The second step in the implementation of genetic information is the synthesis of a protein molecule based on messenger RNA (translation). However, unlike transcription, the nucleotide sequence cannot be translated directly into the amino acid sequence, since these compounds have different chemical nature. Therefore, for the implementation of translation, an intermediary is required in the form of transport RNA (tRNA), the function of which is to translate the genetic code into the "language" of amino acids.
General characteristics of transport RNA
Transport RNAs or tRNAs are small molecules that deliver amino acids to the site of protein synthesis (in the ribosomes). The amount of this type of ribonucleic acid in a cell is approximately 10% of the total RNA pool.
Like other types of ribonucleic acids, tRNA consists of a chain of ribonucleoside triphosphates. The length of the nucleotide sequence is 70-90 units, and about 10% of the composition of the molecule falls on the minor components.
Due to the fact that each amino acid has its own carrier in the form of tRNA, the cell synthesizes a large number of varieties of this molecule. Depending on the type of living organism, this indicator varies from 80 to 100.
TRNA functions
Transport RNA is a supplier of a substrate for protein synthesis that occurs in ribosomes. Due to the unique ability to bind to amino acids and the matrix sequence, tRNA acts as a sense adapter when translating genetic information from the RNA form to the protein form. The interaction of such an intermediary with a coding matrix, as in transcription, is based on the principle of complementarity of nitrogen bases.
The main function of tRNA is to accept amino acid units and transport them to the protein synthesis apparatus. Behind this technical process is a huge biological meaning - the implementation of the genetic code. The implementation of this process is based on the following features:
- all amino acids are encoded by triplets of nucleotides;
- for each triplet (or codon) there is an anticodon that is part of tRNA;
- each tRNA can only bind to a specific amino acid.
Thus, the amino acid sequence of a protein is determined by which tRNAs and in what order will complementarily interact with messenger RNA during translation. This is possible due to the presence of functional centers in the transport RNA, one of which is responsible for the selective attachment of amino acids, and the other for binding to the codon. Therefore, the functions and structure of tRNA are closely interrelated.
The structure of transport RNA
The uniqueness of tRNA lies in the fact that its molecular structure is not linear. It includes spiral double-stranded sections called stems, and 3 single-stranded loops. In shape, this conformation resembles a clover leaf.
The following stems are distinguished in the structure of tRNA:
- acceptor;
- anti-codon;
- dihydrouridyl;
- pseudouridyl;
- additional.
Double spirals of stems contain from 5 to 7 Watson-Crixon pairs. At the end of the acceptor stem is a small chain of unpaired nucleotides, the 3-hydroxyl of which is the site of attachment of the corresponding amino acid molecule.
The structural region for the connection with mRNA is one of the tRNA loops. It contains an anticodon complementary to the sense triplet in messenger RNA. It is the anticodon and the accepting end that provide the adapter function of tRNA.
Tertiary structure of the molecule
The “clover leaf” is the secondary structure of tRNA, however, due to folding, the molecule acquires an L-shaped conformation, which is held together by additional hydrogen bonds.
The L-form is the tertiary structure of tRNA and consists of two almost perpendicular A-RNA helices having a length of 7 nm and a thickness of 2 nm. This form of the molecule has only 2 ends, on one of which there is an anticodon, and on the other - an acceptor center.
Features of binding of tRNA to amino acid
The activation of amino acids (their attachment to transport RNA) is carried out by aminoacyl-tRNA synthetase. This enzyme simultaneously performs 2 important functions:
- catalyzes the formation of a covalent bond between the 3`-hydroxyl group of the acceptor stem and the amino acid;
- provides the principle of selective compliance.
Each of the 20 amino acids has its own aminoacyl-tRNA synthetase. It can interact only with the corresponding type of transport molecule. This means that the anticodon of the latter must be complementary to the triplet encoding this particular amino acid. For example, leucine synthetase will only bind to leucine-targeted tRNA.
The aminoacyl-tRNA synthetase molecule has three nucleotide-binding pockets, the conformation and charge of which are complementary to the nucleotides of the corresponding anticodon in tRNA. Thus, the enzyme determines the desired transport molecule. Much less often, the nucleotide sequence of the acceptor stem serves as a recognition fragment.