7 Types Of RNA Overview

What is RNA?

RNA is Ribonucleic acid, a nucleic acid present in all living cells. The subunits of this polymer are connected by phosphodiester links. It is a single-stranded nucleic acid with uracil as one of the nucleotide bases in place of thymine and ribose sugar in place of deoxyribose sugar, similar to DNA.

7 Types of RNA

RNA polymerase creates RNA from DNA that is functional for either messenger RNA (mRNA), which codes for proteins, or non-coding DNA (RNA genes). These roles make RNA molecules of the following types:

• messenger RNA (mRNA) is the RNA that conveys instructions from DNA to the cell’s ribosomes, which are where proteins are made. The amino acid sequence of the protein that is generated is determined by the mRNA coding sequences.
• rRNA, or ribosomal RNA, is incorporated into ribosomes.
• transfer RNA (tRNA) is used to transfer specific amino acids to growing polypeptide chains during translation at the ribosomal site of protein synthesis.
• smallnuclear RNA,  (snRNA), are critical components of the spliceosome that catalyze the splicing of pre-mRNA.
• microRNA (miRNA), they have tiny (22 nucleotides) RNA molecules which are used to regulate gene activity and to regulate the synthesis of messenger RNA (mRNA) molecules.
• small nucleolar RNA (snoRNA) are a class of small RNA molecules that primarily guide chemical modifications of other RNAs.
• long noncoding RNA (lncRNA), are RNAs that primarily interact with mRNA, DNA, protein, and miRNA and consequently regulate gene expression.
• catalytic RNA RNA molecules called catalytic RNA (ribozymes) are enzymatically active RNA molecules.

1. Messenger RNA (mRNA): Structure and Function.

• To enable protein synthesis and code sequencing on proteins, it is produced inside the cell nucleus and subsequently sent outside.
• During mRNA translation, polypeptides are created.
• It has a wide variety of sizes, which corresponds to the size of the polypeptide it encodes.
• Thousands of different mRNA molecules in tiny amounts, which are then translated into peptides required by the cell, are produced by the majority of cells.
• Most cells share a number of mRNA molecules that code for proteins which protect cellular metabolisms, like the glycolysis-related enzymes.
• Some mRNA types are specialized for some kinds of cells, and they encode the proteins required for that type of cell to function, such as the mRNA for hemoglobin, which is present in Red Blood Cells (RBCs).

