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You are watching: Using the codon table below, what conclusions can be drawn about the genetic code?

Berg JM, Tymoczko JL, Stryer L. Biochemistry. fifth edition. New York: W H Freeman; 2002.


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The genetic code is the relation between the sequence of bases inDNA (or its RNA transcripts) and the sequence of amino acids in proteins.Experiments by Francis Crick, Sydney Brenner, and also others establimelted the followingfeatures of the hereditary code by 1961:

1.

Three nucleotides encode an amino acid. Proteins aredeveloped from a simple set of 20 amino acids, but tbelow are just 4 bases.Simple calculations display that a minimum of three bases is required toencode at leastern 20 amino acids. Genetic experiments proved thatan amino acid is in truth encoded by a team of threebases, or codon.

2.

The code is nonoverlapping. Consider a base sequenceABCDEF. In an overlapping code, ABC mentions the initially amino acid, BCDthe next, CDE the next, and also so on. In a nonoverlapping code, ABCdesignates the initially amino acid, DEF the second, and so forth. Geneticsexperiments again establimelted the code to be nonoverlapping.

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3.

The code has no punctuation. In principle, one base(delisted as Q) could serve as a “comma” in between teams of threebases.

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This is not the instance. Rather, the sequence of bases is readsequentially from a addressed founding point, withoutpunctuation.

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4.

The hereditary code is degeneprice. Some amino acids areencoded by even more than one codon, inasmuch as tright here are 64 possible basetriplets and just 20 amino acids. In truth, 61 of the 64 possibletripallows specify particular amino acids and also 3 tripallows (referred to as stopcodons) designate the termination of translation. Hence, for mostamino acids, tbelow is even more than one code word.


5.5.1. Major Features of the Genetic Code

All 64 codons have actually been deciphered (Table5.4). Because the code is very degeneprice, only tryptophan andmethionine are encoded by just one triplet each. The various other 18 amino acids areeach encoded by two or even more. Indeed, leucine, arginine, and also serine are specifiedby 6 codons each. The number of codons for a particular amino acid correlatesvia its frequency of event in proteins.


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Codons that specify the same amino acid are dubbed synonyms. Forinstance, CAU and CAC are synonyms for histidine. Keep in mind that synonyms are notspread haphazardly throughout the genetic code (depicted in Table 5.4). An amino acid mentioned bytwo or more synonyms occupies a solitary box (unmuch less it is stated by more than4 synonyms). The amino acids in a box are mentioned by codons that have actually theexact same first two bases yet differ in the 3rd base, as exemplified by GUU, GUC,GUA, and GUG. Hence, most synonyms differ only in the last base of thetriplet. Inspection of the code shows that XYC and XYU alwaysencode the very same amino acid, whereas XYG and XYA normally encode the same aminoacid. The structural basis for these equivalences of codons will certainly become evidentas soon as we consider the nature of the anticodons of tRNA molecules (Section 29.3.9).

What is the organic definition of the comprehensive degeneracy of the geneticcode? If the code were not degeneprice, 20 codons would certainly designate amino acids and44 would certainly lead to chain termination. The probcapability of mutating to chaintermination would certainly therefore be much greater with a nondegenerate code.Chain-termicountry mutations usually bring about inenergetic proteins, whereassubstitutions of one amino acid for one more are normally quite harmmuch less. Thus,degeneracy minimizes the deleterious effects of mutations.Degeneracy of the code may additionally be considerable in permitting DNA basecomposition to differ over a vast variety without changing the amino acid sequenceof the proteins encoded by the DNA. The G + C content of bacterial DNA rangesfrom less than 30% to more than 70%. DNA molecules with fairly various G + Ccontents might encode the very same proteins if various synonyms of the geneticcode were repetitively provided.


