The genetic code is the set of rules by which the information encoded in genetic material ( DNA or RNA sequences ) is translated into proteins ( amino acid sequences ) in living cells . The code defines the relationship between three nucleotide sequences, called codons, and amino acids . A codon corresponds to a specific amino acid .
Summary
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- 1 Structure
- 2 History
- 3 The origin of the genetic code
- 4 Features and Decryption
- 1 Characteristics of the genetic code.
- 1.1 The code is organized in triplets or codons:
- 1.2 The genetic code is degenerate
- 1.3 The genetic code is non-overlapping or without overlapping
- 1.4 Reading is “without commas”
- 1.5 The nuclear genetic code is universal
- 2 Decryption of the genetic code
- 1 Characteristics of the genetic code.
- 5 Incorrect uses of the term
- 6 See Also
- 7 Sources
- 8 External Links
Structure
The sequence of the genetic material is made up of four different nitrogenous bases, which have a function equivalent to letters in the genetic code: adenine (A), thymine (T), guanine (G) and cytosine (C) in DNA and adenine ( A), uracil (U), guanine (G) and cytosine (C) in the RNA . Because of this, the number of possible codons is 64, of which 61 encode amino acids(one of them being the start codon, AUG) and the remaining three are stop sites (UAA, called ocher; UAG, called amber; UGA, called opal). The codon sequence determines the amino acid sequence of a specific protein , which will have a specific structure and function.
History
For many years man has been interested in discovering the secrets of inheritance.
Through long and difficult studies the existence of DNA and RNA and their importance for genetics was discovered ; When talking about them, reference is made to the synthesis of proteins that will determine the genotypic and phenotypic characteristics of the organism . Some type of mechanism similar to that of DNA self-duplication was first thought of , but it was not possible to find a satisfactory physicochemical fit. The relationships between DNA and proteins were apparently more complicated. If proteins with their 20 amino acids, Were the “language of life” -to use ‘metaphor of the 40- years molecule of the DNA , with its four nitrogenous bases, he could imagine as a kind of code for this language.
Thus the term ” genetic code ” began to be used . As demonstrated later, the idea of a “code of life” was useful, not only as a good metaphor, but also as a working hypothesis.
The origin of the genetic code
Representative scheme of the genetic code
Despite the variations that exist, the genetic codes used by all known life forms are very similar. This suggests that the genetic code was established very early in the history of life and that it has a common origin in current life forms. Phylogenetic analysis suggests that tRNA molecules evolved earlier than the current set of aminoacyl- tRNA synthetases.
The genetic code is not a random assignment of codons to amino acids . For example, amino acids that share the same biosynthetic pathway tend to have the same first base in their codons, and amino acids with similar physical properties tend to have codon-like.
Recent experiments show that some amino acids have selective chemical affinity for their codons. This suggests that the current complex mRNA translation mechanism that involves the action of tRNA and associated enzymes may be a further development and that, initially, the proteins were synthesized directly on the RNA sequence , acting as a ribozyme and catalyzing the peptide bond formation (such as occurs with ribosome 23S rRNA ).
The hypothesis that the current standard genetic code arose by biosynthetic expansion of a previous simple code has been hypothesized . Primordial life was able to add new amino acids (for example, by-products of metabolism), some of which were later incorporated into the genetic coding machinery . There is evidence, albeit circumstantial, that primitive life forms used fewer different amino acids , although it is not known exactly which amino acids and in what order they entered the genetic code .
Another interesting factor to keep in mind is that natural selection has favored code degeneration to minimize the effects of mutations and is due to the interaction of two different atoms in the reaction. This has led us to think that the primitive genetic code could have consisted of two nucleotide codons, which is quite consistent with the hypothesis of tRNA balancing during its coupling.
Features and Decryption
Characteristics of the genetic code.
The code is organized in triplets or codons:
- Every three nucleotides (triplet) determine an amino acid.
If each nucleotide determined an amino acid , we could only code for four different amino acids since there are only four different nucleotides in DNA . It is much lower than the 20 [[Amino acid | different amino acids that exist. If every two nucleotides will encode an amino acid , the total number of different dinucleotides that we could achieve with the four different nucleotides (A, G, T and C) would be repetition variations of four elements taken two by two VR4.2 = 42 = 16 Therefore, we would have only 16 different dinucleotides, less than the number of different amino acids that exist (20).
If each group of three nucleotides determines an amino acid . Taking into account that there are four different nucleotides (A, G, T and C), the number of groups of three different nucleotides that can be obtained are variations with repetition of four elements (the four nucleotides) taken three by three: VR4, 3 = 43 = 64. Therefore, there are a total of 64 different triplets, more than enough to encode the 20 different amino acids .
The genetic code is degenerate
Functional regions of any transfer RNA molecule
- There are more triplets or codons than amino acids , so that a certain amino acid can be encoded by more than one triplet.
