Personal genetic code. DNA and genes. DNA code system

Ministry of Education and Science Russian Federation Federal agency of Education

State educational institution higher vocational education"Altai State Technical University them. I.I. Polzunov"

Department of Natural Sciences and System Analysis

Abstract on the topic "Genetic code"

1. The concept of genetic code

3. Genetic information

Bibliography

1. The concept of genetic code

Genetic code - characteristic of living organisms one system recording hereditary information in nucleic acid molecules in the form of a sequence of nucleotides. Each nucleotide is designated by a capital letter, which begins the name of the nitrogenous base included in its composition: - A (A) adenine; - G (G) guanine; - C (C) cytosine; - T (T) thymine (in DNA) or U (U) uracil (in mRNA).

The implementation of the genetic code in a cell occurs in two stages: transcription and translation.

The first of them occurs in the core; it consists in the synthesis of mRNA molecules at the corresponding sections of DNA. In this case, the DNA nucleotide sequence is “rewritten” into the RNA nucleotide sequence. The second stage takes place in the cytoplasm, on ribosomes; in this case, the nucleotide sequence of mRNA is translated into the sequence of amino acids in the protein: this stage occurs with the participation of transfer RNA (tRNA) and the corresponding enzymes.

2. Properties of the genetic code

1. Triplety

Each amino acid is encoded by a sequence of 3 nucleotides.

A triplet or codon is a sequence of three nucleotides encoding one amino acid.

The code cannot be monoplet, since 4 (the number of different nucleotides in DNA) is less than 20. The code cannot be doublet, because 16 (the number of combinations and permutations of 4 nucleotides of 2) is less than 20. The code can be triplet, because 64 (the number of combinations and permutations from 4 to 3) is more than 20.

2. Degeneracy.

All amino acids, with the exception of methionine and tryptophan, are encoded by more than one triplet: 2 amino acids of 1 triplet = 2 9 amino acids of 2 triplets = 18 1 amino acid 3 triplets = 3 5 amino acids of 4 triplets = 20 3 amino acids of 6 triplets = 18 Total 61 triplets encode 20 amino acids.

3. Presence of intergenic punctuation marks.

A gene is a section of DNA that encodes one polypeptide chain or one molecule of tRNA, rRNA or sRNA.

The tRNA, rRNA, and sRNA genes do not code for proteins.

At the end of each gene encoding a polypeptide there is at least one of 3 stop codons, or stop signals: UAA, UAG, UGA. They terminate the broadcast.

Conventionally, the AUG codon, the first after the leader sequence, also belongs to punctuation marks. It functions as a capital letter. In this position it encodes formylmethionine (in prokaryotes).

4. Unambiguity.

Each triplet encodes only one amino acid or is a translation terminator.

The exception is the AUG codon. In prokaryotes, in the first position (capital letter) it encodes formylmethionine, and in any other position it encodes methionine.

5. Compactness, or absence of intragenic punctuation marks.

Within a gene, each nucleotide is part of a significant codon.

In 1961 Seymour Benzer and Francis Crick experimentally proved the triplet nature of the code and its compactness.

The essence of the experiment: “+” mutation - insertion of one nucleotide. "-" mutation - loss of one nucleotide. A single "+" or "-" mutation at the beginning of a gene spoils the entire gene. A double "+" or "-" mutation also spoils the entire gene. A triple “+” or “-” mutation at the beginning of a gene spoils only part of it. A quadruple “+” or “-” mutation again spoils the entire gene.

The experiment proves that the code is triplet and there are no punctuation marks inside the gene. The experiment was carried out on two adjacent phage genes and showed, in addition, the presence of punctuation marks between the genes.

3. Genetic information

Genetic information is a program of the properties of an organism, received from ancestors and embedded in hereditary structures in the form of a genetic code.

It is assumed that the formation of genetic information followed the following scheme: geochemical processes - mineral formation - evolutionary catalysis (autocatalysis).

