Overview of RNA Structure, Synthesis, and Processing

RNA Structure, Synthesis, and Processing

OVERVIEW

The genetic master plan of an organism is contained in the sequence of deoxyribonucleotides in its deoxyribonucleic acid (DNA). However, it is through the ribonucleic acid (RNA), the “working copies” of the DNA, that the master plan is expressed (Figure 30.1). The copying process, during which a DNA strand serves as a template for the synthesis of RNA, is called transcription. Transcription produces messenger RNAs (mRNAs) that are translated into sequences of amino acids (polypeptide chains or proteins) and ribosomal RNAs (rRNAs), transfer RNAs (tRNAs), and additional small RNA molecules that perform specialized structural, catalytic, and regulatory functions and are not translated. That is, they are noncoding RNAs (ncRNAs). [Note: Only about 2% of the genome codes for proteins.] The final product of gene expression, therefore, can be RNA or protein, depending upon the gene. A central feature of transcription is that it is highly selective. For example, many transcripts are made of some regions of the DNA. In other regions, few or no transcripts are made. This selectivity is due, at least in part, to signals embedded in the nucleotide sequence of the DNA. These signals instruct the RNA polymerase where to start, how often to start, and where to stop transcription. A variety of regulatory proteins is also involved in this selection process. The biochemical differentiation of an organism’s tissues is ultimately a result of the selectivity of the transcription process. [Note: This selectivity of transcription is in contrast to the “all-or-none” nature of genomic replication.] Another important feature of transcription is that many RNA transcripts that initially are faithful copies of one of the two DNA strands may undergo various modifications, such as terminal additions, base modifications, trimming, and internal segment removal, which convert the inactive primary transcript into a functional molecule.

Figure 30.1 Expression of genetic information by transcription. [Note: RNAs shown are eukaryotic.] tRNA = transfer RNA; rRNA = ribosomal RNA; mRNA = messenger RNA; me-7Gppp = 7-methylguanosine triphosphate “cap;” AAA = poly-A “tail,”.