BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 9. NUCLEIC ACIDS

9.5. Structure and Functions of Ribonucleic Acids

9.5.2. Biological Functions of RNA

RNA molecules are found in both The Nucleus and the Cytoplasm. The total RNA content in any Cell exceeds that of DNA by 5–10 times. The primary role of RNA is to translate Genetic information into Proteins. However, RNA molecules also participate in certain specialized endonuclease Functions, which likely regulate various stages of Gene Expression. Furthermore, RNA molecules constitute the genomes of certain Viruses (Retroviruses, as well as numerous animal, plant, and insect viruses with single- and double-stranded genomes).

The RNA molecules directly involved in METABOLISM/35.html">Protein Biosynthesis include: messenger RNAs (mRNAs), which act as carriers of genetic information regarding Amino acid sequences during Translation; Ribosomal RNAs (rRNAs), which serve as Structural components of Ribosomes—Organelles that ensure template-driven Protein Synthesis; and adaptor molecules, namely Transfer RNAs (tRNAs), which participate in translating mRNA information into the Amino Acid Sequence of proteins; in other words, tRNAs establish the code correspondence between Amino Acids and their respective mRNA codons.

In addition to these Major Types of RNA, Cells contain many other RNA species (Table 9.4). Among them are Heterogeneous nuclear RNAs (hnRNAs), which undergo Processing (maturation) in the nucleus and are transported to the cytoplasm as mature mRNAs.

All Eukaryotic cells contain abundant, discrete, highly conserved, small, and stable RNA molecules, most of which exist as ribonucleoprotein particles and are localized in the nucleus, cytoplasm, or simultaneously in both compartments. These include:

✵ Small nuclear RNAs (snRNAs) - localized in The Nucleus as ribonucleoprotein particles, participating in the processing of mRNA transcripts, Cleavage of polycistronic mRNAs, maintenance of telomere integrity, and Regulation of Transcription;

✵ small cytoplasmic RNAs (scRNAs) - diverse molecules functioning in the cytoplasm that participate in controlling gene activity systems in eukaryotic cells (RNA Interference);

✵ small interfering RNAs (siRNAs) - generated from extended double-stranded RNA regions, mediating mRNA degradation and/or Chromatin modification;

✵ microRNAs - formed from large primary transcripts containing partially complementary regions (inverted repeats) that form numerous hairpin structures; microRNAs induce translational repression or mRNA degradation;

Class="center">Table 9.4

Characteristics of Various RNA Types

* P - prokaryotes, E - eukaryotes.