RNA, mRNA, rRNA, and tRNA. See

RNA, mRNA, rRNA, and tRNA. See Ribonucleic acid Roberts, John D., 928 Robinson, Sir Robert, 4, 402, 724 Robinson annulation, 724, 728 Rotamer, 90. See also Conformation Rotational energy barrier alkenes, 172—173 amides, 779 butane, 94—95 conjugated dienes, 31(r-371 ethane, 93—94... 

Function. In all cells, RNA functions in the transfer of genetic information from DNA to the site of protein biosynthesis (mRNA) and in the translation of this information during protein biosynthesis (mRNA, rRNA and tRNA). In addition, the RNA in . coli and Bacillus subtilis ribonuclease P has been shown to be responsible for the catalytic activity of the ribo-protein [C. Guerrier-Thkada S. Altman Science 223 (1984) 285-286]. The RNA in Small nuclear ribonu-cleoproteins (see) also appears to be catalytically active other examples of catalytically active RNA are known, e.g. the autocatalydc cleavage-ligation of Te-trahymena pre-ribosomal RNA (see Intron). 

Almost all of the RNA in the cell is synthesized in the nucleus, in this process, known as transcription, the information stored in DNA is transcribed into RNA (see p. 242). As mentioned above, ribosomal RNA (rRNA) is mainly produced in the nucleolus, while messenger and transfer RNA (mRNA and tRNA) are formed in the region of the euchromatin. Enzymatic duplication of DNA—replication—also only takes place in the nucleus (see p. 240). 

Transcription is catalyzed by DNA-dependent RNA polymerases. These act in a similar way to DNA polymerases (see p. 240), except that they incorporate ribonucleotides instead of deoxyribonucleotides into the newly synthesized strand also, they do not require a primer. Eukaryotic cells contain at least three different types of RNA polymerase. RNA polymerase I synthesizes an RNA with a sedimentation coef cient (see p. 200) of 45 S, which serves as precursor for three ribosomal RNAs. The products of RNA polymerase II are hnRNAs, from which mRNAs later develop, as well as precursors for snRNAs. Finally, RNA polymerase III transcribes genes that code for tRNAs, 5S rRNA, and certain snRNAs. These precursors give rise to functional RNA molecules by a process called RNA maturation (see p. 246). Polymerases II and III are inhibited by a-amanitin, a toxin in the Amanita phalloides mushroom. 

The bulk of the cellular RNA is ribosomal RNA. Although seven genes exist in E. coli for rRNA, they all lead to essentially the same three ribosomal RNA molecules (see table 28.1) which differ substantially in size. The three rRNAs are always found in a complex with proteins in a functional component known as the ribosome. The ribosome is the site where mRNA and tRNAs meet to engage in protein synthesis. In E. coli, ribosomes are referred to as 70S particles, a measure of their rate of sedimentation and hence their size (S refers to Svedberg units, which are defined in chapter 6). A 70S ribosome consists of two dissociable subunits A 50S subunit and a 30S subunit. Each of these contains both RNA and protein. The 50S subunit contains 23S and 5S rRNAs. The 30S subunit contains a single 16S rRNA (fig. 28.5). Eukaryotic ribosomes are similar in structure, although they are somewhat larger (80S) and con-... 

RNA secondary structure Most RNA molecules are single-stranded but an RNA molecule may contain regions which can form complementary base-pairing where the RNA strand loops back on itself (Fig. 2). If so, the RNA will have some double-stranded regions. Ribosomal RNAs (rRNAs) and transfer RNAs (tRNAs) (see Topics G9 and G10, respectively) exhibit substantial secondary structure, as do some messenger RNAs (mRNAs). 

In prokaryotes, RNA transcribed from protein-coding genes (messenger RNA, mRNA), requires little or no modification prior to translation. In fact, many mRNA molecules begin to be translated even before RNA synthesis has finished. However, ribosomal RNA (rRNA) and transfer RNA (tRNA) are synthesized as precursor molecules that do require post-transcriptional processing (see Topics G9 and G10, respectively). 