5.5.2. Messenger RNA Contains Start and Sheight Signals for Protein Synthesis

Messenger RNA is translated into proteins on ribosomes, largemolecular complexes assembled from proteins and also ribosomal RNA. How is mRNAinterpreted by the translation apparatus? As currently discussed, UAA,UAG, and UGA designate chain termination. These codons are read notby tRNA molecules however fairly by particular proteins dubbed releasecomponents (Section 29.4.4).Binding of the release factors to the ribosomes releases the newly synthesizedprotein. The begin signal for protein synthesis is even more facility. Polypeptidechains in bacteria start with a modified amino acid—namely, formylmethionine(fMet). A specific tRNA, the initiator tRNA, carries fMet. This fMet-tRNArecognizes the codon AUG or, less frequently, GUG. However before, AUG is additionally thecodon for an interior methio-nine residue, and GUG is the codon for an internalvaline residue. Hence, the signal for the initially amino acid in a prokaryoticpolypeptide chain should be even more complex than that for all subsequent ones.AUG (or GUG) is only component of the initiation signal (Figure 5.32). In bacteria, the initiatingAUG (or GUG) codon is preyielded numerous nucleotides away by a purine-richsequence that base-pairs with a complementary sequence in a ribosomal RNAmolecule (Section 29.3.4). Ineukaryotes, the AUG closest to the 5′ end of an mRNA molecule is generally thebegin signal for protein synthesis. This particular AUG is read by an initiatortRNA conjugated to methionine. Once the initiator AUG is located, thereading structure is established—teams of 3 nonoverlappingnucleotides are identified, beginning via the initiator AUG codon.


Figure 5.32

Initiation of Protein Synthesis. Start signals are compelled for the initiation of protein synthesis in(A) prokaryotes and also (B) eukaryotes.


5.5.3. The Genetic Code Is Nbeforehand Universal

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Is the genetic codethe very same in all organisms? The base sequences of many kind of wild-kind and mutant genesare well-known, as are the amino acid sequences of their encoded proteins. In eachcase, the nucleotide adjust in the gene and the amino acid readjust in the proteinare as predicted by the genetic code. In addition, mRNAs have the right to be correctlyinterpreted by the proteinsynthesizing machinery of exceptionally different species. Forexample, human hemoglobin mRNA is effectively analyzed by a wheat germ extract,and also bacteria efficiently expush recombinant DNA molecules encoding humanproteins such as insulin. These experimental findings strongly suggested thatthe genetic code is universal.

A surpclimb was encountered when the sequence of huguy mitochondrial DNA becameknown. Human being mitochondria check out UGA as a codon for tryptophan fairly than as aspeak signal (Table 5.5). Furthermore,AGA and also AGG are read as soptimal signals fairly than as codons for arginine, and also AUAis review as a codon for methionine instead of isoleucine. Mitochondria of otherspecies, such as those of yeast, additionally have actually hereditary codes that differ slightlyfrom the typical one. The hereditary code of mitochondria can differ from that ofthe remainder of the cell bereason mitochondrial DNA encodes a distinct collection of tRNAs.Do any type of cellular protein-synthesizing units deviate from the typical geneticcode? Ciliated protozoa differ from most organisms in analysis UAA and UAG ascodons for amino acids fairly than as speak signals; UGA is their soletermicountry signal. Hence, the genetic code is virtually but not absolutelyglobal. Variations plainly exist in mitochondria and also in species,such as ciliates, that branched off very early in eukaryotic advancement. It isamazing to note that 2 of the codon reassignments in humale mitochondriadiminish the information content of the 3rd base of the triplet (e.g., bothAUA and AUG specify methionine). Most variations from the typical hereditary codeare in the direction of a easier code.


Why has the code continued to be virtually invariant via billions of years ofdevelopment, from bacteria to humale beings? A mutation that transformed the analysis ofmRNA would change the amino acid sequence of most, if not all, proteinssynthesized by that specific organism. Many type of of these changes would undoubtedlybe deleterious, and so there would be strong selection versus a mutation withsuch pervasive aftermath.

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