We drew attention to the fact that heredity and inheritance are two different phenomena that not everyone strictly distinguishes.
Heredity there is a process of material and functional discrete continuity between generations of cells and organisms. It is based on the exact reproduction of hereditarily significant structures.
Inheritance is the process of transferring hereditarily determined characteristics and properties of an organism and cell during the process of reproduction. The study of inheritance allows us to reveal the essence of heredity. Therefore, one should strictly separate these two phenomena.
The patterns of splitting and independent combination we examined relate to the study of inheritance, not heredity. Incorrect when " law of splitting" And " law of independent combination of traits-genes"are interpreted as laws of heredity. The laws discovered by Mendel are the laws of inheritance.
In the time of Mendel, it was believed that when crossing, parental characteristics are inherited in the offspring either together (“fused heredity”) or mosaically—some traits are inherited from the mother, others from the father (“mixed heredity”). The basis of such ideas was the belief that in the offspring the heredity of the parents is mixed, merged, and dissolved. This idea was wrong. It did not make it possible to scientifically argue the theory natural selection, and in fact, if during crossing the hereditary adaptive characteristics in the offspring were not preserved, but “dissolved,” then natural selection would work in vain. To free his theory of natural selection from such difficulties, Darwin put forward the theory of hereditary determination of a character by individual units - the theory of pangenesis. However, she did not give the right decision question.
Mendel's success is due to the discovery of a method genetic analysis individual pairs of hereditary traits; Mendel developed method of discrete analysis of inheritance of traits and essentially created scientific basis genetics, discovering the following phenomena:
- each hereditary characteristic is determined by a separate hereditary factor, a deposit; in the modern view, these inclinations correspond to genes: “one gene - one trait”, “one gene - one enzyme”;
- genes are preserved in their pure form over a number of generations, without losing their individuality: this was proof of the main point of genetics: the gene is relatively constant;
- both sexes participate equally in the transmission of their hereditary properties to offspring;
- reduplication of an equal number of genes and their reduction in male and female germ cells; this position was a genetic prediction of the existence of meiosis;
- hereditary inclinations are paired, one is maternal, the other is paternal; one of them may be dominant, the other recessive; This position corresponds to the discovery of the principle of allelism: a gene is represented by at least two alleles.
Thus, Mendel, having discovered the method of genetic analysis of the inheritance of individual pairs of traits (and not a set of traits) and established the laws of inheritance, was the first to postulate and experimentally prove the principle of discrete (genetic) determination of hereditary traits.
Based on the above, it seems useful to us to distinguish between the laws directly formulated by Mendel and related to the process of inheritance, and the principles of heredity arising from Mendel's work.
The laws of inheritance include the law of splitting hereditary characteristics in the offspring of a hybrid and the law of independent combination of hereditary characteristics. These two laws reflect the process of transmission of hereditary information in cellular generations during sexual reproduction. Their discovery was the first actual evidence of the existence of heredity as a phenomenon.
The laws of heredity have a different content, and they are formulated as follows:
First law- the law of discrete (genetic) hereditary determination of traits; it underlies the gene theory.
Second Law- the law of relative constancy of the hereditary unit - the gene.
Third Law- the law of the allelic state of a gene (dominance and recessivity).
It is these laws that represent the main result of Mendel’s work, since they reflect the essence of heredity.
Mendelian laws of inheritance and laws of heredity are the main content of genetics. Their discovery gave modern natural science unit of measurement of life processes - the gene and thereby created the possibility of unification natural sciences- biology, physics, chemistry and mathematics for the purpose of Analysis of biological processes.
In the future, when defining a hereditary unit, we will use only the term “gene”. The concepts of “hereditary factor” and “hereditary deposit” are cumbersome, and, in addition, the time has probably come when the hereditary factor and the gene should be distinguished and each of these concepts should have its own content. By the concept of “gene” we will further mean an indivisible functionally integral unit of heredity that determines a hereditary trait. The term “hereditary factor” should be interpreted in a broader sense as a complex of a number of genes and cytoplasmic influences on a hereditary trait.