Deoxyribonucleic acid (DNA), the genetic material, carries information to specify the amino acid sequences of proteins. It is transcribed into several types of ribonucleic acid (RNA), including messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), which function in protein synthesis (see Figure 4-1). 

There are many different kinds of RNA manufactured in a cell. Ales-senger RNA (mRNA) is produced to take the information contained in a specific segment of DNA and then use it to make proteins. Ribosomal RNA (rRNA) is part of a large RNA protein complex called the ribosome that binds mRNA and joins amino acids to make a protein. Transfer RNA (tRNA) brings amino acids to the ribosome and ensures that the amino acid used is in the order specified by mRNA. Many other kinds of RNA are also present in a cell. RNA plays an important role in the proper ftmctioning of a cell. SEE ALSO DNA Replication Nucleic Acids Proteins. 

Transfer RNA has a very low molecular weight when compared to those of mRNA and rRNA. Transfer RNA, consequently, is much more soluble than mRNA or rRNA and is sometimes referred to as soluble RNA. The function of tRNA is to transport amino acids to specific areas on the mRNA bound to the ribosome. There are, therefore, many forms of tRNA, more than one for each of the 20 amino acids that is incorporated into proteins, including the redundancies in the genetic code (see Table 25.2). ... 

Types of RNA and their occurrence. There are three main types of RNA, classified on the basis of their function Messenger RNA (mRNA) (see), ribosonuil RNA (rRNA) (see Ribosomes) and Transfer RNA (tRNA) (see) they also have different secondary and tertiary structures. Viral RNA is structurally and functionally very similar to mRNA. In eukaryotic cells, RNA is found in the nucleus, cytoplasm and in the cytoplasmic organelles (ribosomes, mitochondria, chloroplasts). The nucleus is the chief site of RNA... 

Ribosomes are high-molecular-weight complexes of RNA (rRNA) and proteins (Table I), and the electron-dense particles are easily visualized by electron microscopy. Ribosomes from various sources (prokaryotes, eukaryotic cytoplasm, mitochondria, chloroplasts, and kinetoplasts) vary in size from 20 to 30 nm in diameter, but all are composed of a large and a small subparticle or subunit and perform similar functions in protein synthesis. The principal functional domains of the ribosome and associated components are given in Fig. 10. More detailed resolution of the ribosome structure has allowed the placement of mRNA, aminoacyl-tRNA, peptidyl-tRNA, and the nascent polypeptide chain (see Section C and Fig. 11). 

Eukaryotic RNA polymerases - Eukaryotes contain three distinct RNA polymerases, one each for the synthesis of the three larger rRNAs, mRNA, and small RNAs (tRNA plus the 5S species of rRNA). These are called RNA polymerases I (see here), II (here), and III (here), respectively. The enzymes differ in their sensitivity to inhibition by oi-amanitin (Figure 26.4b), a toxin from the poisonous Amanita mushroom. RNA polymerase II is inhibited at low concentrations, RNA polymerase III is inhibited at high concentrations, and RNA polymerase I is quite resistant. 

There are other types of RNA, which have different roles. Transfer RNA (tRNA) contains 73-93 nucleotides and recruits amino acids to participate in the protein synthesis process that the mRNA directs. Ribosomal RNA (rRNA) is part of the cell s machinery that enables the protein synthesis process to occur (by acting as a catalyst, see the discussion of protein synthesis further in this section). These three types of RNA molecules all contain nucleotides and constitute stepping stones along the path from DNA to protein,... 

In addition to tRNA, the cytoplasm also contains rRNA (in the ribosomes) and mRNA (in polysomes). Other nucleoprotein particles have also been demonstrated, which represent transport forms of mRNA (see Informosomes). All cytoplasmic RNA is synthesized in the nucleus. Mitochondria and plasdds also contain mRNA, tRNA and rRNA which are transcribed from the DNA of the organelle. 

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