Messenger ribonucleic acid or mRNA encodes for a protein production. mRNA is produced from a DNA template by a process known as transcription. This mRNA carries all the required codes required for the synthesis of protein to cytoplasm. Here in cytoplasm with the help of ribosome proteins are produced. Just like DNA, mRNA also contains genetic information in the sequence of nucleotides arranged into codons. Each codon consists of three bases, and they encode for a specific amino acid. Only the stop codon terminates the protein synthesis. This process required two types of RNA, transfer RNA for recognizing the codon and also provides the corresponding amino acid, and ribosomal RNA is the central component of the ribosome's protein synthesis process, which is also called as translation.
Structure of Messenger RNA - mRNA:-
Messenger RNA is a single stranded structure, with no base pairing. It contains bases such as adenine, guanine, cytosine and uracil. Since mRNA is transcribed from the DNA molecule, its sequences are complementary to that of DNA on which they are transcribed. Usually each gene transcribes its own mRNA therefore there may be 1000 to 10000 different types of mRNA may be present in a single cell.
The mRNA molecule has the following structural features:
1. Cap: It is present at the 5' end of the mRNA molecule in most of the eukaryotic cells
The rate of protein synthesis depends upon the presence of the cap. Without the cap mRNA molecules bind very poorly to the protein producing factory ribosome.
2. Noncoding region 1 (NC1). The cap is followed by a region of 10 to 100
nucleotides. This region is rich in adenine and uracil bases, and they do not code for any protein so the name noncoding region.
3. Initiation Codon: AUG is the initiation codon in both prokaryotes and eukaryotes.
4. The coding region: This consists of about 1,500 nucleotides on the average and
translates into a functional protein.
Difference between Prokaryotic and Eukaryotic mRNA:
1. The mRNA of many types of bacteria and bacteriophage are polygenic, that is a single mRNA is transcribed by the several structural genes of an operon. It also contains many sites for initiation and termination codons. That is a single mRNA can code for several different protein molecules.
Whereas all the known eukaryotic mRNA have got only one site for initiation and also termination of protein synthesis. Therefore eukaryotic mRNA is monocistronic in nature.
2. In most of the bacterial cells translation of the mRNA begins while the mRNA is still being transcribed from the DNA molecule.
Whereas in eukaryotes the mRNA produced from DNA template are first transported into the cytoplasm via nuclear pores, then it forms complexes with ribosome, then the protein are synthesized. Thus translation process begging only after transcription of mRNA is completed.
3. Life span of the prokaryotic mRNA is very short. mRNA molecules are constantly breakdown into its ribonucleotides by enzyme known as ribonucleases. In E.coli the average half life of mRNA is only about two minutes. That is at one end of the mRNA may be being degraded and on the other end translation may take place simultaneously. Short life span of mRNA enables prokaryotes to synthesize different proteins or enzymes in response to changes in the external environment.
Eukaryote mRNAs have much longer life span than bacterial mRNAs. That is eukaryotic mRNA are metabolically stable. For example mammalian reticulocytes synthesize protein even after hours or days after losing their nuclei.
4. In prokaryotes mRNA undergoes very little post transcriptional changes and also there is a very short time interval between transcription and translational process. For instance translation may occur simultaneously while transcription is going on at one end of mRNA molecule.
In eukaryotes the transcribed mRNA undergoes major post transcriptional modifications.
This process consists of
a. Polyadenylation at the 3' end of the mRNA. This poly adenyl chain helps in giving stability to the mRNA molecule.
b. Capping or formation of a cap at 5' end by condensation of guanylate residue
c. Transcribed mRNA present in the nucleus before post transcriptional modifications are called as heterogeneous mRNA. These heterogeneous mRNA consists of both introns and exon regions. Then by the help of slicing mechanism mature mRNA are produce which consists of only coding region. Therefore the mature mRNA is only a fraction in length of heterogeneous mRNA molecules.
These are some of the major differences between prokaryotic and eukaryotic mRNA molecules.
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