Structure and Functions of mRNA

• The five components that make up a eukaryotic cell’s mature mRNA are:
• 5′ Cap
• This nucleotide was changed as a result of mRNA capping at the 5′ end of the original transcript. mRNA capping is crucial for controlling and producing mature mRNA during protein synthesis (translation). But take note that neither chloroplastic nor mitochondrial mRNA have caps.
• Guanine nucleotides at the 5′ end are linked to mRNA via a 5′ to 5′ triphosphate linkage. Methyltransferase methylate the guanine nucleotide in the seventh position, resulting in a structure known as a 7-methyl guanylate cap (m7G) after capping.
• Chemically, this cap is comparable to the 3′ end of the RNA molecule.
• Small nuclear RNAs (snRNA) with a 5′ trimethylguanosine cap and long non-coding RNAs (lncRNA) with a 5′ monomethyl phosphate cap are the only RNAs that have this cap.
• The mRNA is capped with NAD+, NADH, or 3′ dephospho-coenzyme A in bacteria and certain other species.
• messenger RNA decapping is a process used by all organisms to uncap mRNA molecules.
• 5′ untranslated region (5′ UTR)
• This section of the mRNA is located just before the initiation codon.
• It is necessary for viruses, prokaryotes, and eukaryotes to control the translation of a transcript.
• The major coding sequence on the mRNA is translated according to the main 5′ untranslated region, that is occasionally translated into protein products.
• The 5′ UTR does not translate in certain species, thus it creates the intricate secondary structure that controls translation.
• Coding region
• This area of the RNA codes for proteins.
• The coding region of DNA is located on either side of the promoter sequence, which the RNA polymerase attaches to before travelling down the template strand to the coding region during transcription. The RNA polymerase introduces RNA nucleotides which are complementary to the coding region, producing the mRNA by swapping out thymine for uracil. The process keeps going up until the termination sequence takes place.
• Three prime untranslated region (3′ UTR)
• This section is the part of the mRNA that comes after the translation termination codon. This unit contains the regulatory regions that post-transcriptionally influence gene expression.
• Similar to the 5′ UTR, the 3′ UTR is not translated into proteins.
• However, the 3′ UTR’s regulatory regions have an impact on polyadenylation, translational effectiveness, translation location, and mRNA stability.
• It contains attachment spots for regulatory proteins and miRNAs that regulate and reduce the levels of gene expression for certain mRNAs by either obstructing translation or causing direct transcript destruction.
• A silencer region in the 3′ UTR also works with repressor proteins to halt the mRNA’s transcription.
• Adenine-Uracil (AU) rich elements, such as the sequences (AAUAAA), are found in many 3′ UTRs and they guide the addition of adenine residues known as poly (A) tail to the end of the mRNA transcript.
• Additionally, the 3′ UTR has regions that encourage proteins to associate with mRNA and the cell cytoskeleton.
• It moves it to or from the cell nucleus or carries out various sorts of localization inside the cell.
• In general, the 3′ UTR aids in gene regulation and makes sure the right genes are produced properly and at the appropriate times.
• Poly A tail
• The poly(A) tail is a segment of RNA composed entirely of adenine bases and is composed of many adenosine monophosphates.
• It is bound by poly-A binding proteins (PABP), which are crucial in controlling the translation, stability, and export of mRNA.
• Interacting with proteins is the poly (A) tail, which is connected to the PABP, which is connected to the transcript’s 5′ end, which results in the movement of the mRNA and the stimulation of protein translation.
• Polyadenylation is the process of adding a poly-A tail to mRNA.
• Eukaryotic polyadenylation aids in the synthesis of mature messenger RNA (mRNA), which is necessary for translation.
• The poly-A tail has proved helpful in accelerating the degradation of mRNA in a number of bacteria, which means that poly-A contributes to the overall process of gene expression.
• As the genes’ transcription stops, poly-A begins to play a part in polyadenylation.
• The poly-A tail is essential during nuclear export, translation, and mRNA stability.
• The synthesis of mRNA is deteriorated enzymatically. as the tail gets too short over time.

2. ribosomal RNA (rRNA): Structure and Functions

Structure

• The kind of RNA found in ribosomes is called ribosomal ribonucleic acid (rRNA).
• It is referred to as the molecular device that facilitates protein production.
• Since they are essential for several of the ribosomes’ activities, including binding to mRNA, attracting tRNA, and catalysing the creation of peptide bonds between amino acids, they account for up to 60% of the weight of ribosomes.
• The rRNA core is used to identify the shape of ribosomes as well.
• Internal loops and helices give rRNA its particular three-dimensional form, which produces the specialised A, P, and E sites in the ribosome.
• The A site serves as an anchor for an entering tRNA charged with an amino acid, while the P site serves as a binding site for the expanding polypeptide.
• The tRNA temporarily attaches to the E site following the establishment of the peptide bond before exiting the ribosomes. The tRNA temporarily attaches to the E site following the creation of a peptide bond before exiting the ribosome.
• Additionally, the rRNA analyses and distinguishes RNA residues from protein residues and has a spot for attachment to ribosomal proteins.
• The proteins on the ribosome surface support the structural integrity of the ribosome by connecting with the rRNA core.

Function

• In particular, the nucleoli of the cell’s nucleus are where the rRNA is either generated or translated. Through the sequestration of ribosomal proteins, the nucleoli play a significant part in the synthesis of ribosomes.
• The translation of mRNA brings together the bigger and smaller subunits that make up prokaryotic and eukaryotic ribosomes.
• A RNA molecule with a Svedberg coefficient of 16S and a length of around 1500 nucleotides makes up the tiny subunits of prokaryotes.
• The 30S sedimentation rate is shared by the small subunit and ribosomal proteins.
• This pairs with the bigger component, which contains two RNA molecules: one with an approximately 3000-nucleotide (23S) length and the other with a 120-nucleotide short sequence (5S). The proteins that form the bigger 50S subunit are present with these RNA molecules.
• The big subunit 60S and the tiny subunit 40S make up the two subunits that make up eukaryotic ribosomes.
• The small subunit is formed by two short rRNA molecules, each fewer than 200 nucleotides in length (5S and 5.8S), whereas the big subunit is built up of two longer large molecules, one over 5kb (28S) and another over 2kilobases in length (18S).
• The 28S, 18S, and 5.8S molecules are produced by processing a single primary transcript from a cluster of homologous pairs of a single gene. The same genes in a separate cluster of genes make the 5S molecules.
• The Svedberg coefficient of the eukaryotic ribosome as a whole is 80S.
• rRNA is also present in eukaryotic cells’ mitochondria and chloroplasts.
• Ribosomes may float freely in a cell’s cytoplasm or be connected to the endoplasmic reticulum.

