Replication in biology refers to the duplication of genetic information that the human body stores in the form of deoxyribonucleic acid (DNA). Certain enzymes copy the genes, preserving half of the original DNA strand. Biology therefore also refers to semiconservative replication.
What is replication?
Replication is a biological process that multiplies deoxyribonucleic acid (DNA). DNA is a long chain composed of four types of nucleosides. A nucleoside is composed of a sugar (deoxyribose) and a nuclear acid. In the nucleus, DNA is present in the form of chromosomes, which consist of uncoiled DNA and protein molecules. For replication, the chromosomes uncoil and the DNA double strand smooths out. Then the two complementary DNA strands separate from each other, like the rows of teeth in a zipper. Only then can the actual replication begin. All living organisms multiply their genetic information semiconservatively: one half of the double strand remains, while a second half is newly formed by enzymes. In the first daughter generation, therefore, each copy possesses half of the original DNA of the parent cell; in the second daughter generation, it still makes up a quarter of the genes. As early as 1958, the researchers Meselson and Stahl were able to prove semiconservative replication. To do so, they used a biochemical marker with which they labeled the DNA of bacteria. The analyses confirmed the quantitative ratio of original and new DNA, as the scientists had predicted for semiconservative replication.
Function and task
Most people associate genetics with the inheritance of traits that parents pass on to their children. While this is a very familiar function, it is far from the only function of replication. DNA duplication takes place in the human body not only to form eggs and sperm. Every cell division requires a copy of the DNA. No cell can function without the genes in the nucleus – because genes control metabolic processes and provide the blueprints for biomolecules. Four different nucleic acids occur in human DNA: Adenine, guanine, cytosine and thymine. Two of them form a so-called base pair; they fit together like two pieces of a puzzle. The sequence of nucleosides represents the genetic code that contains all the hereditary information of the human body. The combination of the individual nucleosides is comparable to the combination of letters: although the alphabet contains only a limited number of letters, an almost infinite number of words can be formed from them. Theoretically, cells only need a single strand of DNA to store and pass on information. However, DNA has two strands that complement each other. Each piece of information is thus stored twice. Scientists also refer to the complementary DNA strand as the template. The two chains wind around each other to form the characteristic double helix. Highly specialized enzymes copy the DNA in the cell nucleus. These catalysts are known in biology as DNA polymerases and are composed of protein molecules. So far, scientists have been able to identify three different DNA polymerases, which differ slightly in the functions they perform. The DNA polymerases dock onto a DNA strand at a very specific site, which is marked with a primer. A primer is a start molecule with which the polymerases connect the first nucleoside of the new DNA strand. The enzymes obtain the energy for their work by splitting off two phosphate residues from the nucleosides, which they use as building blocks. From the primer, the polymerases work from the 5′ end to the 3′ end. This occurs on both DNA strands of the original genes simultaneously. On one of the strands, the enzymes can proceed continuously and add the complementary nucleic bases one by one. However, since the opposite strand is mirrored and thus proceeds in the “wrong” order, replication takes place there as a discontinuous synthesis. The polymerases also copy the DNA at the template starting from the primer; however, they can only synthesize fragments because they repeatedly interrupt the process. These so-called Okazaki fragments are later joined by another enzyme – also a DNA polymerase.This DNA polymerase fills the gaps between the Okazaki fragments by also adding the complementary nucleosides to the template strand. Then, a DNA ligase migrates across the new double strand and links the aligned nucleosides into a solid chain.
Diseases and disorders
Errors in replication can lead to the development of genetic diseases without there being a specific disease. Occasionally, DNA polymerase incorporates the wrong nucleoside into the new DNA strand. Such an error is called a point mutation in biology. In another type of mutation, the insertion, the enzymes insert one too many nucleosides during replication. This shifts the grid that divides the nucleosides into groups of three. A group of three forms a gene. Deletion also shifts the reading frame. In contrast to insertion, the enzymes skip a nucleoside during replication: it appears deleted in the DNA copy. This error means that other enzymes cannot read the DNA correctly; the result is incorrectly produced cell building blocks or messenger substances. As a result, metabolic disorders can occur, potentially leading to a variety of physical diseases. However, mutations do not always have to result in diseases. In particular, point mutations pose less risk if they occur within DNA segments that have no practical significance for protein synthesis. Replication errors are especially critical if the defective DNA ends up in egg or sperm cells. An embryo that develops from this DNA does not have error-free DNA in addition to the mutated DNA: each new copy of its DNA then also contains the mutation.
