Main Difference – mRNA vs tRNA
Messenger RNA (mRNA) and transfer RNA (tRNA) are two types of major RNAs functioning in protein synthesis. Protein coding genes in the genome are transcribed into mRNAs by RNA polymerase enzyme. This step is the first step in protein synthesis, and is known as protein encoding. This protein encoded mRNA are translated at the ribosomes into polypeptide chains. This step is the second step in protein synthesis, and is known as protein decoding. The tRNAs are the carriers of specific amino acids encoded in the mRNA. The main difference between mRNA and tRNA is that mRNA serves as the messenger between genes and proteins whereas tRNA carries the specified amino acid into the ribosome in order to process the protein synthesis.
This article explains,
1. What is mRNA
– Structure, Function, Synthesis, Degradation
2. What is tRNA
– Structure, Function, Synthesis, Degradation
3. What is the difference between mRNA and tRNA
What is mRNA
The messenger RNA is a type of RNAs found in cells encoding the protein coding genes. The mRNA is considered as the carrier of the message of a protein into the ribosome which facilitates the protein synthesis. Protein coding genes are transcribed into mRNAs by the enzyme RNA polymerase during the event known as transcription, which occurs in the nucleus. The mRNA transcript following the transcription is referred to as the primary transcript or pre-mRNA. The primary transcript of mRNA undergoes post-transcriptional modifications inside the nucleus. The mature mRNA is released into the cytoplasm for translation. Transcription followed by translation is the central dogma of molecular biology, as shown in figure 1.
Figure 1: Central dogma of molecular biology
mRNA Structure
The mRNA is a linear, single-stranded molecule. A mature mRNA consists of a coding region, untranslated regions (UTR), 5′ cap and a 3′ poly-A tail. The coding region of mRNA contains a series of codons, which are complementary to the protein-coding genes in the genome. The coding region contains a start codon in order to initiate the translation. The start codon is AUG, which specifies the amino acid methionine in the polypeptide chain. The codons followed by the start codon are responsible for determining the amino acid sequence of the polypeptide chain. Translation ends at the stop codon. The codons, UAA, UAG and UGA are responsible for the end of the translation. Other than determining the amino acid sequence of the polypeptide, some regions of the coding region of the pre-mRNA are also involved in the regulation of pre-mRNA processing and serves as exonic splicing enhancers/silencers.
The regions of the mRNA found former and latter to the coding region are called as 5′ UTR and 3′ UTR, respectively. The UTRs controls the mRNA stability by varying the affinity for RNase enzymes which degrade the RNAs. The mRNA localization is performed in the cytoplasm by the 3′ UTR. The translation efficiency of mRNA is determined by the proteins bound to the UTRs. Genetic variations in the 3′ UTR region lead to the disease susceptibility by changing the structure of RNA and protein translation.
Figure 2: Mature mRNA structure
The 5′ cap is a modified nucleotide of guanine, 7-methylguanosine which binds through a 5′-5′-triphosphate bond. The 3’poly-A tail is several hundred of adenine nucleotides added to the 3′ end of the mRNA primary transcript.
The eukaryotic mRNA forms a circular structure by interacting with the poly-A binding protein and the translation initiation factor, eIF4E. Both eIF4E and poly-A binding proteins bind with the translation initiation factor, eIF4G. This circulation promotes a time-efficient translation by circulating the ribosome on the mRNA circle. The intact RNAs will also be translated.
Figure 3: The mRNA circle
Synthesis, Processing, and Function mRNA
The mRNA is synthesized during the event known as transcription, which is the first step of the process of protein synthesis. The enzyme involved in the transcription is RNA polymerase. The protein coding genes are encoded into the mRNA molecule and exported into the cytoplasm for the translation. Only the eukaryotic mRNA undergoes the processing, which produces a mature mRNA from pre-mRNA. Three major events occur during pre-mRNA processing: 5′ cap addition, 3′ cap addition and splicing out of introns.
The addition of 5′ cap occurs co-transcriptionally. The 5′ cap serves as a protection from RNases and is critical in recognition of mRNA by ribosomes. The addition of 3′ poly-A tail/polyadenylation immediately occurs after the transcription. The poly-A tail protects the mRNA from RNases and promotes the export of mRNA from the nucleus to the cytoplasm. Eukaryotic mRNA consists of introns between two exons. Thus, these introns are removed from the mRNA strand during the splicing. Some mRNAs are edited in order to change their nucleotide composition.