Elements of the correct answer
1. Each organism is individual in its hereditary characteristics, this also applies to the structure of proteins.
2. When organs and tissues are transplanted, there is a threat of their rejection due to the incompatibility of the proteins of the donor and recipient.
Answer yourself
What is the relationship between genes and proteins in the body?
What does a gene code and how?
Elements of the correct answer
1. It is necessary that the gene responsible for the phenotypic trait be inherited by the organism.
2. The gene must be either dominant or recessive, but in this case it must be in a homozygous state.
Answer yourself
What conditions contribute to the variability of an organism?
How are variability and heredity related?
Elements of the correct answer
1. Inherited traits do not always appear; for example, a trait may be recessive and be in a heterozygous state.
2. The manifestation of phenotypic traits depends on many factors (for example, the penetrance and expressivity of genes), therefore, despite the presence of the corresponding genes, the inherited trait may not appear.
Answer yourself
What is the relationship between the genotype and phenotype of an organism?
Is it possible to determine its genotype based on the phenotype of an organism? Justify your answer.
Elements of the correct answer
1. These plants differ from each other in one feature - the shape of the seeds.
2. This trait is controlled by one pair of allelic genes.
Answer yourself
Why is crossing pea plants with yellow and smooth seeds with plants that produce green and wrinkled seeds called dihybrid?
Why does the sign of seed wrinkling not appear in the first generation of a monohybrid cross?
Elements of the correct answer
1. In first generation hybrids, only the dominant trait is manifested.
2. The recessive trait is suppressed in these hybrids.
Answer yourself
How is Mendel's first law formulated?
Why, according to Mendel's first law, in F2 (the offspring of crossing F1 hybrids) is the split approximately 3:1?
Elements of the correct answer
1. Mendel’s laws are statistical in nature, i.e. are confirmed on a large number of individuals (large statistical sample).
2. B real life In organisms that produce a small number of descendants, deviations from Mendel’s laws due to statistics occur.
3. Possible incomplete dominance, non-allelic gene interactions.
Answer yourself
Are Mendel's laws confirmed in families with two or three children? Explain your answer.
How can we explain that children in the same family inherit different traits from their parents?
Elements of the correct answer
1. Peas are a plant with pronounced contrasting allelic characteristics.
2. Peas are a self-pollinating plant, which allows you to experiment with clean lines and artificial cross-pollination.
Answer yourself
What patterns underlie the segregation by genotype and phenotype during monohybrid crossing?
What patterns underlie the segregation by genotype and phenotype during dihybrid crossing?
What is the essence of the gamete purity hypothesis?
Elements of the correct answer
1. Donkeys and horses have different karyotypes (donkeys have 62 chromosomes, horses have 64). Horse chromosomes are not homologous to donkey chromosomes.
2. Different chromosomes in meiosis do not conjugate with each other. Therefore, hybrids - mules - are sterile.
Answer yourself
Why is the number and nucleotide composition of chromosomes considered a species characteristic of organisms?
What is the biological meaning of chromosome conjugation and crossing over?
Elements of the correct answer
1. With complete dominance, heterozygous individuals exhibit a dominant trait in their phenotype (plant with red flowers? plant with white flowers = plant with red flowers: AA x ahh = Ahh;Ahh- red flowers).
2. With incomplete dominance in the heterozygous state, an intermediate phenotype appears (plant with red flowers? plant with white flowers = plant with pink flowers: AA x ahh = Ahh;Ahh- pink flowers).
Answer yourself
In what cases is the intermediate nature of inheritance manifested?
Can we say that the phenomenon of incomplete dominance refutes the hypothesis of gamete purity?
Elements of the correct answer
Gametes of one organism - AB, Ab; another - AB, aB.
Answer yourself
What types of gametes does an individual with the genotype produce? SсВbКК?
Write down the results of crossing individuals heterozygous for two traits in a Punnett grid.
Elements of the correct answer
1. Test crossbreeding is carried out to establish the genotype of a certain individual - to identify a recessive gene in it.
