Non-coding deoxyribonucleic acid

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The parts of deoxyribonucleic acid (DNA) that do not code for proteins are referred to as non-coding deoxyribonucleic acid ( English noncoding DNA , also junk DNA ) . In higher organisms such as humans, animals and plants, the vast majority of the DNA is “non-coding” in this sense. Diverse functions have also been found for this non-coding DNA. It is not known how large the proportion of non-coding DNA with function is compared to the non-functional proportion of non-coding DNA.

Occurrence

Protein-coding DNA serves as a template for the messenger RNA , which in turn is used as a template for the synthesis of proteins. The former process is called transcription , the latter called translation . Non-coding DNA regions are also transcribed many times, but the resulting RNAs are not used for translation ( non-coding ribonucleic acid ). Classic, long-known examples are ribosomal and transfer RNA , which both also have essential functions in translation, but do not serve as a template.

Non-coding DNA is particularly characteristic of eukaryotes , in which it makes up the largest part of the genome , while its proportion is only 5-20% in prokaryotic genomes. In human DNA, around 95% of the nucleotides are currently regarded as non-coding DNA, i.e. a maximum of 5% of the nucleotides that make up the DNA encode genetic information for proteins.

The ENCODE project, in which the functional elements of the genome are to be described, has come to the result that these areas are nevertheless largely transcribed, i.e. rewritten into RNA. In a follow-up study, the ENCODE project came to the conclusion that even more than 80% of the human genome has biochemical activity (usually transcription). Another study, which analyzes the frequency of RNA transcripts, comes to the conclusion that non-coding areas are practically not transcribed. This contradiction has led to the hypothesis that the majority of the non-coding RNA transcripts are not stable and are degraded again shortly after transcription.

Examples

The largest proportion of non-coding DNA make up transposable elements , around 45% of the human genome.

The so-called pseudogenes , copies of genes that are no longer functional due to mutations , are still widespread . According to the theory of evolution, they are the starting material for new genes with new functions.

Repetitive sequences that consist of numerous repetitions of a base sequence make up significant parts of the non-coding DNA .

Regular genes also contain non-coding sections: the promoter region, which regulates the activity of the gene ( gene expression ), and the introns , which are also transcribed but whose transcripts are removed before translation ( splicing ). Further non-coding DNA segments that are not themselves part of genes, but which interact with promoters to regulate gene expression, are enhancers and silencers .

The telomeres , the ends of the chromosomes, are also non-coding .

Functions of non-coding DNA

Many sections of non-coding DNA are known which perform essential functions for the organism and are evolutionarily conserved. Non-coding DNA has, among other things, important functions in gene regulation and for chromosomal structure.

A distinction must be made between a direct function in the organism and a long-term evolutionary significance. Some types of non-coding DNA, such as pseudogenes or transposable elements, play an important role in evolution, even if they do not fulfill an immediate function.

Debate about "junk DNA"

The term “junk DNA”, which was popularized in the 1960s by Susumu Ohno (1928–2000), describes DNA that has no function for the organism. The question of whether a significant proportion of human DNA is functionless is the subject of an ongoing scientific debate.

The results of the ENCODE project, the authors of which attributed a function to over 80% of the human genome, have led to media reports in which the concept of junk DNA has been refuted. The definition of the term “function” via biochemical activity by the ENCODE authors (instead of being useful for the organism), however, has led to some sharp criticism of this interpretation of the results.

The arguments of the proponents of junk DNA include the C-value paradox , exemplified by the "Onion Test", which raises the question of why the onion ( Allium cepa ) has a genome 5 times larger than humans, if not large parts of it are non-functional. The result of a study in which two longer sections of non-coding DNA were removed from the genome of mice, which did not lead to any noticeable differences in phenotype, further supports the dispensability of parts of non-coding DNA.

Since it is not known whether the majority of non-coding DNA has a functional task, it is still an open question how large the proportion of non-coding DNA fulfills a function.

literature

Web links

See also

Individual evidence

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