|MoBio||Duplicated Genes||Chapter 3|
Most proteins do not need duplicated genes, because the mRNA molecule transcribed from one gene can be used to produce many identical protein molecules. However, rRNA and tRNA are the final gene products. In order to accelerate the production process, all species contain an array of tandemly repeated RNA genes. The number of repeats ranges from tens to 24,000.
There are four types of rRNA in mammalian cells: 28S, 5.8S, 5S and 18S. In the human genome, 28S, 5.8S and 18S are clustered together. They form a single transcription unit which will be separated by specific enzymes after transcription. "Pre-rRNA" refers to their precursor. In humans, a repeat unit for the pre-rRNA has about 40 kb in length, including a 13-kb transcription unit and a 27-kb untranscribed spacer region. The transcription unit contains three spacers: ETS, ITS1 and ITS2. They will be removed during RNA processing.