| 31 Ribonucleic Acid (RNA)
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After reading this chapter you should be able to:
- Identify the major types of cellular RNA and the function of each.
- Describe the major steps in transcription of an RNA molecule.
- Explain the function of the different RNA polymerase enzymes.
- Describe the different processing and splicing events that occur during synthesis of eukaryotic mRNAs.
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Table 31-1.
General classes of RNA. |
| Body_ID: None |
| General classes of RNA |
| Body_ID: T031001.50 |
| RNA | Size and length | Percent of total cellular RNA | Function |
| Body_ID: T031001.100 |
| rRNA | 28s, 18s, 5.8s, 5s (26s, 16s, 5s)* | 80 | interact to form ribosomes |
| Body_ID: T031001.150 |
| tRNA | 65-110 nt | 15 | adapter |
| Body_ID: T031001.200 |
| mRNA | 0.5-6 kb | 5 | direct synthesis of cellular proteins |
| Body_ID: T031001.250 |
*Size of rRNA in prokaryotic cells. Nt, nucleotides; Kb, kilobases; S, svedbergs.
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| Transcription is defined as the synthesis of a ribonucleic acid (RNA) molecule using deoxyribonucleic acid (DNA) as a template. This rather simple definition describes a series of complicated enzymatic processes that result in the transfer of the genetic information stored in double-stranded DNA into a single-stranded RNA molecule that will be used by the cell to direct the synthesis of its proteins. There are three general classes of RNA molecules found in prokaryotic and eukaryotic cells: ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA). Each class has a distinctive size and function (Table 31.1), described by its sedimentation rate in an ultracentrifuge (S, Svedbergs) or its number of bases (nt, nucleotides, or kb, kilobases). Prokaryotes have the same
three general classes of RNA as eukaryotes, but their RNAs differ in size and in some structural features:
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- ribosomal RNA (rRNA) from prokaryotes consists of three different sizes of RNA, while rRNA from eukaryotes consists of four different sizes of RNA. These RNAs interact with each other, and with proteins, to form a ribosome that provides the basic machinery on which protein synthesis takes place;
- transfer RNAs (tRNAs) consist of one size class of RNA that are 65-110 nucleotides in length; they function as adapter molecules that translate the information stored in the mRNA nucleotide sequence to the amino acid sequence of proteins;
- messenger RNAs (mRNAs) represent the most heterogeneous class of RNAs found in cells, ranging in size from 500 nt to >6 kb; they are carriers of genetic information, defining the sequence of all proteins in the cell.
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| In order to understand the complex series of events that lead to the production of these three classes of RNA, this chapter is divided into four parts. The first part deals with the molecular anatomy of the major types of RNA found in prokaryotic and eukaryotic cells; knowing then the chemical nature of the final products of transcription, you will be better prepared to understand the steps involved in generating these molecules. The second part describes the main enzymes involved in transcription, and their specificities. The third part describes the three steps (initiation, elongation, and termination) required to produce an RNA transcript. Finally, in the last section, the modifications that are made to the primary products of transcription (post-transcriptional processing) are described. This information is expanded in Chapter 33: Control of Gene Expression.
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