Types of RNA

Types of RNA

1. Types of RNA Niloufer M.A and Dr. Kayeen Vadakkan Department of Biotechnology ST.MARY’S COLLEGE, THRISSUR

2. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR INTODUCTION RNA or ribonucleic acid is a polymer of nucleotides that is made up of a ribose sugar, a phosphate, and bases such as adenine, guanine, cytosine, and uracil . It plays a crucial role in gene expression by acting as the intermediate between the genetic information encoded by DNA and proteins. RNA has a structure very similar to that of DNA. The key difference in RNA structure is that the ribose sugar in RNA possesses a hydroxyl (-OH) group that is absent in DNA.

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4. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR Messenger RNA (mRNA ) Messenger ribonucleic acids (mRNAs) transfer the information from DNA to the cell machinery that makes proteins.  mRNA accounts for just 5% of the total RNA in the cell. mRNA is the most heterogeneous of the 3 types of RNA in terms of both base sequence and size. It carries complimentary genetic code copied, from DNA during transcription, in the form of triplets of nucleotides called codons . The message of mRNA is read consecutively in 5’ 3’ direction.

5. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR Molecules of mRNA are composed of relatively short, single strands of molecules made up of adenine, cytosine, guanine and uracil bases held together by a sugar phosphate backbone. When RNA polymerase finishes reading a section of the DNA, the pre-mRNA copy is processed to form mature mRNA and then transferred out of the cell nucleus. Ribosomes read the mRNA and translate the message into functional proteins in a process called translation.

6. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR Depending on the newly synthesized protein’s structure and function, it will be further modified by the cell, exported to the extra-cellular space, or will remain inside the cell. Precursor mRNA contains introns and exons . Introns are removed before  translation , while exons code for the amino acid sequence of proteins. To make mature mRNA, the  cell  machinery removes “non-translatable” introns from the pre-mRNA, leaving only translatable  exon  sequences in the mRNA .

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8. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR Types of mRNA Pre-mRNA and hnRNA Precursor mRNA (pre-mRNA) is the primary transcript of eukaryotic mRNA as it comes off the DNA template. Pre-mRNA is part of a group of RNAs called heterogeneous nuclear RNA ( hnRNA ). hnRNA refers to all single strand RNA located inside the nucleus of the cell where transcription takes place (DNA->RNA) and pre-mRNA form a large part of these ribonucleic acids.

9. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR Pre-mRNA contains sequences that need to be removed or “spliced out” before being translated into a protein. These sequences can either be removed through the catalytic activity of the RNA itself, or through the action of a multi-protein structure called  spliceosome . After this processing step, the pre-mRNA is considered as a mature mRNA transcript. Monocistronic mRNA A monocistronic mRNA  molecule  contains the exon sequences coding for a single protein. Most eukaryotic mRNAs are monocistronic .

10. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR Bicistronic mRNA A bicistronic mRNA molecule contains the exon coding sequences for two proteins . Polycistronic mRNA A polycistronic mRNA molecule contains the exon coding sequences for multiple proteins. Most mRNA of bacteria and bacteriophages (viruses that live in bacterial hosts) are polycistronic .

11. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR Ribosomal RNA ( rRNA ) rRNAs are found in the ribosomes and account for 80% of the total RNA present in the cell. Ribosomes are composed of a large subunit called the 50S and a small subunit called the 30S, each of which is made up of its own specific rRNA molecules. Different rRNAs present in the ribosomes include small rRNAs and large rRNAs , which belong to the small and large subunits of the ribosome, respectively.

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13. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR rRNAs combine with proteins and enzymes in the cytoplasm to form ribosomes , which act as the site of protein synthesis. These complex structures travel along the mRNA molecule during translation and facilitate the assembly of amino acids to form a polypeptide chain. They interact with tRNAs and other molecules that are crucial to protein synthesis. In bacteria, the small and large rRNAs contain about 1500 and 3000 nucleotides, respectively, whereas in humans, they have about 1800 and 5000 nucleotides, respectively.

14. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR Transfer RNA ( tRNA ) tRNA is the smallest of the 3 types of RNA, possessing around 75-95 nucleotides. tRNAs are an essential component of translation, where their main function is the transfer of amino acids during protein synthesis. Therefore, they are called transfer RNAs. tRNAs also act as adapters in the translation of the genetic sequence of mRNA into proteins. Thus, they are also called adapter molecules.

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16. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR tRNAs have a clover leaf structure which is stabilized by strong hydrogen bonds between the nucleotides. They normally contain some unusual bases in addition to the usual 4, which are formed by methylation of the usual bases. Methyl guanine and methylcytosine are two examples of methylated bases. Each of the 20 amino acids has a specific tRNA that binds with it and transfers it to the growing polypeptide chain.

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