rRNA

Ribosomal RNA (rRNA) is the most abundant type of RNA in a cell. It combines with proteins to make ribosomes, the organelles where translation happens.rRNA is transcribed from DNA in the nucleolus, the dense region inside the nucleus. Here it is combined with proteins (made in the cytoplasm) to form the ribosome subunits, which then leave through nuclear pores.Each ribosome is made of two subunits, one large and one small, each containing rRNA and proteins. The rRNA strands fold into complex shapes, held by hydrogen bonds between complementary bases, which give the ribosome its structure.Ribosomes come in two sizes, measured by how fast they settle in a centrifuge (in S units):
• 80S in the cytoplasm of eukaryotic cells, free or attached to the rough endoplasmic reticulum
• 70S in prokaryotes, and in mitochondria and chloroplastsDuring translation:
• the small subunit binds the mRNA
• the large subunit holds two tRNA molecules at a time next to each other
• rRNA in the large subunit catalyses the formation of peptide bonds between the neighbouring amino acids, so rRNA acts as a catalyst as well as a structural moleculeWorked example: a cell's ribosomes are found to be 70S and free in the cytoplasm, with no nucleus present. The cell is prokaryotic.