Less Common Types of 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 cell cycle sequence of growth, replication and division that produces new cells endoplasmic reticulum network of membranes within the cell

Archaea one of three domains of life, a type of cell without a nucleus

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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. see also Evolution, Molecular; Nucleotide; Ribosome; RNA Polymerases; RNA Processing; Transcription; Translation.

Lasse Lindahl

Bibliography

Lodish, Harvey, et al. Molecular Cell Biology, 4th ed. New York: W. H. Freeman, 2000.

Meili, M., B. Albert-Fournier, and M. C. Maurel. "Recent Findings in the Modern RNA World." International Microbiology 4 (2001): 5-11.

Robinson, Richard. Biology. Farmington Hills, MI: Macmillan Reference USA, 2002.

Storz, G. "An Expanding Universe of Non-coding RNAs." Science 296 (2002): 1260-1263.

translation synthesis of protein using mRNA code

RNA Interference

RNA interference is a process in which translation of some of a cell's messenger RNA (mRNA) sequences is prevented, because of the presence of (and consequent destruction of) matching double-stranded RNA sequences. RNA interference is believed to protect the cell against viruses and other threats. "Interference" refers to the interruption of the cell's translation of its own mRNA. RNA interference is also called posttranscriptional gene silencing, since its effect on gene expression occurs after the creation of the mRNA during transcription.

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