Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 132

DNA carries the information for the synthesis of RNA and proteins in regions called genes. The first step in expressing a gene is to transcribe an RNA copy from one strand in that region. The word “transcription” is descriptive, because the information in RNA is copied from DNA in the same language, which is written in a sequence of nucleotides. If the gene carries information for a protein, this RNA transcript is called messenger RNA (mRNA). An mRNA is a messenger because it carries the information encoded in a gene to a ribosome, which is the main machinery for protein synthesis. Once on the ribosome, the information in the mRNA can be translated into the protein. Translation is another descriptive word, because one language—a sequence of nucleotides in DNA and RNA—is translated into a different language—a sequence of amino acids in a protein. The mRNA is translated as it moves through the ribosome, 3 nucleotides at a time. Each 3-nucleotide sequence, called a codon, carries information for a specific amino acid. The assignment of each of the possible codons to an amino acid is called the genetic code.

The actual translation from the language of nucleotide sequences to the language of amino acid sequences is performed by small RNAs called tRNAs and enzymes called aminoacyl-tRNA synthetases (aaRSs). The aaRS enzymes attach specific amino acids to their matching tRNAs. Each aminoacylated tRNA (aa-tRNA) specifically pairs with a codon in the mRNA as it moves through the ribosome, and the amino acid carried by the tRNA is added to the growing protein. The tRNA pairs with the codon in the mRNA through a 3-nucleotide sequence in the tRNA called the anticodon that is complementary to the codon in the mRNA. The base-pairing rules for codons and anticodons are basically the same as the base-pairing rules for DNA replication, and the pairing is antiparallel. The only major differences are that RNA has uracil (U) rather than thymine (T) and that the pairing between the last of the 3 bases in the codon and the first base in the anticodon is less stringent.

This basic outline of gene expression leaves many important questions unanswered. How does mRNA synthesis begin and end at the correct places and on the correct strand in the DNA? Similarly, how does translation start and stop at the correct places on the mRNA? What actually happens to the tRNA and ribosomes during translation? What happens to the mRNA and proteins after they are made? The answers to these questions and many others are important for the interpretation of genetic experiments, so we will discuss the structure of RNA and proteins and the processes by which they are synthesized in more detail.