RNA

RNA molecules are synthesized by enzymes known as RNA polymerases in a process called transcription. Usually, one strand of a double-stranded DNA molecule is used as a template for the RNA. The order of ribonucleotides that are assembled to form the RNA molecule is determined by the order of the deoxyribonucleotides in the DNA strand. The genetic information RNA nucleotides link together to form a chain. The phosphate group, attached to the 5′ carbon of ribose, links to the OH group at the 3′ position. This chain itself has a 5′ end and a 3′ end, as indicated. in the DNA sequence is thus reproduced in the RNA molecule. Sometimes, but rarely, an RNA molecule is synthesized using another RNA molecule as the template.

Often, when RNA molecules are synthesized, they are in a form that prevents them from carrying out their function. To become functional, they must undergo processing, which can involve removing segments of the strands or modifying specific nucleotides. The link between a base and a ribose may be altered, or extra chemical groups may be added to the bases or ribose molecules. Many RNA molecules are associated with proteins during or after their synthesis. Together, the RNA and protein are referred to as RNA-protein particles (RNPs).

The clover-leaf shape (left) is characteristic of transfer RNA. The L shaped figure (right) is the more compacted form the tRNA adopts in the cell. The asterisks within the RNA sequence indicate modified RNA nucleotides that occur in tRNA only. The anticodon binds with a complementary sequence on messenger RNA. Adapted from Robinson, 2001.

In eukaryotes, RNA that is encoded by nuclear chromosomes is synthesized in the nucleus. The processing and assembly of many small RNA molecules in higher eukaryotes is accomplished in Cajal bodies, which are coiled structures in the nucleus that were identified more than 100 years ago but that have begun to be investigated in detail only recently. The synthesis of those RNA molecules that are components of ribosomes occurs in the nucleolus, a part of the nucleus. RNA synthesis and processing also occurs in the mitochondria and chloroplasts, when the RNA will be used in those organelles.

After being processed and assembled, RNPs either remain in the nucleus or are exported to the cytoplasm through the nuclear pores. Some are also exported and modified in the cytoplasm and then imported back into the nucleus. In prokaryotes, where there is no nucleus, the synthesis and processing of RNA, as well as the assembly of RNPs, occurs in the cytoplasm.