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mRNA

 MESSENGER RNA Occurence:  It is present in the nucleus and cytoplasm.  It is synthesised from DNA in the nucleus and carries the genetic information from DNA to the cytoplasm.  So, it is present in large quantity in the cytoplasm. Since it carries genetic message, it is called as messenger RNA. Characteristics: Jacob and Monod were first proposed the name "messenger" to this type of RNA. mRNA molecules exist in a large variety of lengths, depending on the length of the polypeptide chain for which they code. They have high Mol. wt. and accounts for only about 1% of the total RNAs of the cell.  Generally the life span of mRNA is relatively short, although in some eukaryotic cells there is evidence for some degree of stability. The base sequence in the mRNA is always complementary to the portion of DNA strand with the exception that uracil replaces thymine. The 5' end of mRNA is "capped" by a 7-methyl guanosine triphosphate which is linked to an adjacent 2'-O-...
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mRNA

 MESSENGER RNA



Occurence:

  •  It is present in the nucleus and cytoplasm. 
  • It is synthesised from DNA in the nucleus and carries the genetic information from DNA to the cytoplasm. 
  • So, it is present in large quantity in the cytoplasm.
  • Since it carries genetic message, it is called as messenger RNA.

Characteristics:

  • Jacob and Monod were first proposed the name "messenger" to this type of RNA.
  • mRNA molecules exist in a large variety of lengths, depending on the length of the polypeptide chain for which they code.
  • They have high Mol. wt. and accounts for only about 1% of the total RNAs of the cell.
  •  Generally the life span of mRNA is relatively short, although in some eukaryotic cells there is evidence for some degree of stability.
  • The base sequence in the mRNA is always complementary to the portion of DNA strand with the exception that uracil replaces thymine.
  • The 5' end of mRNA is "capped" by a 7-methyl guanosine triphosphate which is linked to an adjacent 2'-O-methyl ribonucleoside at its 5'-OH group through three phosphates.
  • The capping of mRNA is involved in the recognition of mRNA by the translating machinery. 
  • The translation of mRNA into proteins begins at the capped 5' end.
  • The 3' hydroxyl terminus has attached to a polymer of adenylate residues, 20–250 nucleotides in length.
  • When the code has been transcribed from DNA on to RNA, the latter leaves the nucleus, passes through the nuclear membrane into the cytoplasm.  Here, it moves to the ribosomes, the site of protein synthesis. 
  • mRNA attaches reversibly to the smaller subunits of ribosomes. 
  • The Mg++ is involved in the formation of a complex between mRNA and ribosomes.

Types of mRNA:

  • On the basis of size and coding, 2 types of mRNA have been described:
  • Monocistronic mRNA carries the codes for the synthesis of single protein molecule.
  • Polycistronic mRNA carries codes for more than one kind of protein.


Function

  • Its only function is to carry the genetic information from DNA of nucleus to the cytoplasm. 
  • The message in the mRNA is translated in the form of amino acid sequence at the site of protein synthesis.
  • The genetic message is always in the form of triplet nucleotides and each triplet nucleotide codes for an amino acid.  This triplet nucleotide is known as a genetic code or a codon.

References
1.Principles of Biochemistry-Lehinger 
2.Biochemistry-U. Sathyanarayana & U.Chakrapani
3. Lewin's Genes




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