It is possible that the first primitive genes were microcrystalline clay crystals, and each new layer of clay is built in accordance with the structural features of the previous one, as if receiving information about the structure from it.

The implementation of genetic information occurs in the process of synthesis of protein molecules using three RNAs: messenger RNA (mRNA), transport RNA (tRNA) and ribosomal RNA (rRNA). The process of information transfer occurs: - through a direct communication channel: DNA - RNA - protein; and - through the channel feedback: environment - protein - DNA.

Living organisms are capable of receiving, storing and transmitting information. Moreover, living organisms have an inherent desire to use the information received about themselves and the world around them as efficiently as possible. Hereditary information embedded in genes and necessary for a living organism to exist, develop and reproduce is transmitted from each individual to his descendants. This information determines the direction of development of the organism, and in the process of its interaction with the environment, the reaction to its individual can be distorted, thereby ensuring the evolution of the development of descendants. In the process of evolution of a living organism, new information arises and is remembered, including the value of information for it increases.

During the implementation of hereditary information under certain conditions external environment the phenotype of organisms of a given biological species.

Genetic information determines the morphological structure, growth, development, metabolism, mental makeup, predisposition to diseases and genetic defects of the body.

Many scientists, rightly emphasizing the role of information in the formation and evolution of living things, noted this circumstance as one of the main criteria of life. So, V.I. Karagodin believes: “Living is such a form of existence of information and the structures encoded by it, which ensures the reproduction of this information in suitable environmental conditions.” The connection between information and life is also noted by A.A. Lyapunov: “Life is a highly ordered state of matter that uses information encoded by the states of individual molecules to develop persistent reactions.” Our famous astrophysicist N.S. Kardashev also emphasizes the informational component of life: “Life arises thanks to the possibility of synthesizing a special kind of molecules capable of remembering and initially using the simplest information about environment and their own structure, which they use for self-preservation, for reproduction and, what is especially important for us, for obtaining more more information." This ability of living organisms to preserve and transmit information was drawn attention to in her book "Physics of Immortality" by ecologist S.S. Chetverikova on population genetics, in which it was shown that it is not individual traits and individuals that are subject to selection, but the genotype of the entire population, but it is carried out through the phenotypic characteristics of individual individuals. This leads to the spread of useful changes throughout the population. Thus, the mechanism of evolution is realized both through random mutations at the genetic level and through the inheritance of the most valuable characteristics (the value of information!), which determine the adaptation of mutational characteristics. to the environment, ensuring the most viable offspring.

Seasonal climate changes, various natural or man-made disasters on the one hand, lead to changes in the frequency of repetition of genes in populations and, as a consequence, to a decrease in hereditary variability. This process is sometimes called genetic drift. And on the other hand, to changes in the concentration of various mutations and a decrease in the diversity of genotypes contained in the population, which can lead to changes in the direction and intensity of selection.

4. Decoding the human genetic code

In May 2006, scientists working to decipher the human genome published a complete genetic map of chromosome 1, which was the last human chromosome not fully sequenced.

A preliminary human genetic map was published in 2003, marking the formal completion of the Human Genome Project. Within its framework, genome fragments containing 99% of human genes were sequenced. The accuracy of gene identification was 99.99%. However, by the time the project was completed, only four of the 24 chromosomes had been fully sequenced. The fact is that in addition to genes, chromosomes contain fragments that do not encode any characteristics and are not involved in protein synthesis. The role that these fragments play in the life of the body remains unknown, but more and more researchers are inclined to believe that their study requires the closest attention.

