Before mRNA can exit the nucleus and bind to a ribosome in the cytoplasm, it must be modified. First a 5’ cap and a poly-A tail are added to its 5’ and 3’ ends respectively, to protect the mature mRNA from degradation by hydrolytic enzymes in the cytoplasm. Next, noncoding segments of the primary transcript, called introns, are removed by spliceosomes. The exons, untranslated regions (UTR), 5’ cap and poly-A tail are then spliced together to produce the final mRNA, which can direct the synthesis of a particular polypeptide. Since different segments can be considered introns or exons, a single gene may be able to code for different polypeptides by alternative RNA splicing.
During translation, the genetic message carried by mRNA is translated into the appropriate polypeptide or RNA molecule. After the mature mRNA exits the nucleus through a nuclear pore, it binds to a small ribosomal subunit, initiator tRNA and large ribosomal subunit, forming the translation initiation complex. tRNA plays a vital role in the elongation of the polypeptide by transferring cytoplasmic amino acids to the growing polypeptide chain. The anticodon at one end of the tRNA binds to the complementary codon of the mRNA and the corresponding amino acid is covalently bonded to its other end by aminoacyl-tRNA synthetases. As the tRNA moves from the ribosomes A site to P site, its amino acid is added to the polypeptide chain. The tRNA then moves to the E site and exits the ribosome. This process continues until a stop codon is reached and a release factor binds to the mRNA and hydrolyzes the bond between the polypeptide and the tRNA.