Harpers

RNA Synthesis, Processing,& Modification 37Daryl K. Granner, MD, & P. Anthony Weil, PhDBIOMEDICAL IMPORTANCEThe synthesis of an RNA molecule from DNA is acomplex process involving one of the group of RNApolymerase enzymes and a number of associated proteins.The general steps required to synthesize the primarytranscript are initiation, elongation, and termination.Most is known about initiation. A number ofDNA regions (generally located upstream from the initiationsite) and protein factors that bind to these sequencesto regulate the initiation of transcription havebeen identified. Certain RNAs—mRNAs in particular—havevery different life spans in a cell. It is importantto understand the basic principles of messengerRNA synthesis and metabolism, for modulation of thisprocess results in altered rates of protein synthesis andthus a variety of metabolic changes. This is how all organismsadapt to changes of environment. It is also howdifferentiated cell structures and functions are establishedand maintained. The RNA molecules synthesizedin mammalian cells are made as precursor moleculesthat have to be processed into mature, activeRNA. Errors or changes in synthesis, processing, andsplicing of mRNA transcripts are a cause of disease.RNA EXISTS IN FOUR MAJOR CLASSESAll eukaryotic cells have four major classes of RNA: ribosomalRNA (rRNA), messenger RNA (mRNA), transferRNA (tRNA), and small nuclear RNA (snRNA).The first three are involved in protein synthesis, andsnRNA is involved in mRNA splicing. As shown inTable 37–1, these various classes of RNA are differentin their diversity, stability, and abundance in cells.RNA IS SYNTHESIZED FROM A DNATEMPLATE BY AN RNA POLYMERASEThe processes of DNA and RNA synthesis are similarin that they involve (1) the general steps of initiation,elongation, and termination with 5′ to 3′ polarity; (2)large, multicomponent initiation complexes; and (3)adherence to Watson-Crick base-pairing rules. Theseprocesses differ in several important ways, including thefollowing: (1) ribonucleotides are used in RNA synthesisrather than deoxyribonucleotides; (2) U replaces Tas the complementary base pair for A in RNA; (3) aprimer is not involved in RNA synthesis; (4) only a verysmall portion of the genome is transcribed or copiedinto RNA, whereas the entire genome must be copiedduring DNA replication; and (5) there is no proofreadingfunction during RNA transcription.The process of synthesizing RNA from a DNA templatehas been characterized best in prokaryotes. Althoughin mammalian cells the regulation of RNA synthesisand the processing of the RNA transcripts aredifferent from those in prokaryotes, the process of RNAsynthesis per se is quite similar in these two classes oforganisms. Therefore, the description of RNA synthesisin prokaryotes, where it is better understood, is applicableto eukaryotes even though the enzymes involvedand the regulatory signals are different.The Template Strand of DNAIs TranscribedThe sequence of ribonucleotides in an RNA molecule iscomplementary to the sequence of deoxyribonucleotidesin one strand of the double-stranded DNAmolecule (Figure 35–8). The strand that is transcribedor copied into an RNA molecule is referred to as thetemplate strand of the DNA. The other DNA strand isfrequently referred to as the coding strand of that gene.It is called this because, with the exception of T for Uchanges, it corresponds exactly to the sequence of theprimary transcript, which encodes the protein productof the gene. In the case of a double-stranded DNA moleculecontaining many genes, the template strand foreach gene will not necessarily be the same strand of theDNA double helix (Figure 37–1). Thus, a given strandof a double-stranded DNA molecule will serve as thetemplate strand for some genes and the coding strandof other genes. Note that the nucleotide sequence of anRNA transcript will be the same (except for U replacingT) as that of the coding strand. The information in thetemplate strand is read out in the 3′ to 5′ direction.341