must be duplicated (“replicated”) before division. Cell divisions also take place during the maturation of male and female germ cells (egg and sperm cells). However, in the meiotic divisions that take place, the DNA is not duplicated because a reduction by half of the DNA is desired. When the egg and sperm fuse, the complete number of chromosomes, the packaging state of the DNA, is then achieved 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 each represents an amino acid, hence the term triplet code. Each base triplet is “translated” into an amino acid via messenger RNA (mRNA); these amino acids are then linked in the cell plasma to form proteins.The mRNA differs from DNA only in one atom in the sugar residue of the backbone and in some bases. MRNA thus serves mainly as an information carrier for transporting information stored in DNA from the nucleus to the cytoplasm.
Diseases and disorders
An organism incapable of DNA synthesis would not be viable, since new cells must be constantly formed by cell division even during embryonic development. However, errors in DNA synthesis, i.e. individual incorrectly inserted bases that do not follow the principle of complementary base pairing, occur relatively frequently. For this reason, human cells have repair systems. These are based on enzymes that control the DNA double strand and correct incorrectly inserted bases by various mechanisms. For example, the area around the incorrect base can be cut out and rebuilt according to the explained synthesis principle. However, if the cell’s DNA repair systems are defective or overloaded, base mismatches, so-called mutations, can accumulate. These mutations destabilize the genome, increasing the likelihood of ever new errors in the course of DNA synthesis. An accumulation of such mutations can lead to cancer. In this process, some genes acquire a cancer-promoting effect (gain of function) as a result of the mutation, whereas other genes lose their protective effect (loss of function). However, in some cells, an increased error rate is even desirable to make them more adaptable, such as in certain cells of the human immune system.
