Molecular Basis of Heredity: Part 1. Nucleic Acids
RNA Processing in Eukaryotic Cells
The RNA molecules made within eukaryotic cells must be processed before they can be used as messengers of the DNA code. Eukaryotic genes contain introns that must be removed. Introns are non-coding segments of DNA, of variable size, that separate the coding segments, or exons, of the genes of eukaryotic organisms. During RNA processing, the introns are removed from RNA molecules in a complex process called splicing. In addition, a methyl group called a 5' cap is attached to the 5' end of the RNA, and a polyA tail is added to the 3' end of the RNA in a process called polyadenylation. The resulting processed RNA is called a messenger RNA (mRNA). Processed mRNAs migrate from the nucleus (the site of their transcription and modification) to the cytoplasm for translation by ribosomes.
Prokaryotic genes typically do not contain introns, although there are some exceptions. As such, there is usually no splicing of RNA molecules in prokaryotic cells. Prokaryotic RNAs also are not capped or polyadenylated as are eukaryotic RNAs. Further, in prokaryotic cells, there is no nucleus to separate the processes of transcription and translation, so transcription and translation often occur simultaneously, with RNA molecules being translated into proteins as they are being transcribed from the prokaryotic DNA. A newly discovered branch of the tree of life is the domain Archaea. Members of the domain Archaea are prokaryotes, as are bacteria. However, some genes of members of the domain Archaea have introns and share other structural and functional similarities with eukaryotes (organisms with a cell nucleus surrounded by a membrane).
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