Nucleotides DNA and RNA Purines: adenine, guanine Pyrimidine: cytosine, thymine (uracil)	Codon- a triplet of nucleotides encoding a specific amino acid.
tab. 1. Amino acids that are commonly found in proteins
Name	Abbreviation
1. Alanine	Ala
2. Arginine	Arg
3. Asparagine	Asn
4. Aspartic acid	Asp
5. Cysteine	Cys
6. Glutamic acid	Glu
7. Glutamine	Gln
8. Glycine	Gly
9. Histidine	His
10. Isoleucine	Ile
11. Leucine	Leu
12. Lysine	Lys
13. Methionine	Met
14. Phenylalanine	Phe
15. Proline	Pro
16. Series	Ser
17. Threonine	Thr
18. Tryptophan	Trp
19. Tyrosine	Tyr
20. Valin	Val

The genetic code, also called the amino acid code, is a system for recording information about the sequence of amino acids in a protein using the sequence of nucleotide residues in DNA that contain one of 4 nitrogenous bases: adenine (A), guanine (G), cytosine (C) and thymine (T). However, since the double-stranded DNA helix is not directly involved in the synthesis of the protein that is encoded by one of these strands (i.e., RNA), the code is written in RNA language, which contains uracil (U) instead of thymine. For the same reason, it is customary to say that a code is a sequence of nucleotides, and not pairs of nucleotides.

The genetic code is represented by certain code words, called codons.

The first code word was deciphered by Nirenberg and Mattei in 1961. They obtained an extract from E. coli containing ribosomes and other factors necessary for protein synthesis. The result was a cell-free system for protein synthesis, which could assemble proteins from amino acids if the necessary mRNA was added to the medium. By adding synthetic RNA consisting only of uracils to the medium, they discovered that a protein was formed consisting only of phenylalanine (polyphenylalanine). Thus, it was established that the triplet of nucleotides UUU (codon) corresponds to phenylalanine. Over the next 5-6 years, all codons of the genetic code were determined.

The genetic code is a kind of dictionary that translates text written with four nucleotides into protein text written with 20 amino acids. The remaining amino acids found in protein are modifications of one of the 20 amino acids.

Properties of the genetic code

The genetic code has the following properties.

Triplety- Each amino acid corresponds to a triple of nucleotides. It is easy to calculate that there are 4 3 = 64 codons. Of these, 61 are semantic and 3 are nonsense (termination, stop codons).
Continuity(no separating marks between nucleotides) - absence of intragenic punctuation marks;
Within a gene, each nucleotide is part of a significant codon. In 1961 Seymour Benzer and Francis Crick experimentally proved the triplet nature of the code and its continuity (compactness) [show]

