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Difference Between Prokaryotes and Eukaryotes mRNA

Difference Between Prokaryotes and Eukaryotes mRNA - Messenger RNAs of prokaryote and eukaryote cells have several distinguishing features.

(1) The mRNAs of many bacteria and bacteriophages are polygenic or polycistronic. A polycistronic mRNA is transcribed by the several structural genes of an operon. It contains several sites for initiating and terminating polypeptide synthesis. On the other hand all known eukaryotes have only one site for initiation of protein synthesis. Thus eukaryote mRNAs are monocistronic.

(2) In most bacterial mRNAs translation begins while the mRNA is still being transcribed on DNA. In eukaryotes the mRNA transcribed on the chromosomes passes through the nuclear pores into the cytoplasm. Here it forms complexes with ribosomes, which synthesize proteins. Thus translation usually begins only after transcription is completed.

(3) Prokaryote mRNA is very short lived. It is constantly under going breakdown to its constituent ribonucleotides by ribonucleases. In E. coli the average half life of some mRNAs is about two minutes. In bacteria mRNA may be so short lived that while one end is translating proteins the other end may be undergoing breakdown.

The short life of bacterial mRNAs has been explained on the grounds that it provides greater flexibility to the bacteria by adjusting to changing environmental conditions. The short life of its mRNAs enables a bacterium to synthesize different enzymes in response to environmental changes. In general, eukaryote mRNAs have longer half lives than bacterial mRANAs. In other words eukaryote mRNAs are metabolically stable.

Mammalian reticulocytes which have lost their nuclei synthesize haemoglobin for hours or even days by utilizing mRNAs which were transcribed when nuclei were present mRNA stability enables eukaryote cells to have a more permanent protein complement. This permits differentiation of cells to occur.

(4) In prokaryotes the mRNAs undergo very little processing after being transcribed. There is a very short time interval between transcription and translation. In fact considerable translation may already take place before completion of transcription, and degradation of mRNA may begin. In eukaryotes the transcribed mRNA undergoes considerable processing before mature mRNA is formed.

Processing consists of:

(i) polyadenylation or addition of 180-200 adenylate residues at the 3' end forming a long poly(A) chain,

(ii) capping or formation of a 'cap' at the 5' end by condensation of a guanylate residue, and (iii) methylation or the addition of methyl groups to some nucleotides. Transcribed precursor RNA, called heterogenous nuclear RNA (hnRNA), may be 5,000 to 50,000 nucleotides long.

After cleavage and degradation the mature mRNA formed may be just a fraction of the original hnRNA length. Thus in duck reticulocytes the precursor hnRNA is X 10 to X 100 longer than mature haemoglobin mRNA.

(5) With the exception of histone mRNAs, most mRNAs of eukaryotic cells have the 3' terminal poly (A) chain of 180-200 nucleotides referred to above. This poly (A) chain shortens with age and probably functions as an mRNA stabilizer. Prokaryotic mRNAs do not have a poly (A) tail at the 3' end.

(6) Eukaryote mRNAs which are capable of being translated begin with the sequence m7G(5')ppp(5')N, where m7G is a methylguanosine residue and (5')ppp(5') a 5'-5' triphosphate linked to a base at the 5' end.

 
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