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Every living cell contains DNA composed of four nucleotides. The precise order of these nucleotides encodes genes that instruct the cell to produce the proteins and RNAs essential for growth and replication. While each chromosome carries a single copy of DNA per cell, the genes it harbors are often transcribed into numerous RNA molecules.
Cells rely on three primary RNA species: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). mRNA serves as the blueprint for protein synthesis, whereas tRNA and rRNA facilitate the assembly of amino acids into polypeptide chains. Ribosomes—complex machines made of proteins and rRNA—translate mRNA into functional proteins. In this process, tRNA brings the correct amino acid to match the codon on the mRNA, while rRNA catalyzes the peptide bond formation that links amino acids together.
Typical animal cells contain billions of proteins, each synthesized on a ribosome. Rapidly dividing cells can harbor up to ten million ribosomes, underscoring the high demand for these molecular factories.
A ribosome consists of two subunits that converge around an mRNA strand to build a protein. The subunits are composed of over 50 proteins that provide structural support, organized around four large rRNA molecules that both maintain ribosomal shape and catalyze the peptide‑bonding reaction. Ribosome assembly begins in the nucleus, where rRNA is transcribed from DNA, processed into fragments, and combined with proteins. The partially assembled ribosomes are then exported to the cytoplasm for final maturation and ready for translation.
To meet the requirement for up to ten million ribosomes, cells repeat rRNA genes in tandem arrays—about 100 copies of each rRNA gene reside in a typical animal genome. Despite this amplification, the cell must transcribe thousands of rRNA molecules to assemble the necessary ribosomes, which explains the abundance of rRNA relative to its singular DNA template per cell.