3. transfer RNA (tRNA): Structure and Functions

• The amino acids are transported to the ribosomes by this non-coding RNA molecule from the growing peptide chain (mRNA nucleotide sequence).
• Therefore, between nucleotide and amino acid sequences, the tRNA serves as an intermediary.
• Since they are ribonucleotides, they combine mRNA and amino acids during translation by forming ester linkages with amino acids and a hydrogen bond with mRNA.

Structure and Function

• It is a short (80 nucleotide or so) RNA chain.
• The developing polypeptide chain in the ribosome receives particular amino acids from the tRNA during translation that match the mRNA sequence.
• Each of tRNA’s base pairs has three nucleotides coupled to mRNA, and these pairings occur in parallel.
• Short molecules of 70–90 nucleotides are used to code for tRNAs (5nm).
• The three-nucleotide sequence on the mRNA is referred to as a codon, and the identical sequence on the tRNA is referred to as an anticodon.
• A translation mechanism is formed by the anticodon and codon pairing at bases.
• The anticodon amino acid sequence that is connected to the tRNA 3′ hydroxyl base links the ribosomes to form a peptide bond, lengthening the polypeptide chain.
• The anticodon and the 3′ hydroxyl group terminal are thus the two primary components of tRNA.
• The D-arm and the T-arm, two additional components of the tRNA structure, are extremely effective and highly specific.
• A sugar-phosphate backbone provides tRNAs their directionality.
• The reactive phosphate group on one end of the tRNA is connected to the ribose’s fifth carbon atom (5′), and the free hydroxyl group on the other end is coupled to the third carbon (3′), giving rise to the 5′ to 3′ ends of RNA.
• The molecule’s acceptor arm, which is bound covalently to the hydroxyl group on the ribose sugar, is made up of the three bases CCA (Cytosine, Cytosine, and Adenine), which are present at the 3′ terminal end.
• Parts of the 5′ end of the tRNA, which is composed of 7-9 nucleotides on opposing ends of the molecule that base pair with one another, are also found in the acceptor arm.
• The amino acid that connects to the acceptor’s arm is determined by the anticodon loop, which is recognised by the aminoacyl tRNA synthetase (AATS) when it is coupled with mRNA.
• The D-arm from the 5′ end of tRNA is read by and recognized by the AATS.
• The D-arm regulates and influences the speed and precision of translation at the ribosomes, and it plays a significant role in maintaining the structure of RNA.
• By interacting with the ribosomes, the T-arm also affects how tRNA affects translation.
• The anticodon loop, T-arm, and D-arm together resemble a cloverleaf.
• The acceptor stem, T-arm, anticodon loop, and D-arm stretch to form an L-shaped structure when RNA folds into a tertiary structure.

4. small nuclear RNA (snRNA): Structure and Functions

• When DNA is being transcribed, primary transcripts for mRNA, rRNA, and tRNA are processed in the nucleus to produce operational elements that will be transferred to the cytoplasm. Some of these processes are mediated by small nuclear RNA (snRNA), which plays this role.
• About 150 nucleotides of RNA polymerase II or RNA polymerase III are used to synthesize snRNA.
• They help in the processing of pre-messenger RNA and are a component of the spliceosomes into mRNA by excising the introns and splicing the exons. They include numerous copies of various genes that perform unique functions in the production of snRNA and other RNA types.
• Additionally, they operate as mediators in the control of RNA polymerase II and transcription factors.
• They also keep the telomeres intact.
• Small nuclear ribonucleoproteins, or snRNPs, are proteins and complexes that interact specifically with snRNA.

5. small nucleolar RNA (snoRNA): Structure and Functions

• They are tiny RNAs that range in size from 60 to 300 nucleotides and are involved in a variety of cellular processes.
• Through the cutting of the 28S, 18S, and 5.8S big RNA precursors, they contribute to the creation of ribosomes.
• By adding groups like methyl groups to ribose, they chemically change many of the nucleotides in rRNA, tRNA, and snRNA molecules.
• Additionally, they support the splicing of pre-mRNA into several mature mRNA forms.
• The production of telomeres uses a specific form of snoRNA as a template.
• The snoRNAs in vertebrates are created from introns that are excised during transcription.

