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Molecular Basis of Heredity: Part 1. Nucleic Acids

Author(s): Raye L. Alford, PhD


After processing, messenger RNA (mRNA) is transported from the nucleus of eukaryotic cells to the cytoplasm, where it comes into contact with ribosomes, the protein-making machinery of the cell. Ribosomes progress down the mRNA strand, in a 5’ to 3’ direction, creating a protein as they go. Ribosomes are complex structures made of protein and RNA, and can be found floating freely within the cytoplasm of eukaryotic cells or attached to the rough endoplasmic reticulum (RER).

Transfer RNA (tRNA) is the mediator for translation. tRNAs of eukaryotes are transcribed by an enzyme called RNA polymerase III. Like mRNAs, tRNAs undergo processing, but the processing of tRNAs is somewhat different from that of mRNA. tRNAs are cleaved from precursor RNAs and modified by the addition of nucleotides to their 3’ ends, and by methylation and other enzymatic modifications of certain ribonucleotides. The tRNA precursors of eukaryotes also contain an intron that must be removed by splicing.

tRNAs include an anti-codon loop, which contains a three-nucleotide long segment of RNA that binds to a complementary three-nucleotide long segment in an mRNA called a codon. tRNAs also have an attached amino acid. When a tRNA anti-codon associates, under the guidance of the ribosome, with its complementary mRNA codon, the amino acid attached to the tRNA is transferred from the tRNA to the growing protein molecule. As the ribosome moves down the mRNA molecule, additional tRNA molecules come in, bind to the codons embedded within the nucleotide sequence of the mRNA, and add amino acids to the growing protein, as specified by the nucleotide sequence of the mRNA and the gene from which it was transcribed.

Another class of RNAs, called ribosomal RNAs, are components of ribosomes. The ribosomal RNAs, rRNAs, of eukaryotes are transcribed by an enzyme called RNA polymerase I and also are processed before assembly into ribosomes.