Translation is the event where the mature mRNAs are decoded in order to synthesize an amino acid chain. The prokaryotic mRNAs do not possess post-transcriptional modifications and are exported to the cytoplasm. Prokaryotic transcription occurs in the cytoplasm itself. Therefore, prokaryotic transcription and the translation are considered to occur simultaneously, reducing the time taken for the synthesis of proteins.The eukaryotic mature mRNAs are exported to the cytoplasm from the nucleus just after their processing. Translation is facilitated by the ribosomes which are either freely-floating in the cytoplasm or bound to the endoplasmic reticulum in eukaryotes.
mRNA Degradation
Prokaryotic mRNAs generally have a comparatively long lifetime. But, eukaryotic mRNAs are short-lived, allowing the regulation of gene expression. Prokaryotic mRNAs are degraded by different types of ribonucleases including endonucleases, 3′ exonucleases and 5′ exonucleases. RNase III degrade small RNAs during RNA interference. RNase J also degrades prokaryotic mRNA from 5′ to 3′. Eukaryotic mRNAs are degraded after the translation only by either exosome complex or decapping complex. Eukaryotic untranslated mRNAs are not degraded by ribonucleases.
What is tRNA
tRNA is the second type of RNA which is involved in protein synthesis. The anticodons are individually borne by the tRNAs which are complementary to a particular codon on the mRNA. tRNA carries specified amino acid by the codons of the mRNA into the ribosomes. The ribosome facilitates the formation of peptide bonds between the existing and incoming amino acids.
tRNA Structure
The tRNA consists of primary, secondary and tertiary structures. The primary structure is a linear molecule of tRNA. It is around 76 to 90 nucleotides long. The secondary structure is clover-leaf shaped structure. The tertiary structure is an L-shaped 3D structure. The tertiary structure of the tRNA allows it to fit with the ribosome.
Figure 4: The mRNA secondary structure
The tRNA secondary structure consists of a 5′ terminal phosphate group. The 3′ end of the acceptor’s arm contains the CCA tail which is attached to the amino acid. The amino acid is contently linked to the 3′ hydroxyl group of the CCA tail by the enzyme, aminoacyl tRNA synthetase. Amino acid loaded tRNA is known as the aminoacyl-tRNA. The CCA tail is added during the processing of tRNA. Secondary structure tRNA consists of four loops: D-loop, TΨC loop, variable loop and the anticodon loop. The anticodon loop contains the anticodon which is a complementary bound with the codon of the mRNA inside the ribosome. The secondary structure of the tRNA becomes its tertiary structure by coaxial stacking of the helices. The tertiary structure of the aminoacyl-tRNA is shown in figure 5.
Figure 5: Aminoacyl tRNA
Functions of tRNA
An anticodon makes up of a nucleotide triplet, containing individually in each tRNA molecule. It is capable of base pairing with more than one codon through wobble base pairing. The first nucleotide of the anticodon is replaced by the inosine. The inosine is capable of hydrogen bonding with more than one specific nucleotide in the codon. Anticodon is in the 3′ to 5′ direction in order to base pair with the codon. Therefore, the third nucleotide of the codon varies in the redundant codon specifying the same amino acid. For example, the codons, GGU, GGC, GGA and GGG code for the amino acid glycine. Thus, a single tRNA brings the glycine for all of the above four codons. Sixty-one distinct codons can be identified on the mRNA. But, only thirty-one distinct tRNAs are required as the amino acid carriers due to the wobble base pairing.
The translation initiation complex is formed by the assembling of two ribosomal units with theaminoacyl tRNA. The aminoacyl tRNA binds to the A site and the polypeptide chain binds to the P site of the large subunit of the ribosome. Translation initiation codon is AUG which specifies the amino acid methionine. The translation processes through the translocation of the ribosome on the mRNA by reading the codon sequence. The polypeptide chain grows by forming polypeptide bonds with the incoming amino acids.
