Messenger RNA (mRNA)

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Messenger RNA (mRNA)

Messenger RNA (mRNA)
The RNA molecules which are transcribed from large number of genes of the total genome. (i.e ., 99 percent genes of the total genome of E.coli) and have sequence complementary to DNA carry DNA's genetic informations for the assembly of amino acids into the polypeptide chains (protein molecules) to the cytoplasmic sites of protein synthesis the ribosomes to which they become associated to participate in codon-anticodon inter-actions with tRNA are called informational or messenger RNAs (mRNA). The name messenger RNA has been proposed by Jacob and Menod (1961). The molecule of a mRNA is single-stranded like the rRNA molecule and it is DNA-like in its base composition so that GC contents of mRNA correspond to the GC contents of the genome's total DNA.

Synthesis of mRNA- The origin and fate of mRNA in eukaryotic cells is much more complex than in bacteria. Very recently, it has been found that the formation of a functionally active mRNA is the consequence of a complex series of steps that comprise: (1) the actual transcription 'of' DNA into mRNA precursors, (2) the intranuclear processing (tailoring) of these precursors, and (3) the transport of the mRNAs into the cytoplasm and their association with ribosomes to initiate the process of translation or protein synthesis.
(a) Heterogeneous nuclear RNA-It has been suggested that mRNA is synthesized in the nucleus as part of a heterogeneous population of large RNA molecules which constitute the so-called heterogeneous nuclear RNA (het-RNAs or Hn-RNAs). The Hn-RNAs are also known as high molecular weight RNA (HMW RNA) or DNA-like RNA (dRNA).

The Hn-RNA molecules range in size from 5 x 105 to 107 daltons and are degraded, for the most part within the nucleus at a relatively rapid rate (Darnell, 1968). Only about 20 per cent of the Hn-RNA, in terms to total nucleotide, is not degraded and has been postulated to be converted into mRNAs. The Hn-RNA occurs in nucleoplasm outside the nucleolus and it contains DNA-like base composition and readily hybridizes with DNA.
(b) Polyadenylation and transport of mRNA-Recently it has been discovered that Hn-RNA contain a sequence of polyadenylic acid (poly A), approximately 200 nucleotides long at their 3' ends (Kates, 1970; Lim and Canellakis, 1910; Edmons et.al., 1971; and Darnell et.al., 1971). This poly A sequence is added to the Hn-RNA after the transcription is completed (Darnell et.al., 1971) and is attached to the 3' end of the RNA molecule (Perry et.al., 1973).

DeRobertis et.aI., (1975) have suggested that once the Hn-RNA molecule is transcribed from DNA, the poly A sequence is added, in a otherwise fashion, by the action of a poly synthetase.Simultaneous with or-after' polyadenylation, and starting from the S' end a selective degradation of the Hn-RNA molecules takes place by the action of nucleases. The final product is Poly A(+ )mRNA, which will finally reach the cytoplasm. Most of mRNA molecules of eukaryotes contain a 3' terminal poly A chain. The post transcriptional attachment of poly A may be a special step in the processing of mRNA and may be related to its transport to the cytoplasm.

Synthesis and processing of heterogenous nuclear rna

A. Nucleus	B.histone-mRNA	C. Nuclear envelope	D.Cytoplasm
E.Transcription	F.Polyadenylation	G.Selective degradation	H.Transport
I.Genome(DNA and chromosome proteins)	J.Hn-RNA	K.Turnover	L.polyA(+)Hn RNA
M.polyA(+)mRNA	N.polyribosome polyA(+)mRNA

Recently, it is also discovered that the mRNAs for the this tones (called histone mRNAs) are directly transcribed from repititive sequences in the DNA wit40ut the need for a giant precursor molecule (Hn-RNA) or polyadenylation. It has been demonstrated that the histone mRNA enter the cytoplasm without delay, whereas the poly A( + )mRNAs have a 15 minute delay before appearing attached to the polyribosomes. This lag period may be attributed to the processing steps that poly A( +) mRNAs probably undergo before passing into the cytoplasm.

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