RNA means RiboNucleic Acid. There are three types of RNA molecules present in every cell which are produced by transcription from DNA. They are messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (tRNA). Transfer RNA (soluble RNA) molecule contains 71 to 80 nucleotides (mostly 75) with a molecular weight of about 25,000. There are at least 20 species of tRNA, corresponding to 20 amino acids present in protein structure. The structure of tRNA (for alanine) was first elucidated by Holley. In this topic explaining tRNA structure and function.
Base-pairing occurs between complementary part of the nucleotide sequence resulting in a structure known as the Cloverleaf which is characteristic of tRNA molecule.
Parts of transfer RNA
The cloverleaf is composed of series of stem-loop structures known as arms which include acceptor, DHU, Variable and TØC arms.
- The Acceptor arm: It is formed by base-pairing between nucleotides at the 5’ and 3’ end of the tRNA. The sequence CCA, which occurs at the 3’ terminus, is not base-paired and is the point of attachment for amino acids. It contains 7 bp.
- The D or DHU arm: It is a stem-loop structure containing dihydrouracil (the unusual pyrimidine nucleotide). It has 4 bp.
- The anticodon arm: It is responsible for recognizing and binding codons in the messenger RNA. It has 5 bp.
- The Extra, optional or variable arm: it occurs only in certain tRNAs. It may be small containing 13 to 21 nucleotides with up to 5 base pairs in s stem-loop (Class-II tRNAs). Based on this variability, tRNA are classified into two categories.
- Class I tRNAs: The most predominant (about 75%) form with 3-5 base pairs length.
- Class II tRNAs: They contain 13 – 20 base pair long arm.
- The TψC arms: It contains the sequence TψC, where ψ is a modified nucleotide called pseudouracil. T means Thymine and C is Cytosine. The arm has 5 bp.
The cloverleaf structure is a two-dimensional description of the tRNA molecule. The three-dimensional structure of tRNA seems L-shaped.
Role of tRNA in Translation:
Transfer RNAs are small molecules that act as adapters during protein synthesis; they link the nucleotide sequence of the mRNA to the amino acid sequence of the polypeptide. Cells contain a number of tRNA each of which can bind a specific amino acid. Each tRNA recognizes a codon in the mRNA allowing it to place its amino acid in the appropriate position in the growing polypeptide chain as determined by the sequence of the mRNA.
tRNA synthesis and Processing:
tRNAs are synthesized by transcription of rRNA genes by the RNA polymerase-III enzyme. The tRNAs are produced as precursor RNA molecules called pre-tRNA which are processed to give mature tRNAs.
Several tRNA genes many be transcribed together as a single pre-tRNA which is then processed by ribonucleases that cleave at the 5’ end and 3’ end of each tRNA sequence in eukaryotes. In prokaryotes, processing is carried out in an ordered series of steps by the ribonucleases, RNase D and P.RNase P, which is found in prokaryotes and eukaryotes is usual in that it has an RNA component with the catalytic activity known as Ribozyme.
In eukaryotes, CCA is not presentin the DNA of the tRNA gene but is added later by a tRNA nucleotidyl transferase. In prokaryotes, the CCA is present in the coding sequence but is sometimes removed by RNase D and then replaced by a prokaryotic nucleotidyl transferase enzyme.