2. To do this, an individual homozygous for the recessive gene is crossed with an individual whose genotype is unknown.
Answer yourself
Is it possible to determine the genotype of an individual based on its phenotype? Explain your answer.
How can you accurately determine the genotype of an individual?
Elements of the correct answer
1. The law is valid for genes localized on one chromosome.
2. The law is violated when homologous chromosomes cross over.
Answer yourself
Under what conditions does crossing over occur?
Between which chromosomes does crossing over not occur?
What are the causes of combinational variability?
Elements of the correct answer
1. These structures include mitochondria, chloroplasts, and the cell center.
2. These organelles contain DNA.
Answer yourself
Is there heredity that is not transmitted through the chromosomal apparatus of a cell?
What do the nucleus, mitochondria and chloroplasts have in common?
Elements of the correct answer
1. Sex is determined by a pair of sex chromosomes located in human nuclear cells.
2. For men, this pair consists of a set designated XY, among women - XX.
Answer yourself
What is homo- and heterogamety?
How does sex-linked inheritance manifest itself?
Why are there no tortoiseshell cats?
Elements of the correct answer
1. Consanguineous marriages.
2. Age of the woman giving birth to the child (38–42 years).
3. Parents work in hazardous enterprises (nuclear, chemical, etc.).
Answer yourself
What are the risks of increasing frequency hereditary diseases can you name?
Explain your choice.
Elements of the correct answer
How does Down syndrome manifest and what are the causes of this disease?
1. Gene mutations affect one of the gene sections. For example, one nucleotide in a triplet may drop out or be replaced. A mutation may turn out to be neutral, or it may be harmful or beneficial.
2. Chromosomal mutations can lead to serious health consequences. They are associated with chromosome rearrangement.
3. A genomic mutation affects the genome. As a result of such a mutation, the number of chromosomes in the karyotype changes. If one or more haploid sets are added to the chromosome set, the phenomenon is called polyploidy. The phenomenon of polyploidy allows one to overcome interspecific sterility.
C2 level questions Genetics questions are not usually found in exam papers
Elements of the correct answer
Unified State Examination at level C2. However, we present tasks corresponding to this level for better understanding of genetic concepts by schoolchildren.
Errors were made in sentences 2, 5, 6.
Sentence 2 incorrectly indicates the number of characteristics by which the plants differed.
Proposition 5 erroneously indicates the proportion of hybrids with yellow seeds.
2. 1. There is reproductive isolation between species. 2. This factor contributes to the preservation of the species as an independent evolutionary unit. 3. It is especially important that genetically distant species be isolated. 4. The possibility of crossing between them is higher than with close, related species. 5. Protection from foreign genes is achieved by: a) different periods of maturation of gametes, b) similar habitats, c) the ability of the egg to distinguish between its own and foreign sperm. 6. Interspecific hybrids are often nonviable or sterile. |
Elements of the correct answer
Errors were made in sentences 3, 4, 5.
In sentence 3 there is an error in indicating the nature of the genetic proximity of species.
Proposition 4 erroneously states the probability of interbreeding between certain species.
In sentence 5, one of the factors of protection against foreign genes is incorrectly named.
3. Find errors in the given text. Indicate the numbers of the sentences in which they are allowed, explain them. 1. A gene is a section of an mRNA molecule that determines the structure of a protein and the corresponding characteristic of an organism. 2. Somatic cells contain a haploid set of chromosomes. 3. Genes that store information about one trait are located in strictly defined regions of homologous chromosomes and are called allelic. 4. Individuals that carry two allelic genes that are identical in expression and produce identical gametes are called dominant. 5. Individuals carrying allelic genes of different manifestations and, accordingly, different gametes are called heterozygous. 6. The patterns of independent inheritance of traits were established by T. Morgan. |
Elements of the correct answer
Errors were made in sentences 1, 2, 4, 6.
Sentence 1 has an incorrect definition of a gene.
Sentence 2 incorrectly indicates the number of chromosomes in somatic cells.
Sentence 4 incorrectly defines dominance.
Elements of the correct answer
Errors were made in the recording of the gametes produced by the parental individuals and in the recording of one of the genotypes.
