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Genes take up about 30% of the pufferfish genome and the coding DNA is about 10%. (Non-coding DNA = 90%.) The reduced size of the pufferfish genome is due to a reduction in the length of introns and less repetitive DNA. [8] [9] Utricularia gibba, a bladderwort plant, has a very small nuclear genome (100.7 Mb) compared to most plants.
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Junk DNA (non-functional DNA) is a DNA sequence that has no known biological function. [ 1 ] [ 2 ] Most organisms have some junk DNA in their genomes —mostly, pseudogenes and fragments of transposons and viruses—but it is possible that some organisms have substantial amounts of junk DNA.
The total amount of coding DNA is about 1-2% of the genome. [18] [16] Many people divide the genome into coding and non-coding DNA based on the idea that coding DNA is the most important functional component of the genome. About 98-99% of the human genome is non-coding DNA.
Eventually pseudogenes may be deleted from their genomes by chance of DNA replication or DNA repair errors, or they may accumulate so many mutational changes that they are no longer recognizable as former genes. Analysis of these degeneration events helps clarify the effects of non-selective processes in genomes.
A conserved non-coding sequence (CNS) is a DNA sequence of noncoding DNA that is evolutionarily conserved. These sequences are of interest for their potential to regulate gene production. [1] CNSs in plants [2] and animals [1] are highly associated with transcription factor binding sites and other cis-acting regulatory elements.
The onion test is a way of assessing the validity of an argument for a functional role for junk DNA.It relates to the paradox that would emerge if the majority of eukaryotic non-coding DNA were assumed to be functional and the difficulty of reconciling that assumption with the diversity in genome sizes among species. [1]
Novel genes can emerge from ancestrally non-genic regions through poorly understood mechanisms. (A) A non-genic region first gains transcription and an open reading frame (ORF), in either order, facilitating the birth of a de novo gene. The ORF is for illustrative purposes only, as de novo genes may also , or lack an ORF, as with RNA genes.