Genetický kód: Porovnání verzí

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Degenerovanost genetického kódu a z ní plynoucí existence tichých [[mutace|mutací]] značně zvyšuje toleranci substitučních mutací v degenerovaných kodonech. Např. kodony kódující alanin (GCG, GCA, GCU, GCC) mohou po libosti mutovat na své třetí pozici, aniž by došlo k záměně [[Aminokyselina|aminokyseliny]], kterou kódují. Naproti tomu [[aminokyselina]] [[histidin]] je kódována pouze dvěma kodony, takže bez změny [[Aminokyselina|aminokyseliny]] je pouze jedna z možných tří mutací na třetí pozici.
<!--The codons attempt to ensure that minor errors in the genetic code either only causes a silent mutation or an error that would not affect the amino acid's [[hydrophilic]]/[[hydrophobic]] property, eg. a codon of XUX (where X = any nucleotide) tends to code for hydrophobic amino acids. However, the genetic code is not perfect as minor changes such as the single-base substitution in [[sickle cell anaemia]] is disasterous. The hydrophilic [[glutamate]] (Glu) is substituted by the hydrophobic [[valine]] (Val) which reduces the solubility of ß-globin. This causes haemoglobin to form linear polymers linked by the hydrophobic interaction between the valine groups causing sickle cell deformation of erythrocytes.
These variable codes for amino acids are possible because of modified bases in the first base of the [[anticodon]], and the basepair formed is called a [[wobble base pair]]. The modified bases include [[inosine]] and the U-G basepair.
Only two amino acids are specified by a single codon; one of these is the amino-acid [[methionine]], specified by the codon AUG, which also specifies the start of transcription; the other is [[tryptophan]], specified by the codon UGG.
=== Phase or reading frame of a sequence ===
Note that a „codon“ is entirely defined by your starting position. For example, the string GGGAAACCC, if read from the first position, contains the codons GGG, AAA and CCC. If read from the second position, it contains the codons GGA and AAC (partial codons being ignored). If read starting from the third position, GAA and ACC. Every DNA sequence can thus be read in three '''reading frames''', each of which will produce a radically different amino acid sequence (in our example, Gly-Lys-Pro, Gly-Asp, and Glu-Thr, respectively). The actual frame a protein sequence is translated in is defined by a '''start codon''', usually the first occurrence of AUG in the RNA sequence. Mutations that disrupt the reading frame (i.e. insertions or deletions of one or two nucleotide bases) severely impair the function of a protein and are thus exceedingly rare in protein-coding sequences, since they do not often survive [[purifying selection]].
==Origin of the genetic code==
Numerous variations of the standard genetic code are found in [[mitochondrion|mitochondria]], which are energy-producing [[organelles]]. [[Ciliate]] [[protozoa]] also have some variation in the genetic code: UAG and often UAA code for Glutamine (a variant also found in some [[green alga]]e), or UGA codes for Cysteine. Another variant is found in some species of the [[yeast]] [[Candida (rod)|candida]], where CUG codes for Serine. In some species of [[bacterium|bacteria]] and [[archaea]], a few non-standard amino acids are substituted for standard stop codons; UGA can code for [[selenocysteine]] and UAG can code for [[pyrrolysine]]. There may be other non-standard interpretations that are not known.
Despite these variations, the genetic codes used by all known forms of life on Earth are very similar. Since there are many possible genetic codes that are thought to have similar utility to the one used by Earth life, the theory of [[evolution]] suggests that the genetic code was established very early in the history of life.
One can ask the question: is the genetic code completely random, just one set of codon-amino acid correspondences that happened to establish itself and be „frozen in“ early in evolution, although ''functionally'' any other of the near-infinite set of possible transcription tables would have done just as well? Already a cursory look at the table shows patterns that suggest that this is not the case.
Recent [[aptamer]] experiments have shown that amino acids have indeed a selective chemical affinity for the base triplets that code for them.{{fn|1}} This suggests that the current, complex transcription mechanism involving [[tRNA]] and associated enzymes is a later development, and that originally, protein sequences were directly templated on base sequences. Also, evidence has been found that originally the number of different amino acids used may have been considerably smaller than today.{{fn|2}}
== Reference ==
There are several books available online that go into great detail on this topic. They are available through the [ NCBI Bookshelf], maintained by the [[National_Institutes_of_Health|United States National Institutes of Health]]. In particular the following books would be useful to consult:
* Griffiths, Anthony J.F.; Miller, Jeffrey H.; Suzuki, David T.; Lewontin, Richard C.; Gelbart, William M. (1999). [ ''Introduction to Genetic Analysis'' (7th ed.)]. New York: W. H. Freeman & Co. ISBN 0-7167-3771-X
* Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter. (2002). [ ''Molecular Biology of the Cell'' (4th ed.)]. New York: Garland Publishing. ISBN 0-8153-3218-1
* Lodish, Harvey; Berk, Arnold; Zipursky, S. Lawrence; Matsudaira, Paul; Baltimore, David; Darnell, James E. (1999). [ ''Molecular Cell Biology'' (4th ed.)]. New York: W. H. Freeman & Co. ISBN 0-7167-3706-X
== Reference ==
== Literatura ==
* Knight, R.D. and Landweber, L.F. (1998). [ Rhyme or reason: RNA-arginine interactions and the genetic code.] ''Chemistry & Biology'' '''5'''(9), R215-R220. [ PDF version of manuscript]
* Brooks, Dawn J.; Fresco, Jacques R.; Lesk, Arthur M.; and Singh, Mona. (2002). [ Evolution of Amino Acid Frequencies in Proteins Over Deep Time: Inferred Order of Introduction of Amino Acids into the Genetic Code]. ''Molecular Biology and Evolution'' '''19''', 1645-1655.