mRNA or messenger RNA is a single stranded RNA molecule. It is complementary to the DNA and carries genetic information present in the DNA. It is translated to form proteins. The genetic codes (triplet) present on mRNA get translated to amino acids, giving rise to the functional product of a gene (proteins).
In eukaryotes, mRNA is synthesised in the nucleus and after processing, it gets transported to the cytoplasm, where translation or protein synthesis takes place.
Types of RNA
RNA or ribonucleic acids are a type of nucleic acids present inside the cell. They take part in various biological processes such as protein synthesis or translation, gene expression and regulation, some RNA act as catalyst, cell signaling, etc. Some of the viruses contain RNA as the genome.
RNA are synthesised from DNA by the enzyme RNA polymerase, which is a DNA-dependent enzyme. The process of RNA synthesis is called transcription.
There are three main types of RNA present in a cell. All three types of RNA are involved in the synthesis of proteins. The three main types of RNA are:
- mRNA – It is a single stranded RNA and provides the template for protein synthesis. It carries the gene complementary to one strand of the DNA.
- tRNA – It is a small RNA molecule of about 80 nucleotides long. As the name suggests, the main function of tRNA is to read the code on mRNA template and transfer the specific amino acid to the ribosome for protein synthesis.
- rRNA – It plays a structural and catalytic role in the protein synthesis. It forms the catalytic part of ribosomes. Eukaryotic ribosomes contain different types of rRNA, which are 18S, 5.8S, 28S and 5S rRNA.
Other types of non-coding regulatory RNAs are microRNA (miRNA), small nuclear RNA (snRNA), Small interfering RNA (siRNA), etc.
Now let us learn about mRNA in detail.
mRNA or messenger RNA is a type of RNA. It is the single-stranded RNA template that is translated for protein synthesis by ribosomes.
Jacob and Monad gave the name “messenger RNA”. It is a long polymeric molecule made up of nucleotides.
mRNA is a long, single-stranded molecule consisting of nucleotides attached by phosphodiester bonds. It contains four nitrogenous bases, adenine, guanine, cytosine and uracil. The RNA is complementary to one of the DNA strands except for uracil instead of thymine in DNA.
The mature mRNA consists of the following regions: 5′ cap, 5′ UTR, coding region, 3′ UTR, and poly(A) tail.
- Coding region – It is made up of codons, i.e. triplet of nucleotides. Each codon codes for specific amino acids and codons in mRNA get translated by ribosomes to amino acids in a polypeptide chain. The coding region begins with the start codon, i.e. ‘AUG’ and ends with any of the stop codons, i.e. UAG, UAA or UGA. A portion of the coding sequence may also have a regulatory function.
- Untranslated regions or UTRs – They are present at 5’ and 3’ regions before and after the coding region, respectively. Untranslated regions are present before the start codon and after the stop codon. They have a role to play in gene expression. They are involved in the stability of RNA, the efficiency of translation and mRNA localisation.
- 5’ Cap – A cap of methyl guanosine triphosphate is present at the 5’-end.
- Poly(A) Tail – Polyadenylate tail is present at the 3’-end.
When a coding region of mRNA codes for a single protein, it is known as monocistronic as in most eukaryotic mRNAs. In prokaryotes, most mRNAs code for more than one protein, it is called polycistronic. These proteins mostly have related functions and are regulated by a single regulatory region having promoter and operator regions. In humans, the mitochondrial genome is polycistronic.
Synthesis of mRNA – Transcription and Processing
RNA is synthesised from DNA by the process known as transcription.
In prokaryotes, a single DNA-dependent RNA polymerase catalyses the transcription of all types of RNA. Here, mRNA does not require any processing therefore transcription and translation can be coupled as both the processes occur in the cytosol.
In eukaryotes, pre-mRNA is transcribed by RNA polymerase II. mRNA is synthesised as pre-mRNA or primary transcript in the nucleus. It is called hnRNA or heterogeneous nuclear RNA. After processing, the mature mRNA is transported to the cytosol and undergoes translation.
The synthesis of RNA from DNA is called transcription. The DNA-dependent RNA polymerase synthesises RNA using DNA as a template in the 5’ to 3’ direction.
The DNA strand having 3’ to 5’ polarity acts as a template strand and the DNA strand having 5’ to 3’ polarity is called the coding strand. The RNA transcribed has the same sequence of nucleotides as the coding strand.
A transcription unit in DNA consists of three main regions and all the references are made with respect to the coding strand.
- Promoter – present towards 5’-end of the structural gene.
- Structural gene – present between promoter and terminator.
- Terminator – present towards 3’-end of the structural gene.
The transcription process is carried out in three steps. They are:
- Initiation – RNA polymerase binds to the promoter region. It starts transcription by forming a transcription bubble by unwinding the DNA double helix.
- Elongation – RNA polymerase continues to use complementary nucleoside triphosphate as substrates and the elongation continues with the opening of the DNA double helix. RNA polymerase transcribes RNA in the 5’ to 3’ direction.
- Termination – On reaching the terminator region, transcription stops and mRNA detaches from RNA polymerase.
The newly formed mRNA or primary transcript is called hnRNA and needs to undergo processing to transform into mature mRNA. The hnRNA undergoes post-transcriptional processing to produce mature mRNA. The functional mRNA is transported out of the nucleus. The translation or protein synthesis takes place in the cytoplasm.
The hnRNA contains both exons and introns, which are coding and non-coding regions respectively. It undergoes splicing, capping and tailing.
- Splicing – In this process, introns are removed from the primary transcript and exons are joined together.
- Capping – In this process, a cap of an unusual nucleotide, i.e. methyl guanosine triphosphate is added to the 5′-end of hnRNA.
- Tailing – In the tailing process, 200-300 adenylate residues are added at 3’-end. The poly(A) tail protects RNA from degradation by exonucleases and is also involved in the termination of transcription, transport of mRNA and translation.
After processing of hnRNA, the fully functional mRNA is transported to the cytoplasm through the nuclear pore complex and translated into a polypeptide chain.
Also Read: RNA Splicing – Process and Importance
Functions and Applications
The main function of mRNA is to provide a template for the synthesis of protein. The translation may be carried out by ribosomes present freely in the cytoplasm or present on the rough endoplasmic reticulum (RER).
The genetic information present in the RNA is translated into amino acids present in the polypeptide chain. The sequence of nucleotides or codons in the mRNA specifies the sequence of amino acids in a polypeptide chain. Each triplet codon, which is a sequence of three nucleotides is specific for one amino acid it codes for. An amino acid can be coded by more than one codon. Codons do not overlap.
mRNA directs a cell to synthesise a protein. The administration of modified mRNA sequence can be utilised to cure diseases and also can be used as a vaccine. Several mRNA vaccines are developed and are being worked upon to protect against COVID-19 infection and have got approval for restricted usage, e.g. Pfizer-BioNTech and the Moderna vaccine.
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