As we have previously said, there are 64 different triplets and 20 different amino acids , so that an amino acid can be encoded by more than one codon. This type of code is called: degenerate. Wittmann ( 1962 ) inducing base substitutions by deamination with nitrites made substitutions of C U and A for G in the RNA of the virus of the mosaic of the snuff (TMV), showing that serine and isoleucine were determined by more than one triplet.
The molecules responsible for transporting the amino acid to the ribosome and recognize codons the RNA messenger for the translation process are the RNA transferors ( RNA -t). T- RNAs have a cloverleaf structure with several functional sites:
- End 3: amino acid binding site (always contains the ACC sequence).
- Dihydrouracil loop(DHU) binding site aminoacyl RNA -t synthetase or enzymes responsible for binding an amino acid to its corresponding RNA -t.
- T ψ C loop: ribosome binding site .
- Loop of the anticodon: place of recognition of the messenger codons.
Yeast Elanin Transfer RNA Sequence
Normally, the t- RNA adopts an L-shaped or a boomerang-shaped folded cloverleaf structure.
The degeneration of the code is explained taking into account two reasons:
- Some amino acids may be carried by different molecular species (types) of RNA transferors (ARN-t) containing different anticodons.
- Some molecular species of t- RNA can incorporate their specific amino acid in response to several codons, so that they possess an anticodon that is capable of pairing with several different codons. This permissive pairing is called the 3rd base of the anticodon or wobble.
Genetic code is non-overlapping or without overlaps
- A nucleotide only belongs to a single triplet.
A nucleotide is only part of one triplet and therefore not part of multiple triplets, indicating that the genetic code does not have overlaps. Therefore, the code is non-overlapping. Wittmann ( 1962 ) by inducing mutations with acid nitroso in the RNA of the virus of the mosaic of the snuff (TMV) could demonstrate that mutations usually produced a change in a single amino acid . the acidnitroso produces deaminations causing base substitutions, if the code was one nucleotide overlap and form part of two or three triplets, a nucleotide substitution would give rise to two or three amino acids altered in the protein capsid of the TMV.
Another way to check that the code is no overlap is that there is no restriction on the sequence of amino acids of the protein , so that a certain amino acid may be preceded or followed by any of the 20 amino acids that exist. If two successive codons shared two nucleotides, any triplet could only be preceded or followed by four determined codons. Therefore, if the code were superimposed, a certain amino acid could only be preceded or followed by four other amino acids at most.
Reading is “without commas”
- The triplets are read continuously “without commas” or without blank spaces.
Taking into account that the reading is done from three to three bases, from a starting point the reading is carried out without interruptions or empty spaces, that is, the reading is followed “without commas”. Thus, if we add a nucleotide (addition) to the sequence, the reading frame is altered from that point and all amino acids are modified . The same is true if a nucleotide in the sequence is lost (deleted). From the deleted nucleotide the reading frame is altered and all the amino acids change . If the addition or deletion is three nucleotides or a multiple of three, an amino acid is addedor more than one to the sequence that remains the same as of the last addition or deletion. Successive addition and deletion restore the reading box again.
The nuclear genetic code is universal
- The same triplet in different species codes for the same amino acid . The main exception to universality is the mitochondrial genetic code .
The deciphering of the genetic code has been carried out mainly in the E. coli bacteria , therefore, it is worth asking whether the genetic code of this bacterium is the same as that of other prokaryotic and eukaryotic organisms . Experiments carried out to date indicate that the nuclear genetic code is universal, so that a given triplet or codon carries information for the same amino acid in different species.
Deciphering the genetic code
Assigning an amino acid to each triplet or deciphering the genetic code . It seems logical to think that the deciphering of the genetic code should have been done comparing the nucleotide sequences of a gene and the amino acid sequence of the polypeptide encoded by that gene. However, at the time these works were carried out, it was not yet possible to obtain the nucleic acid sequence.
Most of the work done by the research groups consisted of synthesizing RNA messengers ( RNA -m) for later use as artificial messengers in acellular system of translation “in vitro”. These “in vitro” acellular translation systems came from the E. coli bacteria and contained everything necessary to carry out the translation: ribosomes , all the transfer RNAs , amino acids , enzymes , etc. However, these acellular systems had the E. coli messenger RNAs removed and an RNA addedartificially synthesized. In these acellular systems, a polypeptide was synthesized.
Incorrect uses of the term
The expression ” genetic code ” is frequently used in the media as a synonym for genome , genotype, or DNA . Phrases like:
- “The genetic code of the remains was analyzed and coincided with that of the disappeared one”
- «A database will be created with the genetic code of all citizens»
They are scientifically incorrect. It is foolish, for example, to refer to the ” genetic code of a certain person”, because the genetic code is the same for all individuals. However, each organism has its own genotype, although it is possible that it shares it with others if it has originated from some asexual multiplication mechanism.
