tailieunhanh - Nanotechnology Science and Computation Part 15

Tham khảo tài liệu 'nanotechnology science and computation part 15', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Insights into a Biological Computer 357 these small RNAs are produced in the macronucleus and their function is to direct degradation of homologous mRNAs before conjugation. Another class of small RNAs similar to the scnRNAs in Tetrahymena could be produced by transcription of the micronucleus and might not scan the macronuclear chromosomes but rather the macronuclear transcripts. This model can explain the seemingly opposite mechanisms of excision overloading the macronucleus with an excess of a gene would lead to higher production of the 22 to 23 bp RNAs which would in turn destroy the mRNAs of the gene. This would have the same effect as if the gene were absent from the mother macronucleus and therefore providing no mRNA transcripts 9 . 4 Gene Unscrambling in Spirotrichs Much less is known about the number types and distribution of MIC-limited sequences in spirotrichs. The level of fragmentation of the macronucleus in spirotrichs is much higher than in oligohymenophorans. Oxytricha has 24 000 to 26 000 different chromosomes in its MAC. Thus the number of eliminated sequences involved in chromosome breakage must be very high. The mean number of IESs in the Oxytricha genes sequenced thus far is 14 and an estimate of 26 000 genes would put the number of intragenic IESs at approximately 350 000. The IESs in spirotrichs are also much smaller than those in oligohymenophorans. IESs of size 0 excluding the pointers occur in some scrambled genes and more than half of all known IESs have sizes 30 bp. Because most of these IESs occur in coding regions they must be excised precisely or else the gene product might contain deleterious deletions and or frameshifts. In addition approximately 5 of the micronucleus is composed of 4 kbp transposable elements of the TBE family T. Doak personal communication which are also eliminated as IESs 25 . One should be careful in extrapolating findings in oligohymenophorans to spirotrichs as their evolutionary distance is very large as much