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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. 5th edition. New York: W H Freeman; 2002.


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The hereditary code is the relation between the succession of bases inDNA (or its RNA transcripts) and the succession of amino mountain in proteins.Experiments through Francis Crick, Sydney Brenner, and others created the followingfeatures the the genetic code through 1961:

1.

Three nucleotides encode an amino acid. Protein arebuilt native a basic collection of 20 amino acids, but there room only four bases.Simple calculations display that a minimum of 3 bases is forced toencode at the very least 20 amino acids. Hereditary experiments showed thatan amino mountain is in reality encoded through a team of threebases, or codon.

2.

The code is nonoverlapping. Think about a basic sequenceABCDEF. In an overlapping code, ABC specifies the very first amino acid, BCDthe next, CDE the next, and so on. In a nonoverlapping code, ABCdesignates the very first amino acid, DEF the second, and also so forth. Geneticsexperiments again established the code to be nonoverlapping.

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3.

The code has no punctuation. In principle, one base(denoted together Q) can serve together a “comma” between groups the threebases.

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This is no the case. Rather, the sequence of bases is readsequentially from a fixed starting point, withoutpunctuation.

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4.

The genetic code is degenerate. Part amino mountain areencoded by an ext than one codon, inasmuch as there space 64 feasible basetriplets and also only 20 amino acids. In fact, 61 of the 64 possibletriplets specify specific amino acids and also 3 triplets (called stopcodons) designate the discontinuation of translation. Thus, because that mostamino acids, there is an ext than one code word.


5.5.1. Major Features that the genetic Code

All 64 codons have been deciphered (Table5.4). Since the password is extremely degenerate, only tryptophan andmethionine space encoded by just one triplet each. The various other 18 amino acids areeach encoded by two or more. Indeed, leucine, arginine, and serine space specifiedby 6 codons each. The number of codons because that a specific amino acid correlateswith that frequency of event in proteins.


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Codons the specify the very same amino acid are dubbed synonyms. Forexample, CAU and also CAC space synonyms for histidine. Keep in mind that synonyms room notdistributed haphazardly throughout the hereditary code (depicted in Table 5.4). One amino acid specified bytwo or much more synonyms rectal a single box (unless it is mentioned by more thanfour synonyms). The amino mountain in a box are specified by codons that have actually thesame first two bases yet differ in the third base, together exemplified through GUU, GUC,GUA, and GUG. Thus, many synonyms differ only in the critical base the thetriplet. Inspection of the code shows that XYC and XYU alwaysencode the exact same amino acid, vice versa, XYG and XYA commonly encode the very same aminoacid. The structure basis for these equivalences the codons will become evidentwhen we take into consideration the nature that the anticodons that tRNA molecule (Section 29.3.9).

What is the biological significance of the substantial degeneracy of the geneticcode? If the code were not degenerate, 20 codons would certainly designate amino mountain and44 would cause chain termination. The probability that mutating to chaintermination would because of this be much greater with a nondegenerate code.Chain-termination mutations usually result in inactive proteins, whereassubstitutions that one amino mountain for an additional are usually quite harmless. Thus,degeneracy minimizes the deleterious impacts of mutations.Degeneracy that the password may also be far-ranging in permitting DNA basecomposition to differ over a wide variety without changing the amino acid sequenceof the protein encoded by the DNA. The G + C contents of bacterial DNA rangesfrom less than 30% to more than 70%. DNA molecules v quite different G + Ccontents might encode the same proteins if different synonyms the the geneticcode were continuously used.


5.5.2. Messenger RNA includes Start and Stop Signals because that Protein Synthesis

Messenger RNA is interpreted into proteins on ribosomes, largemolecular complexes assembled from proteins and also ribosomal RNA. How is mRNAinterpreted through the translate in apparatus? As currently mentioned, UAA,UAG, and UGA clues chain termination. This codons are check out notby tRNA molecules but rather by particular proteins referred to as releasefactors (Section 29.4.4).Binding that the release factors to the ribosomes publication the newly synthesizedprotein. The begin signal because that protein synthetic is more complex. Polypeptidechains in bacteria begin with a modification amino acid—namely, formylmethionine(fMet). A particular tRNA, the initiator tRNA, tote fMet. This fMet-tRNArecognizes the codon AUG or, much less frequently, GUG. However, AUG is likewise thecodon for an interior methio-nine residue, and GUG is the codon because that an internalvaline residue. Hence, the signal because that the very first amino acid in a prokaryoticpolypeptide chain should be more facility than the for all succeeding ones.AUG (or GUG) is only component of the initiation signal (Figure 5.32). In bacteria, the initiatingAUG (or GUG) codon is preceded several nucleotides far by a purine-richsequence the 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 normally thestart signal because that protein synthesis. This particular AUG is check out by an initiatortRNA conjugated to methionine. Once the initiator AUG is located, thereading framework is established—groups of three nonoverlappingnucleotides space defined, beginning with the initiator AUG codon.


Figure 5.32

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


5.5.3. The genetic Code Is practically Universal

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Is the genetic codethe same in all organisms? The base sequences of many wild-type and also mutant genesare known, as space the amino mountain sequences of your encoded proteins. In eachcase, the nucleotide change in the gene and the amino acid readjust in the proteinare together predicted by the hereditary code. Furthermore, mRNAs can be correctlytranslated by the proteinsynthesizing machine of really different species. Forexample, human hemoglobin mRNA is correctly translated by a wheat germ extract,and bacteria efficiently express recombinant DNA molecule encoding humanproteins such as insulin. These speculative findings strongly said thatthe genetic code is universal.

A surprise was encountered when the sequence of human being mitochondrial DNA becameknown. Human being mitochondria read UGA as a codon for tryptophan fairly than as astop signal (Table 5.5). Furthermore,AGA and also AGG are review as protect against signals fairly than together codons for arginine, and also AUAis check out as a codon because that methionine rather of isoleucine. Mitochondria that otherspecies, such as those the yeast, likewise have hereditary codes that differ slightlyfrom the typical one. The genetic code of mitochondria deserve to differ from that ofthe rest of the cell since mitochondrial DNA encodes a distinct collection of tRNAs.Do any cellular protein-synthesizing systems deviate from the traditional geneticcode? Ciliated protozoa different from most organisms in analysis UAA and UAG ascodons because that amino acids rather than as prevent signals; UGA is your soletermination signal. Thus, the genetic code is nearly but no absolutelyuniversal. Variations clearly exist in mitochondria and in species,such together ciliates, the branched off an extremely early in eukaryotic bio evolution. The isinteresting to keep in mind that two of the codon reassignments in human mitochondriadiminish the information content that the 3rd base the the triplet (e.g., bothAUA and also AUG specify methionine). Most variations from the standard genetic codeare in the direction of a easier code.


Why has actually the password remained almost invariant through billions of years ofevolution, native bacteria to human beings? A mutation that transformed the analysis ofmRNA would readjust the amino mountain sequence of most, if not all, proteinssynthesized through that details organism. Plenty of of these transforms would undoubtedlybe deleterious, and also so there would certainly be strong choice against a mutation withsuch pervasive consequences.

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