Figure 6: Translation
In addition to its role in protein synthesis, it also plays a role in the regulation of gene expression, metabolic processes, priming reverse transcription and stress responses.
tRNA Degradation
The tRNA is reactivated by attaching to a second amino acid specific to it after releasing its first amino acid during translation. During the quality control of RNA, two surveillance pathways are involved in degradation of hypo-modified and miss-processed pre-tRNAs and mature tRNAs which are lack modifications. The two pathways are nuclear surveillance pathways and the rapid tRNA decay (RTD) pathway. During the nuclear surveillance pathway, miss-modified or hypo-modified pre-tRNAs and mature tRNAs are subjected to 3′ end polyadenylation by TRAMP complex and undergo rapid turnover. It was first discovered in the yeast, Saccharomyces cerevisiae. The rapid tRNA decay (RTD) pathway was first observed in trm8∆trm4∆ yeast mutant strain which is temperature sensitive and lacks tRNA modification enzymes. Most of the tRNAs are correctly folded under the normal temperature conditions. But, variations of the temperature lead to hypo-modified tRNAs and they are degraded by the RTD pathway. The tRNAs containing mutations in the acceptor stem as well as the T-stem are degraded during the RTD pathway.
Difference Between mRNA and tRNA
Name
mRNA: The m stands for messenger; messenger RNA
tRNA: The t stands for transfer; transfer RNA
Function
mRNA: The mRNA serves as the messenger between genes and proteins.
tRNA: The tRNA carries the specified amino acid into the ribosome in order to process the protein synthesis.
Location of Function
mRNA: The mRNA functions at the nucleus and the cytoplasm.
tRNA: The tRNA functions at the cytoplasm.
Codon/Anticodon
mRNA: The mRNA carries a codon sequence which is complementary to the codon sequence of the gene.
tRNA: The tRNA carries an anticodon which is complementary to the codon on the mRNA.
Continuity of the Sequence
mRNA: The mRNA carries an order of sequential codons.
tRNA: The tRNA carries individual anticodons.
Shape
mRNA: The mRNA is linear, single-stranded molecule. Sometimes the mRNA forms the secondary structures like hair pin loops.
tRNA: The tRNA is L-shaped shaped molecule.
Size
mRNA: The size depends on the sizes of the protein coding genes.
tRNA: It is around 76 to 90 nucleotides long.
Attachment to Amino Acids
mRNA: The mRNA do not attach with the amino acids during protein synthesis.
tRNA: The tRNA carries a specific amino acid by attaching to its acceptor arm.
Fate after Functioning
mRNA: The mRNA is destroyed after the transcription.
tRNA: The tRNA is reactivated by attaching it to a second amino acid specific to it after releasing its first amino acid during translation.
Conclusion
The messenger RNA and transfer RNA are two types of RNAs involved in the protein synthesis. Both of them are composed of four nucleotides: adenine (A), guanine (G), cytosine (C) and thymine (T). Protein coding genes are encoded into mRNAs during the process known as transcription. The transcribed mRNAs are decoded into an amino acid chain with the aid of ribosomes during the process known as translation. The specified amino acid required for the decoding of mRNAs into proteins are carried by distinct tRNAs into the ribosome. Sixty-one distinct codons can be identified on the mRNA. Thirty-one distinct anticodons can be identified on distinct tRNAs specifying the twenty essential amino acids. Therefore, the main difference between mRNA and tRNA is that mRNA is a messenger of a specific protein whereas tRNA is a carrier of a specific amino acid.
Reference:
1.“Messenger RNA.” Wikipedia. N.p.: Wikimedia Foundation, 14 Feb. 2017. Web. 5 Mar. 2017.
2.“Transfer RNA.” Wikipedia. N.p.: Wikimedia Foundation, 20 Feb. 2017. Web. 5 Mar. 2017.
3.“Structural biochemistry/nucleic acid/RNA/transfer RNA (tRNA) – Wikibooks, open books for an open world.” n.d. Web. 5 Mar. 2017
4.Megel, C. et al “Survaillence and cleavage of eukaryotic tRNAs”. International Journel of Molecular Sciences, . 2015, 16, 1873-1893; doi:10.3390/ijms16011873. Web. Accessed 6 Mar. 2017
Image Courtesy:
1. “MRNA-interaction”- original uploader: Sverdrup at English Wikipedia. (Public Domain) via Commons Wikimedia
2. “Mature mRNA” (CC BY-SA 3.0) via Commons Wikimedia
3. “MRNAcircle”By Fdardel – Own work (CC BY-SA 3.0) via Commons Wikimedia
4. “TRNA-Phe yeast en” By Yikrazuul – Own work (CC BY-SA 3.0) via Commons Wikimedia
5. “Peptide syn”By Boumphreyfr – Own work (CC BY-SA 3.0) via Commons Wikimedia
6. “Aminoacyl-tRNA”By Scientific29 – Own work (CC BY-SA 3.0) via Commons Wikimedia
Leave a Reply