The most abundant type of ribonucleic acid (RNA) is ribosomal RNA, or rRNA. It constitutes around 85 to 90 percent of total RNA in a cell.
RNA functions by regulating gene expression in cells. It can be classified into three major types: messenger RNA (mRNA), transfer RNA (tRNA) and rRNA. Compared to rRNA, mRNA and tRNA are significantly smaller in size.
The rRNA in cells are housed in specialized structures called ribosomes, which function as the sites for protein synthesis. In a prokaryotic cell, these organelles comprise a quarter of the cell's total weight. The large quantity of ribosomes required for protein production necessitates multiple copies of rRNA genes. In the human genome, there are approximately 150 to 300 copies of rRNA genes. These duplicate genes are grouped into clusters on various chromosomes.