The structure and function of RNA. RNA types
RNA molecules are found in the nucleus, cytoplasm, ribosomes, mitochondria and plastids of the cell.
Ribonucleic acid (RNA) is a linear polymer consisting of a single strand of nucleotides. RNA monomers (nucleotides) are composed of a five-carbon sugar – ribose, a phosphoric acid residue and a nitrogenous base.
Three nitrogenous bases in RNA molecules are the same as in DNA – adenine, guanine, cytosine, and the fourth is uracil.
The RNA polymer is formed (as in DNA) through covalent bonds between ribose and the phosphoric acid residue of adjacent nucleotides.
Informational, or messenger, RNA (mRNA) make up about 5% of all cellular RNA. They are synthesized in the nucleus (at the site of one of the chains of the DNA molecule) with the participation of the RNA polymerase enzyme.
The function of mRNA is to remove information from DNA and transfer it to the site of protein synthesis – to ribosomes.
Ribosomal (ribosomal) RNA (rRNA) – synthesized in the nucleolus, are part of the ribosome. They participate in the formation of the active center of the ribosome, where the process of protein biosynthesis takes place. rRNAs make up approximately 85% of all RNA in a cell.
Transport RNA (tRNA) – are formed in the nucleus on DNA, then pass into the cytoplasm. They make up about 10% of cellular RNA and are the smallest in size (70–90 nucleotides).
tRNAs transport amino acids to the site of protein synthesis on the ribosome. For the transfer of each type of amino acid to the ribosome, a separate type of tRNA is needed.
The structure of all tRNAs is similar. Their molecules form peculiar structures resembling a clover leaf in shape.
The types of tRNA are distinguished by the nucleotide triplet located “at the top”. This triplet (anticodon) is genetically complementary to the mRNA codon encoding the corresponding amino acid.
The amino acid is attached by a special enzyme to the “leaf petiole” and transported to the active center of the ribosome.
Thus, different types of RNA represent a single functional system aimed at the implementation of hereditary information through protein synthesis.