Prior to understanding the details of transcription and translation, geneticists predicted that DNA could encode amino acids only if a code of at least three nucleotides was used.
The logic is that the nucleotide code must be able to specify the placement of 20 amino acids. Since there are only four nucleotides, a code of single nucleotides would only represent four amino acids, such that A, C, G and U could be translated to encode amino acids.
A doublet code could code for 16 amino acids 4 x 4. The genetic code is almost universal. In the rare exceptions to this rule, the differences from the genetic code are fairly small.
Differential codon usage. Various species have different patterns of codon usage. The pattern of codon usage may be a predictor of the level of expression of the gene. In general, more highly expressed genes tend to use codons that are frequently used in genes in the rest of the genome. This has been quantitated as a "codon adaptation index". Thus in analyzing complete genomes, a previously unknown gene whose codon usage profile matches the preferred codon usage for the organism would score high on the codon adaptation index, and one would propose that it is a highly expressed gene.
Likewise, one with a low score on the index may encode a low abundance protein. The observation of a gene with a pattern of codon usage that differs substantially from that of the rest of the genome indicates that this gene may have entered the genome by horizontal transfer from a different species. The preferred codon usage is a useful consideration in "reverse genetics".
If you know even a partial amino acid sequence for a protein and want to isolate the gene for it, the family of mRNA sequences that can encode this amino acid sequence can be determined easily.
Because of the degeneracy in the code, this family of sequences can be very large. Since one will likely use these sequences as hybridization probes or as PCR primers, the larger the family of possible sequences is, the more likely that one can get hybridization to a target sequence that differs from the desired one. Thus one wants to limit the number of possible sequences, and by referring to a table of codon preferences assuming they are known for the organism of interest , then one can use the preferred codons rather than all possible codons.
This limits the number of sequences that one needs to make as hybridization probes or primers. Wobble in the anticodon. In contrast, the first two positions of the codon form regular Watson-Crick base pairs with the last two positions of the anticodon.
This flexibility at the "wobble" position allows some tRNAs to pair with two or three codons, thereby reducing the number of tRNAs required for translation. Wobble rules. Types of mutations. Base substitutions. Just as a reminder, there are two types of base substitutions. The same class of nucleotide remains.
Examples are A substituting for G or C substituting for T. Over evolutionary time, the rate of accumulation of transitions exceeds the rate of accumulation of transversions. Effect of mutations on the mRNA. Depending on the particular replacement, it may or may not have a detectable phenotypic consequence. Some replacements, e.
Other replacements, such as valine for a glutamate at a site that causes hemoglobin to polymerize in the deoxygenated state, cause significant pathology sickle cell anemia in this example. They almost always have serious phenotypic consequences. Not all base subsitutions alter the encoded amino acids. However, there are several exceptions to this rule. This is one of the strongest supporting arguments in favor of model of neutral evolution, or evolutionary drift, as a principle cause of the substitutions seen in natural populations.
The template strand of a sample of double-helical DNA contains the sequence:. Will the resulting amino acid sequence be the same as in b? Explain the biological significance of your answer.
In sickle-cell hemoglobin there is a Val residue at position 6 of the b -globin chain, instead of the Glu residue found in this position in normal hemoglobin A.
Can you predict what change took place in the DNA codon for glutamate to account for its replacement by valine? What is the sequence of the original codon for Lys? Deduce the sequence of the wild-type codon in each instance. What is the codon for Gln? What is the codon for Leu? Design a DNA probe that would allow you to identify the gene for a protein with the following amino-terminal amino acid sequence.
In consequence, such trait are not expected to evolve. Hope that makes sense to you. I estimate that for E. This doesn't strike me as the basis of a strong selective pressure. Space taking up by DNA and the machinery having to deal with these big sequences in bacteria?
And what about timing issue? I was once told that the limit to growth is the exponential increase needed in ribosome number as more and more ribosomes are tied up just making ribosomes.
Show 3 more comments. Featured on Meta. Now live: A fully responsive profile. Linked Improve this question. Community Bot 1. Luke Luke 6, 4 4 gold badges 27 27 silver badges 63 63 bronze badges. There are now much better alternative answers, so I have asked the question I actually answered here!
I think your query confused me a bit. An article I found online explains this in a more-or-less feasible way: biologie. Add a comment. Active Oldest Votes. Theory One - evolvability If codons were only 2 bases in length then the variety of codons that could be created would be less only 16 unique sequences if there are still 4 nucleotides. Theory Two - "redundancy" of codons conveys additional information This theory is not mutually exclusive to the one described above.
D'Onofrio and Abel, Frontiers in Genetics. Improve this answer. I take issue though with saying "determined that the most efficient". We have no idea that other combinations ever existed. That's all we know. I have given the answer a general re-work because I don't think it flowed properly having been adapted from a different question, and have taken your comment into account. Many thanks. I know everybody is talking about the central dogma of molecular biology but it is such a bad name.
Dogma has no place in science.
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