6. microRNAs (miRNAs): Structure and Functions

• A microRNA is a single-stranded, short non-coding RNA of 22 nucleotides. It is believed to be the same size as siRNAs.
• It is present in all animals, including humans and certain viruses. Its main functions include post-transcriptional gene expression control and RNA silencing.
• About 1000 miRNAs are produced by humans.
• These stand-alone genes or specific regions of the introns of the genes that control the mRNA encode the miRNAs in the genome.
• When various cell types are differentiating, they are expressed in some cell types at specific periods.
• They accomplish this by controlling the expression of mRNA in one of two ways:
• The mRNA is destroyed while the sequences are uniformly paired, particularly in plants.
• By inhibiting mRNA translation when the sequences are partly similar,
• Two of miRNA’s characteristics are responsible for these defining functions.
• Due to their small size, they can control mRNA gene expression for both partial and perfectly paired gene sequences without needing to be translated into a protein element, which makes it simple to quickly transcribe from their gene.
• According to genomic analyses of mammalian gene expression, one or more miRNAs are linked to an mRNA that has been translated from DNA.
• Numerous miRNA binding sites on mRNA make it possible for a single miRNA to bind to around 200 distinct mRNA targets.
• As a result, mRNA translation may be coordinated.

7. long non-coding RNA(lncRNA): Structure and Functions

• This is a diverse collection of 200 nucleotide long non-coding transcript RNA.
• They make up the biggest non-coding transcriptome in mammals.
• In the human genome, there are 8000 lncRNAs, according to estimates.
• Although the majority of lncRNA’s activities are still unclear, some scientific evidence points to its participation in physiological mechanisms and gene regulation.
• Its recognized roles in gene regulatory pathways include some of the following:
• splicing
• translation
• imprinting
• transcription
• XIST-RNA, a specific kind of lncRNA, inactivates one of the two X chromosomes in female vertebrates.
• They contribute to the control of gene transcription through looping, which brings the enhancer and promoter regions of genes closer together.

NOTE:

Three-quarters of transcription that occurs in the cell nucleus is carried out by non-coding RNAs (tRNA, rRNA, snoRNA, snRNA, miRNA, and lncRNA).

References and Sources

• Microbiology by Prescott
• https://www.futurelearn.com/courses/translational-research/0/steps/14201
• The Biology Dictionary-rRNA Notes
• The Biology Dictionary -tRNA Notes
• https://www.news-medical.net/life-sciences/-Types-of-RNA-mRNA-rRNA-and-tRNA.aspx
• http://www.phschool.com/science/biology_place/biocoach/transcription/difgns.html
• https://bio.libretexts.org/Bookshelves/Genetics/Book%3A_Working_with_Molecular_Genetics_(Hardison)/Unit_III%3A_The_Pathway_of_Gene_Expression/10%3A_Transcription%3A_RNA_polymerases
• https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Biology_(Kimball)/06%3A_Gene_Expression/6.02%3A_The_Transcription_of_DNA_into_RNA
• https://www.ebi.ac.uk/chebi/searchId.do?chebiId=74035
• https://www.genome.gov/genetics-glossary/Transfer-RNA
• http://bioscience.jbpub.com/cells/MBIO5245.aspx
• https://en.wikipedia.org/wiki/Small_nuclear_RNA#:~:text=Small%20nuclear%20RNA%20(snRNA)%20is,snRNA%20is%20approximately%20150%20nucleotides.
• https://www.futurelearn.com/courses/translational-research/0/steps/14201
• https://www.news-medical.net/life-sciences/-Types-of-RNA-mRNA-rRNA-and-tRNA.aspx
• https://www.britannica.com/science/RNA#ref340177
• https://en.wikipedia.org/wiki/Polyadenylation
• https://en.wikipedia.org/wiki/Three_prime_untranslated_region
• https://en.wikipedia.org/wiki/Coding_region
• https://en.wikipedia.org/wiki/Five_prime_untranslated_region
• https://en.wikipedia.org/wiki/Five-prime_cap
• https://en.wikipedia.org/wiki/Messenger_RNA
• https://www.genome.gov/genetics-glossary/messenger-rna#:~:text=Messenger%20RNA%20(mRNA)%20is%20a,cytoplasm%20where%20proteins%20are%20made.