Codogenic strand
The codogenic strand is called the single DNA strand of the DNA double helix of a protein-coding gene that is used during transcription to build a single RNA strand.
The codogenic or template strand is referred to here with “antisense”, with “sense” the non- template strand of the DNA.
(Each chromatid on a chromosome contains a long double strand of DNA ; however, in the condensed state, the transcription activity is very low.)
The codogenic strand contains the DNA segment that an RNA polymerase uses as a template for the transcript to be constructed from ribonucleotides . The base sequence of the RNA strand formed is therefore complementary to the codogenic DNA strand used - and thus resembles the other unused DNA strand (which is therefore often also called “coding”). The sequence of the bases of the DNA segment on this non-template strand differs from the sequence of the RNA copy produced only in T instead of U.
The genetic information actually coding for proteins is present on the mRNA within an open reading frame (OLR or ORF). This sequence area is read on ribosomes in the cytoplasm during translation : as a sequence of base triplets , each representing a codon that can each stand for an amino acid . It is only in this context between the start codon and the stop codon that the base sequence indicates the encoded amino acid sequence with which a polypeptide chain is to be built up. The counterpart on the codogenic DNA strand, which is complementary to a codon on the RNA strand, is also called a codogen (“codon generator ”).
Which of the two DNA strands is codogenic and acts as a template is determined by the location of the asymmetric promoter of a gene; this can vary from gene to gene in the DNA double strand of a chromosome .
More detailed explanations
The DNA double helix consists of two single strands, the anti-parallel (5 '→ 3' or 3 '→ 5') in opposite directions about nucleobases complementary paired are. Between the external phosphate-sugar framework of both strands there are grooves in which an RNA polymerase complex glides over the double strand and can recognize a promoter region on the DNA by its sequence. A transcription can only begin after a firm connection to this promoter.
The promoter sequence is not symmetrical and therefore only allows binding in one direction. The bound RNA polymerase is thus positioned as oriented: its starting point and direction of transcription are indicated via the promoter. The RNA polymerase can only synthesize an RNA strand in the 5 '→ 3' direction. The sequence of its ribonucleotides is determined by complementary base pairings with the opposing DNA strand (3 '→ 5'). Using this template, the RNA transcript is built up until the terminator is reached, where the transcription ends. The RNA polymerase is then available for another transcription process. Depending on how the promoter of a gene is located on the DNA, the following transcription then proceeds in one or the other direction with respect to the double strand. The codogenic strand is therefore not always the same DNA strand, but rather the one opposite to the direction of synthesis.
The base sequence transcribed from DNA to RNA is always complementary to the codogenic strand. A base triplet on the coding RNA, which represents the codon CUG (5 '→ 3'), for example, was created on the codogen GAC (3 '→ 5') of the codogenic strand. This corresponds to a triplet with the base sequence CTG(5 ′ → 3 ′) on the non-codogenic other strand of the DNA . The codon of CUGthe mRNA can be read on the ribosome and interpreted from a tRNA with a suitable anticodon , such as GAC(3 ′ → 5 ′). If this tRNA has been loaded with leucine , then this amino acid is incorporated into the resulting polypeptide chain of a protein. Only CUGthen does the genetic information of a gene coding for protein become clear - since the codon on the mRNA then codes for Leu (L) , or the codogen GAC(3 ′ → 5 ′) on the DNA corresponds to the anticodon GACon a tRNA Leu .
When speaking of a coding strand, it should be borne in mind that a new change in an individual nucleobase on the non-template strand of DNA - for example the conversion of cytosine to thymine (a transition C → T) - does not have that effect on the reproduction of the genetic information has, as if this event the template strand is concerned, the codogenic template for the mRNA encoding single strand.
Other names
The term pairs of codogenic and non- codogenic as well as non-coding and coding or also codogenic versus coding are used for the various nucleic acid single-strand segments during transcription . Other common names are, for example, template strand and sense strand , minus strand and plus strand . The English words antisense and sense ( English for 'sense') are also used in German specialist literature, more rarely, mostly in relation to a uniform representation of both strands from 5 '→ 3', the eponyms Crick-Strang (sense) and Watson-Strang (antisense); however, these terms are not always used with the same meaning.
It should also be noted that in numerous cases - when transcribing non (pre) mRNA - terms such as 'codogen' and 'coding' are empty. In addition, since the same DNA strand is not always used as the codogenic strand in the transcription of genes in an organism, it makes more sense to speak of single strand sections in the transcription. This is also necessary to distinguish it from special cases of virus or organelle genomes in which the strand or strands of the entire genome are designated with “+” or “plus” or “-” or “minus”.
In common use, the terms codogenic strand , template strand , minus strand , nonsense strand and antisense denote the DNA strand segment complementary to the RNA, which serves as a template for the transcript for a transcribing RNA polymerase . Nichtcodogener strand , non-template , plus strand , sense strand , and sense or "coding strand" is then designations for those nucleic acid strand portion, the sequence of that of the primary RNA -Produkts resembles the gene. Occasionally, however, the protein or the tRNA is viewed as " sense " with a different meaning , which can then reverse the meaning assigned to the expressions.
literature
- B. Alberts, A. Johnson, J. Lewis et al .: Molecular Biology of the Cell. 4th edition. Garland Science, New York 2002, Chapter 6: How Cells Read the Genome: From DNA to Protein. Online on the NCBI bookshelf
- Rolf Knippers: Molecular Genetics. 8th revised edition. Georg Thieme Verlag, New York NY et al. 2001, ISBN 3-13-477008-3 .
Individual evidence
- ↑ B. Alberts, et al .: Molecular Biology of the Cell. 4th edition. Garland Science, New York 2002, Chapter: From DNA to RNA. here online
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↑ a b JCBN / NC-IUB Newsletter 1989
Nomenclature conventions
coding strand in Lexikon der Biochemie on Spektrum.de , accessed on October 22, 2017. - ^ Reed Cartwright, Dan Graur: The multiple personalities of Watson and Crick strands . In: Biology Direct . 6, February 2011, p. 7. doi : 10.1186 / 1745-6150-6-7 . PMID 21303550 . PMC 3055211 (free full text).