In this section, you will learn the types of RNA. In both types of cells i.e. prokaryotic and eukaryotic cells there are three types of RNA, mRNA,(messenger RNA,) rRNA,( ribosomal RNA), and tRNA (transfer RNA).
The structure of RNA is similar to the DNA, with only one difference ribose sugar in RNA has a (-OH) hydroxyl group which is absent in DNA.
mRNA: (Messenger RNA)
This type of RNA only accounts for 5% of the total RNA present in a cell. It contains genetic code copied, from DNA during transcription, in the form of a triplet of nucleotides called codons. Each codon specifies a specific amino acid. There are 64 possible codons in the genetic code, and only 20 of them represent amino acids. There are 3 stop codons( AUG), (UAA), and (UGA) which show that ribosomes stop protein synthesis by the process of translation.
Now let us start to understand, how mRNA produces. What are parts of mRNA? This concept was suggested by Jacob and Monad. They predicted that the storage house of information in our cells is DNA. The function of DNA is to transfer information to ribosomes. The agent (carrier) that carries information is mRNA. Actually, DNA is a double-stranded molecule. Its one end is 5’ and the second is 3’. There are two regions in DNA. One is the gene also called the coding region and the other is the spacer also called a non-coding region. The spacer region or non-coding region is present between two or more gene regions. The DNA which is a double-stranded molecule produces a single-stranded mRNA. mRNA has complementary regions of DNA molecules. RNA polymerase helps in the transcription of mRNA. In the prokaryotic cell, there is only one RNA polymerase. On the other hand in a eukaryotic cell, there are three types of RNA polymerase i.e. RNA polymerase 1, RNA polymerase 2, and RNA polymerase 3. RNA polymerase is 5’ to 3’. mRNA which is transcribed from DNA has two parts. One is intron and the other is exon. Such mRNA is called hnRNA which stands for heteronuclear RNA. It is also called PT primary transcript because it is first transcribed. Introns and exons are two parts of a gene. The intron is the coding part while the exon is the non-coding part of the gene. Exons the non-coding parts split from mRNA with the help of an enzyme. This process is called splicing. The concept of splicing was presented by Philip Sharp. mRNA after splicing becomes ST, a secondary transcript. mRNA after splicing comes in its active form. Then it leaves the nucleus and comes into the cytoplasm where it binds with the ribosomes and starts its function i.e. synthesizes protein.
tRNA: (Transfer RNA)
This is another type of RNA. tRNA stands for transfer RNA. It was discovered by Paul Zamecnik and collaborators. There are 75-90 nucleotides present on tRNA. Generally, it is one of the smallest among all types of RNA. It is also called soluble RNA because it dissolves in one molar solution of sodium chloride (NaCl).
Shape: A scientist Robert Holley described tRNA as a clover leaf because it looks like a plant. It is composed of two parts. Loop portion and a stem portion. Complementary sequences are present on the stem portion and absent on the loop portion of tRNA. Hydrogen bonding occurs due to the presence of complementary sequences on the stem portion. tRNA looks like a double-stranded molecule, but actually, it is a single-stranded molecule, like other types of RNA. It is due to its folding habit. On 3’ OH group is present, which acts as a binding site for the amino acids. When amino acids are attached to the OH group it transfers them to the ribosome. That is why tRNA is also called an adaptor molecule.
Function: The main function of tRNA is to read the message of nucleotides, and translate it into protein or amino acids. The process of making protein from mRNA is known as translation. A tRNA reads mRNA in three-letter nucleotide sequences called codons. Each codon corresponds to an amino acid. There are 4 nucleotides in mRNA, so taking a cube of 4 will answer 64 (4^3=64). So there are 64 codons exist. Moreover, there is only one tRNA for each codon. Interestingly, there are only 21 amino acids. This brings up the idea that our genetic code is redundant. That is, we have 64 codons but 21 amino acids. Actually, our genetic code is redundant but not ambiguous. For example, the codons, GUU, GUC, GUA, and GUG all code for Valine, and not of them specify any other amino acid. As we know that the function of tRNA is to bring amino acids to the ribosome, each codon has its own tRNA and each tRNA has its own amino acid attached to it. We also know that the basic function of tRNA is to transfer amino acids to ribosomes for protein synthesis. The tRNA doesn’t become part of the protein which indicates that the tRNA can either be attached to an amino acid or remains free.
rRNA: ( Ribosomal RNA)
rRNA stands for ribosomal RNA. It is 80% of the total RNA of the cell. rRNA was discovered by Kuntze. It is the most stable form of RNA. rRNA is present in ribosomes and produced in nucleolus which is a granular substance present in the nucleus. At the time of protein synthesis, it provides a binding site to the tRNA and mRNA and attaches the amino acids to the ribosome. It attaches tRNA to the larger subunit and mRNA to the smaller subunit of the ribosome on which both RNAs interact to translate information of gene into specific protein, thus tRNA acts as the machinery of protein synthesis.
When the messenger RNA is complete, the final proteins are translated by the ribosomal RNA and then allow the conversion of RNA into amino acid peptide sequences.
The mammalian ribosomes have a sedimentation velocity of 80S unit. It has a larger 60S subunit and another smaller 40S subunit. They contain different amino acids ribosomal RNAs and specific proteins. Ribosomal RNA has catalytic activity. Peptidyl transferase activity is carried out by 28S RNA which acts as a ribozyme. The ribosomal RNA is transcribed by RNAP-1 as a single precursor.