Ribosomal RNA (rRNA) is a type of RNA that plays a crucial role in protein synthesis. It is transcribed from ribosomal DNA (rDNA) and then bound to ribosomal proteins to form small and large ribosome subunits. rRNA is the physical and mechanical factor of the ribosome that forces transfer RNA (tRNA) and messenger RNA (mRNA) to process and translate the latter into proteins. The rRNAs form extensive secondary structures and play an active role in recognizing conserved portions of mRNAs and tRNAs. During the translation of mRNA, rRNA functions to bind both mRNA and tRNA. Each group of three nucleotides on the mRNA is called a codon and there is a corresponding tRNA with the anticodon. Within the ribosome, the tRNA briefly attaches to the mRNA, and a peptide bond is formed between this and the preceding amino acid. The polypeptide grows in this fashion until the end of the mRNA is reached.