Correct the mistakes you made using the Punnett grid.
5. Find errors in the given text. Indicate the numbers of the sentences in which they are allowed, explain them. 1. Gene – a section of a chromosome that encodes information about the sequence of amino acids in one protein molecule. 2. When passed on from parents to children, genes change (mutate). 3. The set of all genes of an organism is called a phenotype. 4. The totality of all external and internal characteristics of an organism is called genotype. 5. It is not so much the trait itself that is inherited as the possibility of its manifestation. 6. The implementation of the trait depends on both the genotype and the environmental conditions in which the organism is formed. |
Elements of the correct answer
Errors were made in sentences 2, 3, 4.
Sentence 2 erroneously indicates the nature of the transmission of genes from parents to offspring.
Sentence 3 incorrectly defines a phenotype.
Sentence 4 gives the wrong definition of genotype.
Elements of the correct answer
1. An entry in a gene expression has letter designations.
2. The entry in the chromosomal expression is shown in letter and graphic form.
Answer yourself
Find the error in the problem statement.
In dogs, the trait of black coat color is dominant over the trait of brown coat color. When crossing two black dogs, they got black and brown puppies. In the second generation, 3 black and two brown puppies were obtained from brown parents. What are the genotypes of the first pair of parents?
Find errors in the given text.
Two sons were born into the family of retired hussar colonel Ivan Aleksandrovich Prilezhaev. The boys grew up to be energetic kids and took part in all boyish fun. However, here's the problem - one of them, Peter, suffered from hemophilia, but Stepan did not have it. The boys' mother, Polina Arkadyevna, blamed her husband for Petenka's illness. Ivan Aleksandrovich did not consider himself guilty. When the boys grew up, according to tradition, they had to go serve in the hussar regiment. However, both were rejected for medical reasons, telling their father that the boys had severe heredity and could not be served. Any scratch is dangerous for both, and even more so an injury. After some time, Peter married a healthy girl with hemophilia, in whose family there were no hereditary diseases. They had two boys and two girls. All children suffered from hemophilia.
Stepan also married his second daughter from the same family. He gave birth to a hemophiliac boy and two healthy girls. Nothing is known about the health of the grandchildren in this family.
What process is shown in the picture?
Label the resulting gametes and explain the reason for the appearance of different gametes.
C6 level questions
Monohybrid crossing problems
Algorithm for solving problems in genetics
1. Select the letter designations of the alleles.
2. Write down all the given conditions of the problem.
3. Write the genotypes of the crossed individuals.
The most important condition for correctly solving a problem is a complete understanding of what is known and what is being asked. For example, if the condition says that 9 mice were obtained from two gray mice, of which one or two were white, then this means that both parents were heterozygous for the dominant trait of gray color, and white coat color is a recessive trait. This example shows how, based on the conditions of the problem, output the data necessary to solve it. Having understood the meaning of the problem and received additional data from its conditions, correctly record the solution. In the above problem, the entry will look like this:
If the problem does not ask about the splitting of traits in the offspring according to the ratio, then you don’t need to show it. It is enough to present all possible genotypes in F1.
Examples of simple problems
1. What F1 offspring can be expected from crossing a red-flowered heterozygous pea plant ( A) with a white-flowered plant? Will character splitting be observed and in what proportion?
2. From Drosophila flies with normal wings and flies with shortened wings, flies with normal and shortened wings were obtained in a 1:1 ratio. Determine the genotypes of parents and offspring.
3. The black plumage of Andalusian chickens is not completely dominant over the white plumage. A rooster with black feathers was crossed with a hen with white feathers. Some of the chickens born from this crossing had blue plumage. Write down the genotypes of all individuals mentioned in the condition. What kind of genotype and phenotype splitting should be expected in the offspring from these parents, provided that there are quite a lot of chickens? Is it possible to breed a pure line of chickens with blue feathers?
4. When crossing two tall ( WITH) plants, 25% of the seeds were obtained, from which stunted plants grew. What are the genotypes of low-growing plants?
