Structure and Types of RNA

RNA comprises polyribonucleotide chains in which the bases are usually adenine, guanine, uracil, and cytosine. It is found in both the nucleus and the cytoplasm of cells. 

Question What are the main differences, other than chemical composition, between RNA and DNA  

There are a greater variety of RNA forms, with molecular weights in the range 25,000 to several million. Most RNAs contain a single polynucleotide chain, but this can fold back on itself to form double-helical regions containing A U and G C base pairs. 

Question What types of RNA occur in a typical cell and what are their functions  

There are three major types, transfer RNA (tRNA), ribosomal RNA (rRNA), and messenger RNA (mRNA) their roles in the expression of genetic information are treated in detail in Chap. 17. 

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Transcription starts with the process hy which the genetic information is transcribed onto a form of RNA, called mRNA. Ribonucleic acid, RNA, is structurally similar to DNA with the exceptions that its nucleotides contain ribose, instead of a 2 -deoxyribose, and the base thymine is replaced by uracil. There are three major types of RNA depending on their specific functions. However, all three types of RNA are much smaller than DNA and they are single stranded, rather than double stranded. 

RNA has a variety of functions within a cell for each function, a specific type of RNA is required. Messenger RNA (mRNA) serves as intermediaries for carrying genetic messages from the DNA to the ribosomes where protein synthesis lakes place. Ribosomal RNA (rRNA) serves both structural and functional roles in the ribosome it is diverse, both in terms of its size and structure. Transfer RNAs (tRNAs) are small molecules that have a central role in protein synthesis. Other RNA molecules, called ribozymes, function as enzymes to catalyze chemical transformations. Although ribozymes most often catalyze cleavage of the... 

Nuclear extracts can be fractionated by chromatography on DEAE-cellulose to give three peaks of RNA polymerase activity (the use of column chromatography is explained in chapter 6). These three peaks correspond to three different RNA polymerases (I, II, and III), which differ in relative amount, cellular location, type of RNA synthesized, subunit structure, response to salt and divalent cation concentrations, and sensitivity to the mushroom-derived toxin a-amanitin. The three polymerases and some of their properties are summarized in table 28.4. 

Besides having a much lower molar mass than DNA, RNA generally forms only single-strand helices. RNA is often found associated with proteins inside cells. The most prevalent bases in RNA are the same as those in DNA, except that uracil is present instead of thymine. Three common types of RNA are ribosomal (rRNA), transfer (tRNA), and messenger RNA (mRNA). They are all involved in protein synthesis, controlling the sequence of amino acids that make up the primary structure. Thus the base sequence in RNA is related to the amino acid sequence in the protein that is made from it. 

Ribosomal RNA is present in the ribosomes, which contain approximately an equal mass of protein. Ribosomal RNA makes up about 80 percent of the total RNA in the cell and is of several types, distinguished from one another by their sedimentation rates in an ultracentrifuge (Chap. 4). Bacterial ribosomes, for example, contain three types of RNA 5S, 16S, and 23S. The details of ribosome structure and function are treated in Chap. 17. 

The mRNA transcript is a linear molecule but can have secondary structure through autocomplementarity as indicated above. In addition to mRNA there are other types of RNA, notably ribosomal RNA (rRNA) and transfer RNA (tRNA). The rRNAs in eukaryotes include 18S, 5.8S, 28S and 5S rRNAs (S, the Svedberg, being a measure of rate of sedimentation in ultracentrifugation and hence of relative size). The rRNAs have extensive secondary structure. The rRNAs and a number of proteins make up the ribosome upon which translation occurs. 

As a close relative to the brome mosaic virus the cowpea chlorotic mottle vims (CCMV) possesses an outer diameter of 28 nm, is assembled from 180 subunits that create an icosahedral type structure, and contains an RNA core. " Another similarity is the cationic interior surface of the viral capsid which is formed from subunits with highly basic N-termini (6 positively charged arginine and 3 lysine residues) that project into the cavity and stabilize the RNA core. However as a potential biomineralization template, the electrostatic environment of the virion cavity precludes any cationic metal precursors and thus limits its applicability for nanoparticle synthesis. 

In deoxyribonucleic acid (DNA) the carbohydrate is 2-deoxy-D-ribose, while in ribonucleic acid (RNA) the carbohydrate residue is ribose. Three types of RNA were recognized, and they can be messenger RNA (mRNA), transfer RNA (tRNA), or ribosomal RNA (rRNA), which is the most abundant in cells. Values between 10 and 10 Dalton have been reported for the molecular weight of DNA, and the molecular weight is about 10 for rRNA, 10 for mRNA, and lOMor rRNA. The simplified structures of DNA and RNA are the following ... 

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