<P> Introns are non-coding sections of a gene, transcribed into the precursor mRNA sequence, but ultimately removed by RNA splicing during the processing to mature messenger RNA . Many introns appear to be mobile genetic elements . </P> <P> Studies of group I introns from Tetrahymena protozoans indicate that some introns appear to be selfish genetic elements, neutral to the host because they remove themselves from flanking exons during RNA processing and do not produce an expression bias between alleles with and without the intron . Some introns appear to have significant biological function, possibly through ribozyme functionality that may regulate tRNA and rRNA activity as well as protein - coding gene expression, evident in hosts that have become dependent on such introns over long periods of time; for example, the trnL - intron is found in all green plants and appears to have been vertically inherited for several billions of years, including more than a billion years within chloroplasts and an additional 2--3 billion years prior in the cyanobacterial ancestors of chloroplasts . </P> <P> Pseudogenes are DNA sequences, related to known genes, that have lost their protein - coding ability or are otherwise no longer expressed in the cell . Pseudogenes arise from retrotransposition or genomic duplication of functional genes, and become "genomic fossils" that are nonfunctional due to mutations that prevent the transcription of the gene, such as within the gene promoter region, or fatally alter the translation of the gene, such as premature stop codons or frameshifts . Pseudogenes resulting from the retrotransposition of an RNA intermediate are known as processed pseudogenes; pseudogenes that arise from the genomic remains of duplicated genes or residues of inactivated genes are nonprocessed pseudogenes . Transpositions of once functional mitochondrial genes from the cytoplasm to the nucleus, also known as NUMTs, also qualify as one type of common pseudogene . Numts occur in many eukaryotic taxa . </P> <P> While Dollo's Law suggests that the loss of function in pseudogenes is likely permanent, silenced genes may actually retain function for several million years and can be "reactivated" into protein - coding sequences and a substantial number of pseudogenes are actively transcribed . Because pseudogenes are presumed to change without evolutionary constraint, they can serve as a useful model of the type and frequencies of various spontaneous genetic mutations . </P>

Which part of the dna transcribes for 'non'-coding sequences