In biology, the process of transcription involves the replication of a segment of a DNA strand into a messenger RNA strand (mRNA). The mRNA then contains the nucleic base sequence complementary to the fragment of DNA. Subsequent transcription occurs within the nucleus in all eukaryotes, including humans, whereas subsequent translation, the translation of the mRNA into a concrete protein in the cytoplasm, occurs at ribosomes.
What is transcription?
In biology, the process of transcription involves the process of replicating a segment of a DNA strand into a messenger RNA (mRNA) strand. The transcription process represents the first stage in the conversion of genetic information into proteins. In contrast to replication, it does not involve the production of a copy of the entire genome, but always only specific segments of a DNA strand. At a specific section of a DNA strand, the bond to its complementary sub-strand in the double helix is first broken by eliminating the hydrogen bonds. At the region to be copied, a new complementary sub-strand is formed by the addition of free RNA nucleotides, which, however, consists of ribonucleic acids and not of deoxyribonucleic acids as in DNA. The resulting RNA fragment is effectively the working copy of the DNA fragment and is called messenger RNA (mRNA). The mRNA, which is formed inside the nucleus, splits off from the DNA and is transported through the nuclear membrane into the cytosol, where translation, the conversion of the RNA codons into the corresponding amino acid sequence, i.e. the synthesis of the protein, takes place. Three sequences (triplets) of nucleic bases on the mRNA, called codons, each determine one amino acid. According to the sequence of mRNA codons, the corresponding amino acids are assembled via peptide bonds to form polypeptides and proteins.
Function and task
In biology, transcription fulfills the first of two main processes that convert genetic information, available as sequences of DNA nucleic bases, into the synthesis of proteins. The genetic information consists of sequences of three, called triplets or codons, each representing an amino acid, although some amino acids may be defined by different codons. The function of transcription is to produce an mRNA strand whose nucleic bases – in this case ribonucleic bases and not deoxyribonucleic bases – correspond to the complementary pattern of the expressed DNA segment. The generated mRNA thus corresponds to a kind of negative template of the expressed gene segment, which can be used for the one-time synthesis of the encoded protein and then recycled. The second main process for converting the genetic information into concrete proteins is translation, in the course of which amino acids are strung together to form proteins according to the coding of the mRNA and linked peptidically. Transcription allows genetic information to be selectively read and transported out of the nucleus into the cytosol in the form of complementary copies and, independent of the corresponding DNA segment, to build the proteins. One of the advantages of transcription is that portions of a single DNA strand can be expressed to make the mRNA without having to expose the entire gene to constant physiological milieu changes and thereby run the risk of mutating or otherwise altering its properties. Another advantage of transcription is the so-called splicing and other types of processing of the mRNA. The mRNA is first freed from so-called introns, functionless codons that do not code for amino acids, by the process of splicing. In addition, adenine nucleotides can be added to the mRNA by means of the enzyme poly(A) polymerase. In humans, as in other mammals, this appendage, called the poly(A) tail, consists of about 250 nucleotides. The poly(A) tail shortens as the mRNA molecule ages and determines its biological half-life. Although not all functions and tasks of the poly(A) tail are sufficiently known, it appears at least certain that it protects the mRNA molecule from degradation and improves the convertibility (translatability) into a protein.
Diseases and ailments
Similar to cell division, where errors can occur in the replication of the genome, the most common problem associated with transcription is a “copying error.” Either a codon is “forgotten” during the synthesis of mRNA or an incorrect mRNA codon is created for a particular DNA codon. It is estimated that such a copying error occurs in about one in one thousand copies. In both cases, a protein is synthesized that integrates an unintended amino acid at at least one site. The spectrum of effects ranges from ‘not noticeable’ to total failure of the synthesized protein. If gene mutation occurs during replication or due to other circumstances, the mutated nucleic base sequence is transcribed, since the process of transcription does not include checking the codons for ‘correctness’. However, the body has a distinct DNA repair mechanism, involving more than 100 genes in humans. The mechanism consists of a sophisticated system of immediate repair of the gene mutation or replacement of a damaged nucleic base sequence, or minimization of the effects if the former two possibilities are eliminated. The fact that transcription occurs without prior testing of genes carries the risk that transcription may also be involved in the spread of viruses, if the viruses introduce their own DNA into the host cell and cause the host cell to replicate the genome of the viruses, or parts thereof, by replication or transcription. These can then cause the corresponding disease. This is true in principle for all types of viruses.
