RNA

Several types of less abundant, small RNA molecules perform essential functions in both the nucleus and the cytoplasm. All organisms contain cytoplasmic RNPs that are involved in exporting proteins from cells. During the synthesis of proteins that are destined to be exported, the ribosome and mRNA associate with an "export-RNP," which helps them dock at an export pore in the cell membrane. As it formed, the protein is threaded through the membrane to the outside of the cell. In eukaryotes, this same strategy is used to transport proteins into the endoplasmic reticulum, where some newly synthesized proteins are sorted and modified.

RNase P is another RNP found in all forms of life. This RNA-containing enzyme helps turn precursor tRNA into mature tRNA molecules. It does so by cleaving a section off the 5′ end of the precursor molecules.

Small nucleolar RNAs, which are known as snoRNAs and which are found in the nucleoli of eukaryotes and in Archaea, are required for the processing of precursor rRNA. During the assembly of new ribosomes, snoRNAs help remove regions of the precursor molecules and modify specific nucleotides.

Often, mRNA molecules in eukaryotes and in Archaea contain sequences that do not code for amino acids. These sequences, called introns, must be Proposed mechanism for RNA interference. Double-stranded RNA from a virus is recognized and degraded by the enzyme dicer. These fragments can then pair up with complementary host messenger RNA (mRNA), causing further degradation and gene silencing. spliced out before translation begins. In eukaryotes, small nuclear RNAs (snRNAs) in the nucleus remove these introns. Once the introns are removed, the mature mRNA molecules are exported, through nuclear pores, into the cytoplasm, where they associate with ribosomes for translation.

Some viral genomes consist of single-stranded or double-stranded RNA, not DNA. Examples are found among both prokaryotic and eukaryotic viruses and include HIV, as well as viruses causing some forms of cancer.