The essence of the experiment: “+” mutation - insertion of one nucleotide. "-" mutation - loss of one nucleotide.
A single mutation ("+" or "-") at the beginning of a gene or a double mutation ("+" or "-") spoils the entire gene.
A triple mutation ("+" or "-") at the beginning of a gene spoils only part of the gene.
A quadruple “+” or “-” mutation again spoils the entire gene.
The experiment was carried out on two adjacent phage genes and showed that
1. the code is triplet and there is no punctuation inside the gene
2. there are punctuation marks between genes
Presence of intergenic punctuation marks- the presence among triplets of initiating codons (they begin protein biosynthesis), and terminator codons (indicating the end of protein biosynthesis);
Conventionally, the AUG codon, the first after the leader sequence, also belongs to punctuation marks. It functions as a capital letter. In this position it encodes formylmethionine (in prokaryotes).
At the end of each gene encoding a polypeptide there is at least one of 3 stop codons, or stop signals: UAA, UAG, UGA. They terminate the broadcast.
Colinearity- correspondence of the linear sequence of codons of mRNA and amino acids in the protein.
Specificity- each amino acid corresponds only to certain codons that cannot be used for another amino acid.
Unidirectionality- codons are read in one direction - from the first nucleotide to the subsequent ones
Degeneracy or redundancy, - one amino acid can be encoded by several triplets (amino acids - 20, possible triplets - 64, 61 of them are semantic, i.e., on average, each amino acid corresponds to about 3 codons); the exceptions are methionine (Met) and tryptophan (Trp).
The reason for the degeneracy of the code is that the main semantic load is carried by the first two nucleotides in the triplet, and the third is not so important. From here code degeneracy rule : If two codons have the same first two nucleotides and their third nucleotides belong to the same class (purine or pyrimidine), then they code for the same amino acid.
However, there are two exceptions to this ideal rule. This is the AUA codon, which should correspond not to isoleucine, but to methionine, and the UGA codon, which is a stop codon, whereas it should correspond to tryptophan. The degeneracy of the code obviously has an adaptive significance.
Versatility- all of the above properties of the genetic code are characteristic of all living organisms.
Codon Universal code Mitochondrial codes
Vertebrates Invertebrates Yeast Plants
U.G.A. STOP Trp Trp Trp STOP
AUA Ile Met Met Met Ile
CUA Leu Leu Leu Thr Leu
A.G.A. Arg STOP Ser Arg Arg
AGG Arg STOP Ser Arg Arg
IN Lately the principle of code universality was shaken in connection with Berrell's discovery in 1979 of the ideal code of human mitochondria, in which the rule of code degeneracy is satisfied. In the mitochondrial code, the UGA codon corresponds to tryptophan, and AUA to methionine, as required by the code degeneracy rule.
Perhaps at the beginning of evolution, all simple organisms had the same code as mitochondria, and then it underwent slight deviations.
Non-overlapping- each of the triplets of the genetic text is independent of each other, one nucleotide is included in only one triplet; In Fig. shows the difference between overlapping and non-overlapping code.
In 1976 The DNA of phage φX174 was sequenced. It has single-stranded circular DNA consisting of 5375 nucleotides. The phage was known to encode 9 proteins. For 6 of them, genes located one after another were identified.
It turned out that there is an overlap. Gene E is located entirely within gene D. Its start codon appears as a result of a frame shift of one nucleotide.
Gene J begins where gene D ends. The start codon of gene J overlaps with the stop codon of gene D as a result of a two-nucleotide shift. The construction is called a “reading frameshift” by a number of nucleotides not a multiple of three. To date, overlap has only been shown for a few phages. Noise immunity
- the ratio of the number of conservative substitutions to the number of radical substitutions.
Nucleotide substitution mutations that do not lead to a change in the class of the encoded amino acid are called conservative. Nucleotide substitution mutations that lead to a change in the class of the encoded amino acid are called radical.
Since the same amino acid can be encoded by different triplets, some substitutions in triplets do not lead to a change in the encoded amino acid (for example, UUU -> UUC leaves phenylalanine). Some substitutions change an amino acid to another from the same class (non-polar, polar, basic, acidic), other substitutions also change the class of the amino acid.
By direct calculation using the genetic code table, you can verify that of these: 23 nucleotide substitutions lead to the appearance of codons - translation terminators.

134 substitutions do not change the encoded amino acid.

230 substitutions do not change the class of the encoded amino acid.

162 substitutions lead to a change in amino acid class, i.e. are radical.

Of the 183 substitutions of the 3rd nucleotide, 7 lead to the appearance of translation terminators, and 176 are conservative.

Of the 183 substitutions of the first nucleotide, 9 lead to the appearance of terminators, 114 are conservative and 60 are radical.

Of the 183 substitutions of the 2nd nucleotide, 7 lead to the appearance of terminators, 74 are conservative, 102 are radical. is a method characteristic of all living organisms of encoding the amino acid sequence of proteins using the sequence of nucleotides in a DNA molecule. The implementation of genetic information in living cells (that is, the synthesis of a protein encoded in DNA) is carried out using two matrix processes: transcription (that is, the synthesis of mRNA on a DNA matrix) and translation (the synthesis of a polypeptide chain on an mRNA matrix).

