![]() ![]() Transcription is simple to understand as a means of information transfer: since DNA and RNA are chemically and structurally similar, the DNA can act as a direct template for the synthesis of RNA by complementary base-pairing. Once an mRNA has been produced, by transcription and processing the information present in its nucleotide sequence is used to synthesize a protein. And indeed, not only has the code been cracked step by step, but in the year 2000 the elaborate machinery by which cells read this code-the ribosome-was finally revealed in atomic detail.Īn mRNA Sequence Is Decoded in Sets of Three Nucleotides Here was a cryptogram set up by nature that, after more than 3 billion years of evolution, could finally be solved by one of the products of evolution-human beings. This feat of translation first attracted the attention of biologists in the late 1950s, when it was posed as the “coding problem”: how is the information in a linear sequence of nucleotides in RNA translated into the linear sequence of a chemically quite different set of subunits-the amino acids in proteins? This fascinating question stimulated great excitement among scientists at the time. In this section we examine how the cell converts the information carried in an mRNA molecule into a protein molecule. However, most genes in a cell produce mRNA molecules that serve as intermediaries on the pathway to proteins. Reading frame: The way a sequence of genetic code (DNA or RNA) is split into groups of three nucleotides (codons) from the beginning of the sequence the reading frame for translation is set by the start codon AUG.In the preceding section we have seen that the final product of some genes is an RNA molecule itself, such as those present in the snRNPs and in ribosomes. ![]() Start codon: The codon AUG, which both signals the start of translation and encodes the amino acid methionine. Stop codon: One of three codons (UAA, UAG, and UGA) that signals the end of translation. Translation: The process where RNA is used to create a new polypeptide chain (protein). It also encodes the amino acid methionine.Ĭodon: A sequence of three adjacent nucleotides that encode a specific amino acid or stop signal during protein synthesis (translation). ![]() One codon (AUG) is known as a start codon and initiates the process of translation.In special cases, the stop codons UGA and UAG can encode special 21st and 22nd amino acids.Three codons (UAA, UAG, and UGA) are known as stop codons and they signal the termination of translation.Myelogenous leukemia and two chemotherapeutic agents.The reading frame for translation is set by the AUG start codon. In addition to specifying the amino acid methionine, it also serves as the start codon to initiate translation. One of the 61 codons that encode amino acids, AUG, has a special function. The stop codon UAG is also sometimes used by a few species of microorganisms to encode a 22nd amino acid called pyrrolysine (Pyl). However, the stop codon UGA is sometimes used to encode a 21st amino acid called selenocysteine (Sec), but only if the mRNA additionally contains a specific sequence of nucleotides called a selenocysteine insertion sequence (SECIS). In general, these do not encode amino acids. These triplets are called stop codons: UGA, UAG, and UAA. Three of the 64 codons terminate protein synthesis and release the polypeptide from the translation machinery. Most of these amino acids can be encoded by more than one codon. Of the 64 possible codons present in the genetic code, three encode stop signals that terminate translation and one encodes both a start signal that initiates translation and the amino acid methionine.Ħ1 of the possible 64 codons encode twenty different amino acids. ![]()
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