Three of the 64 possible codons do not code for specific amino acids; these have been termed nonsense codons. Nonsense codons are utilized in the cell as translation termination signals by specifying where the polymerization of amino acids into a protein molecule is to stop. The remaining 61 codons code for the 20 naturally occurring amino acids (see Table 1). Thus, there is “degeneracy” in the genetic code—that is, multiple codons decode the same amino acid. Some amino acids are encoded by several codons; for example, six different codons, UCU, UCC, UCA, UCG, AGU, and AGC all specify serine. Other amino acids, such as methionine and tryptophan, have a single codon. In general, the third nucleotide in a codon is less important than the first two in determining the specific amino acid to be incorporated, and this accounts for most of the degeneracy of the code. However, for any specific codon, only a single amino acid is specified; with rare exceptions, the genetic code is unambiguous—that is, given a specific codon, only a single amino acid is indicated. The distinction between ambiguity and degeneracy is an important concept.

Table1. The Genetic Codea (Codon Assignments in Mammalian Messenger RNAs)

The unambiguous but degenerate code can be explained in molecular terms. The recognition of specific codons in the mRNA by the tRNA adapter molecules is dependent on the tRNA anticodon region and specific base-pairing rules that dictate tRNA–mRNA codon binding. Each tRNA molecule contains a specific sequence, complementary to a codon, which is termed its anticodon. For a given codon in the mRNA, only a single species of tRNA molecule possesses the proper anticodon. Since each tRNA molecule can be charged with only one specific amino acid, each codon therefore specifies only one amino acid. However, some tRNA molecules can utilize the anticodon to recognize more than one codon. With few exceptions, given a specific codon, only a specific amino acid will be incorporated—although, given a specific amino acid, more than one codon may be used.

As discussed in the following section, the reading of the genetic code during the process of protein synthesis does not involve any overlap of codons. Thus, the genetic code is non overlapping. Furthermore, once the reading is commenced at a specific start codon, there is no punctuation between codons, and the message is read in a continuing sequence of nucleotide triplets until a translation stop codon is reached.

Until recently, the genetic code was thought to be universal. It has now been shown that the set of tRNA molecules in mitochondria (which contain their own separate and distinct translation machinery) from lower and higher eukaryotes, including humans, reads four codons differently from the tRNA molecules in the cytoplasm of even the same cells. As noted in a footnote to Table 37–1, in mammalian mitochondria the codon AUA is read as Met, and UGA codes for Trp. In addition, in mitochondria, the codons AGA and AGG are read as stop or chain terminator codons rather than as Arg. As a result of these organelle-specific changes in genetic code, mitochondria require only 22 tRNA molecules (see Figure 35–8 for the location of these genes in mtDNA) to read their genetic code, whereas the cytoplasmic translation system possesses a full complement of 31 tRNA species. These exceptions noted, the genetic code is universal. The frequency of use of each amino acid codon varies considerably between species and among different tissues within a species. The specific tRNA levels generally mirror these codon usage biases. Thus, a particular abundantly used codon is decoded by a similarly abundant specific tRNA which recognizes that particular codon. Tables of codon usage are quite accurate now that many genomes have been sequenced and such information is vital for large-scale production of proteins for therapeutic purposes (ie, insulin, erythropoietin). Such proteins are often produced in nonhuman cells using recombinant DNA technology.

A summary of the main features of the genetic code are listed in Table 2.

Table2. Features of the Genetic Code

اخر الاخبار

اخبار العتبة العباسية المقدسة

لزيادة المساحات الخضراء.. قسم الحزام الأخضر الثاني يشرع بحملة تشجير في جامعة العميد

اللجنة التحضيرية لمهرجان روح النبوة تعلن تمديد موعد تسلم البحوث الخاصة بالمسابقة البحثية

المجمع العلمي يطلق جلستين قرآنيتين لطلبة العلوم الدينية في النجف

قسم الشؤون الفكرية يقدم محاضرة حول أساليب الحوار الإيجابي لوفد شبابي من البصرة

المزيد

الآخبار الصحية

هل ممارسة الرياضة على معدة فارغة تحرق الدهون أكثر؟

العلم يوضح الوقت "الأمثل" لشرب القهوة

دراسة: "طريقة سهلة" لحماية رئتيك من آثار تلوث الهواء

بالذكاء الاصطناعي.. أداة جديدة لتحسين علاج الصرع عند الأطفال

المزيد

الآخبار التكنلوجية

إيلون ماسك يعتزم إطلاق "غروكيبيديا" كمنافس لـ "ويكيبيديا"

"قرار" من أبل لحماية هواتف آيفون 17 من الخدوش

ميزة جديدة على "تشات جي بي تي".. شراء المنتجات من المحادثة

الصين تكشف عن توربين رياح طائر يغير قواعد الطاقة المتجددة

المزيد

مواضيع ذات صلة

Posttranslational Processing Affects the Activity of many Proteins

Like Transcription, Protein synthesis can be described in three phases: Initiation, Elongation, & Termination

Mutations result when changes occur in the Nucleotide Sequence

RNAs can be Extensively Modified

RNA Molecules are Extensively Processed Before They Become Functional

Phosphorylation Activates Pol II

Formation of the Pol II Transcription Complex

Eukaryotic Promoters Are More Complex

Bacterial Promoters Are Relatively Simple

RNA Synthesis is A Cyclical Process that Involves RNA Chain Initiation, Elongation, & Termination

RNA is Synthesized From A DNA Template by RNA Polymerases

DNA & Chromosome Integrity Is Monitored Throughout the Cell Cycle