DNA uses four nucleotides - adenine (A), guanine (G), cytosine (C), thymine (T). These “letters” make up the alphabet of the genetic code. RNA uses the same nucleotides, except for thymine, which is replaced by uracil (U). In DNA and RNA molecules, nucleotides are arranged in chains and, thus, sequences of “letters” are obtained.

The DNA nucleotide sequence contains code “words” for each amino acid of the future protein molecule - the genetic code. It consists in a certain sequence of arrangement of nucleotides in a DNA molecule.	Three consecutive nucleotides encode the “name” of one amino acid, that is, each of the 20 amino acids is encrypted by a significant unit of code - a combination of three nucleotides called a triplet or codon.
Currently, the DNA code has been completely deciphered, and we can talk about certain properties characteristic of this unique	biological system
, providing the translation of information from the “language” of DNA into the “language” of protein.	The carrier of genetic information is DNA, but since mRNA, a copy of one of the DNA strands, is directly involved in protein synthesis, the genetic code is most often written in the “RNA language.”
Amino acid	RNA coding triplets
Alanin	GCU GCC GCA GCH
Arginine	TsGU TsGTs TsGA TsGG AGA AGG
Asparagine	AAU AAC
Aspartic acid	GAU GAC
Valin	GUU GUTS GUA GUG
Histidine	TsAU TsATs
Glycine	AUU AUC AUA
Leucine	TSUU TSUTS TSUA TSUG UUA UUG
Lysine	AAA AAG
Methionine	AUG
Proline	TsTsU TsTs TsTsTsG
Serin	UCU UCC UCA UCG ASU AGC
Tyrosine	UAU UAC
Threonine	ACU ACC ACA ACG
Tryptophan	UGG
Phenylalanine	UUU UUC
Cysteine	UGU UGC
STOP	UGA UAG UAA

Properties of the genetic code

Three consecutive nucleotides (nitrogen bases) encode the “name” of one amino acid, that is, each of the 20 amino acids is encrypted with a significant code unit - a combination of three nucleotides called triplet or codon.

Triplet (codon)- a sequence of three nucleotides (nitrogen bases) in a DNA or RNA molecule that determines the inclusion of a certain amino acid in the protein molecule during its synthesis.

Uniqueness (discreteness)

One triplet cannot encode two different amino acids; it encrypts only one amino acid. A specific codon corresponds to only one amino acid.

Each amino acid can be defined by more than one triplet. Exception - methionine And tryptophan. In other words, several codons can correspond to the same amino acid.

Non-overlapping

The same base cannot appear in two adjacent codons at the same time.

Some triplets do not encode amino acids, but are a kind of “road signs” that determine the beginning and end of individual genes (UAA, UAG, UGA), each of which means the cessation of synthesis and is located at the end of each gene, so we can talk about the polarity of the genetic code.

In animals and plants, fungi, bacteria and viruses, the same triplet codes for the same type of amino acid, that is, the genetic code is the same for all living things. In other words, versatility - the ability of the genetic code to work equally in organisms of different levels of complexity from viruses to humans.The universality of the DNA code confirms the unity oforigin of all life on our planet. Genetic engineering methods are based on the use of the property of the universality of the genetic code.

From the history of the discovery of the genetic code

For the first time the idea of existence genetic code formulated by A. Down in 1952 - 1954. Scientists have shown that the nucleotide sequence that uniquely determines the synthesis of a particular amino acid must contain at least three units. It was later proven that such a sequence consists of three nucleotides called codon or triplet .

The questions of which nucleotides are responsible for the inclusion of a particular amino acid in a protein molecule and how many nucleotides determine this inclusion remained unresolved until 1961. Theoretical analysis showed that the code cannot consist of one nucleotide, since in this case only 4 amino acids can be encoded. However, the code cannot be a doublet, that is, a combination of two nucleotides from a four-letter “alphabet” cannot cover all amino acids, since only 16 such combinations are theoretically possible (4 2 = 16).

