Concept intervening sequence / split gene ? Explain structure of globin mRNA.?

 

The DNA of eukaryotes is double stranded & helical in structure. This DNA molecules are present in the chromosome of the nucleus which is nucleoprotein in nature. The DNA is associated with histone protein & present in the complex manner in the chromosome in linear thread like manner.


Eukaryotic DNA is known as"junk DNA" as it has separate stress of 'introns'& 'exons' unlike these of prokaryotic DNA. The exons formed expressed sequences i.e. the coding region of most eukaryotic genes are interupted by few to several non-coding sequences called intervening sequence or intron. Such genes are called split genes or interrupted genes. But sme genes of eukaryotes i.e. 96% in yeast, 17% of drosophila & 6% in mammals are not split. On the other hand, split genes have been found in an archae bacterium & the phase of E.coli. The first split gene was describe in 1977 by Chambton & Coworkers it was the ovalboumin gene of chicken.
P Both introns & exons are transcribed to yield HnRNA which is then processed to removed introns & produce mature functional Mrna, This is called RNA splicing.


The split gene consist of two categories of sequences. 

1) The sequence that containing actual
information of the gene called exon.

2) The intervening sequences that r not coded for Proteins & r removed out after transcription from the primary transcript r called introns.


Research group of P. Chambon, P. Leoder & R.A. Flavel studied the B-globin split genes n structure of globin mRNA. The components of haemoglobin molecules of mice, rabbits, man, birds, & frogs consist of two polypeptide chains alpha & bita. These 2 types of globin genes share a common type of structures.


INTERRUPTION occurs at homologous positions in all known active globin genes. The first intron is always fairly short. The 2
nd introns is usually longer but the actual length can vary . In mouse, the 2nd  introns is the alpha globin gene is only 150bp long. So, the overall length of the gene is 850bp long.
Whereas the B-globin mRNA is 620bases. Thus about ⅔ of the bases r disordered during splicing.







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