To encode 20 amino acids, as well as a stop signal indicating the end of the protein sequence, three consecutive nucleotides are sufficient, when the number of possible combinations is 64 (4 3 = 64).

In this lesson we will learn about the importance of protein biosynthesis for living organisms, about the two stages of protein biosynthesis in a cell, transcription and translation, and we will show how the sequence of nucleotides in DNA encodes the sequence of amino acids in a polypeptide. We will also characterize the genetic code and its main properties from the standpoint of the unity of origin of all living organisms on Earth, and consider the features of transcription in eukaryotes.

Transcription- a mechanism by which a sequence of bases in one of the chains of a DNA molecule is “rewritten” into a complementary sequence of mRNA bases.

Transcription requires the presence of the enzyme RNA polymerase. Since one DNA molecule can contain many genes, it is very important that RNA polymerase begins the synthesis of messenger RNA from a strictly defined place in the DNA, otherwise information about a protein that does not exist in nature (not needed by the cell) will be recorded in the structure of the mRNA. Therefore, at the beginning of each gene there is a special specific sequence of nucleotides called promoter(see Fig. 7). RNA polymerase “recognizes” the promoter, interacts with it and, thus, begins the synthesis of the mRNA chain from the right place. The enzyme continues to synthesize mRNA, adding new nucleotides to it, until it reaches the next “punctuation mark” in the DNA molecule - terminator. This is a sequence of nucleotides indicating that mRNA synthesis should be stopped.

Rice. 7. mRNA synthesis

In prokaryotes, synthesized mRNA molecules can immediately interact with ribosomes and participate in protein synthesis. In eukaryotes, mRNA first interacts with nuclear proteins and enters the cytoplasm through nuclear pores, where it interacts with ribosomes and protein biosynthesis occurs.

Bacterial ribosomes differ from ribosomes in eukaryotic cells. They are smaller and contain a simpler set of proteins. This is widely used in clinical practice, since there are antibiotics that selectively interact with the ribosomal proteins of prokaryotes, but have no effect on the proteins of eukaryotic organisms. In this case, the bacteria either die or their growth and development stops.

There are antibiotics that selectively affect one of the stages of protein synthesis, for example transcription. These include rifamycins, which are produced by actinomycetes of the genus Streptomyces. The best antibiotic in this class is Rifampicin.

Bibliography

Kamensky A.A., Kriksunov E.A., Pasechnik V.V. General biology 10-11 grade Bustard, 2005.
Biology. Grade 10. General biology. A basic level of/ P.V. Izhevsky, O.A. Kornilova, T.E. Loshchilina and others - 2nd ed., revised. - Ventana-Graf, 2010. - 224 pp.
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Sbio.info().

Homework

Questions 1, 2 at the end of paragraph 26 (p. 101) Kamensky A.A., Kriksunov E.A., Pasechnik V.V. "General Biology", grades 10-11 ()
What is the role of the enzyme RNA polymerase in the process of mRNA synthesis?
What is a promoter and what is its role in mRNA synthesis?
What is a terminator and what is its role in mRNA synthesis?
What is further fate synthesized mRNA in the cell of prokaryotes and eukaryotes?

After the discovery of the rules of the genetic code, according to which hereditary information is rewritten from the language of nucleotides to the language of amino acids, they were considered universal. There are at least 30 known cases where the genetic code is used in a slightly modified form. Changes can be very diverse: the meaning of a codon will change, a stop codon will begin to encode some amino acid, a regular codon will begin to act as a start codon. We offer you ten cases of the most interesting deviations from the standard genetic code.

Despite the generally accepted “standard” nature of the genetic code, there are several dozen examples of living organisms using a slightly modified version of it. Some changes are common to entire taxa, while others are found in just a few species. There are known cases when part of the mRNA of a certain gene is translated according to standard rules, and the other - according to modified ones. For example, when translating human malate dehydrogenase mRNA, which is encoded in the nucleus, in 4% of cases the standard stop codon encodes tryptophan and arginine. Very often, deviations from the standard genetic code are observed only in some organelles. Thus, for the first time the existence of such deviations was confirmed back in 1979, showing that the genetic code of human mitochondria differs from the nuclear code. Our article is devoted to the most surprising cases of deviation of the genetic code from the standard.

“Biomolecule” has written more than once about the genetic code. Article " Such different synonyms"is devoted to the phenomenon of codon preference. In the articles " " And " Evolution of the genetic code" talks about the evolution of the genetic code, and the publications " Expanded genome" And " Four letter word"You can read about the prospects for its artificial expansion.

Blastocrithidia

In protozoa of the genus Blastocrithidia, related to trypanosomes (Fig. 1), the genetic code used in the translation of nuclear genes, in literally“no brakes”: all three stop codons code for amino acids. The UGA codon codes for tryptophan, while UAG and UAA code for glutamate. However, UAA and, less commonly, UAG can still act as stop codons. It turned out that one of the proteins necessary for the release of the ribosome from mRNA after translation eRF1, the extremely important serine residue is replaced by another amino acid, which reduces its affinity for UGA, allowing this stop codon to function as a sense codon. However, it is not completely known why UAG and UAA can act as both sense and terminator codons.

Condylostoma magnum

In ciliates Condylostoma magnum each of the standard stop codons is capable of acting as a sense codon: UAA and UAG can encode glutamine, and UGA can encode tryptophan. However, the dual coding mechanism in this organism is completely different from Blastocrithidia: The meaning of each of the standard stop codons depends on their position in the mRNA. Stop codons, located in the middle part of the transcript, encode amino acids, and stop codons, located near the 3′ end of the mRNA, work “in their specialty” and act as terminators. Probably 3′ untranslated gene regions Condylostoma magnum very short and conserved and play a role in stop codon recognition.

Acetohalobium arabaticum

Rhabdopleura compacta

Scenedesmus obliquus

Genetic code of green algae mitochondria Scenedesmus obliquus(Figure 3) is unusual in that the UCA codon, which normally codes for leucine, functions as a stop codon. The mitochondrial genome of this alga lacks a gene encoding tRNA corresponding to the UCA codon. Instead in mitochondria Scenedesmus obliquus leucine encodes the standard UAG stop codon.

Flatworms class Rhabditophora

Radopholus similis

Ciliates-slippers

Mitochondrial genetic code of slipper ciliates (genus Paramecium) differs from the standard one primarily in the number of start codons. As many as five or six can act as start codons: AUG, AUA, AUU, AUC, GUG, and possibly GUA. Because the mitochondrial genome of these organisms contains genes for only three tRNAs, most of the tRNA comes from the cytoplasm. In this regard, in the mitochondria of slipper ciliates, as in the nucleus of many ciliates, stop codons UAG and UAA encode glutamine.

Ashbya gossypii

In yeast Ashbya gossypii in mitochondria, the codon CUU, which normally codes for leucine, codes for alanine. Surprisingly, the other two leucine codons, CUC and CUG, are completely absent from the mitochondrial genome, so in these organisms leucine is encoded by only two codons - UUG and UUA - instead of the standard five.

Mycobacterium smegmatis

In a bacterium Mycobacterium smegmatis aspartic codons acquire additional meaning in the stationary growth phase, as well as under low pH conditions. Even more curious is that, due to the ambiguity of aspartic codons, substitutions occur in the β-subunit of RNA polymerase that preserve its functionality, but make the enzyme resistant to the antibiotic rifampicin, which normally blocks its work.

Of course, variations in the standard genetic code are not limited to these examples. However, exceptions only prove the rule, and this is also true for the genetic code. Despite the enormous diversity of living organisms, exceptions to the genetic code are so rare that they seem to be nothing more than curious curiosities. However, these exceptions provide valuable material for reconstructing the evolution of the genetic code and help to better understand its fundamental properties.

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