Dihybrid crossing problems
When solving problems of this type it is necessary:
a) carefully read the conditions of the task;
b) make the necessary notes as you read the task;
c) having understood the condition of the problem, you need to designate the alleles with the corresponding letters, draw a Punnett grid and fill it in in accordance with the logic of the solution;
d) make sure that general form record solution met the requirements.
An example of a problem covered in textbooks
Pea plants producing yellow ( A) smooth ( IN) seeds crossed with plants that produce green ( A) wrinkled ( b) seeds. Both lines were clean. What will the hybrid offspring in F1 and F2 be like in terms of genotypes and phenotypes?
The logic of the reasoning is as follows.
1. If the lines are pure, it means the parents are homozygous for both traits.
2. Each parent produces one type of gamete.
Genotype AABB gives gametes AB.
Genotype aabb gives gametes ab.
Therefore, all first generation hybrids will have the genotype AaBb.
Individuals with this genotype form 4 types of gametes: AB,
aB, Ab, ab.
3. To determine the genotypes of individuals of the second generation, it is necessary to draw a Punnett grid and write down the types of gametes formed by the parents in the upper horizontal row and the left vertical column. After that, write down the resulting genotypes of the offspring in the remaining free fields.
AABB and. Ch. |
AaBB and. Ch. |
AABb and. Ch. |
AABb and. Ch. |
|
AaBB and. ch |
aaBB h. Ch. |
AaBb and. Ch. |
aaBb h. Ch. |
|
AABb and. ch |
AaBb and. Ch. |
ААbb and. wrinkle |
Ааbb and. wrinkle |
|
AaBb and. Ch. |
aaBb h. Ch. |
Ааbb and. wrinkle |
aabb h. wrinkle |
– both dominant genes;
– dominant gene of one of the traits;
– a dominant gene for another trait;
– only recessive genes.
The result in this case will be as follows: 9 AB : 3Ab : 3aB : 1ab.
5. Answer: hybrid offspring in F1 – AaBb, in the second generation there will be 16 genotypes (shown in a Punnett lattice) and 4 phenotypes:
– plants with yellow smooth seeds;
– plants with yellow wrinkled seeds;
– plants with green smooth seeds;
- plants with green wrinkled seeds.
Problems found in exam papers
Elements of the correct answer
To make a correct decision, you need to prove that:
1) flies with a genotype XY(males) can be red-eyed or white-eyed;
2) heterozygous females are always red-eyed, females homozygous for a recessive trait are white-eyed, and females homozygous for a dominant trait are red-eyed.
To prove these two points, it is necessary to cross a red-eyed heterozygous female with a white-eyed male. Some of the males resulting from this cross will have eyes white. Therefore, the recessive trait is linked to X-chromosome.
2. Make a diagram to illustrate the text below, showing the genotypes and patterns of inheritance of color blindness. If a woman suffering from color blindness marries a man with normal vision, then their children exhibit a very peculiar pattern of cross-inheritance. All daughters from such a marriage will receive the sign of their father, i.e. they have normal vision, and all sons, receiving the mother’s trait, suffer from color blindness (a-color blindness linked to X-chromosome). |
Elements of the correct answer
Girls are carriers, boys are colorblind.
Girls are carriers, boys are healthy.
Half of the boys and girls are healthy, half of the girls are carriers, half of the boys are color blind.
Elements of the correct answer for independent decision
1. Write down the letter designations of the alleles of the parents’ genotypes and the crossing scheme.
2. Determine all genotypes specified in the condition.
3. Draw up a diagram of a new crossing and write down its results.
Elements of the correct answer
1. Genotypes of parents X f X And XY.
2. Genotypes of children X f Y, X f X, XX, XY.
3. The nature of inheritance is dominant, linked to X-chromosome.
Elements of the correct answer
1. According to the condition, the baldness gene was inherited only by boys.
2. All women in the families in question had normal hair.
3. Consequently, this gene was passed on from fathers, i.e. in the male line.
4. Conclusion: the trait is linked to U-chromosome and is passed on from fathers to sons.
P1 XY l x XX
F1 2 XY l and 4 XX
P2 XY l x XX
F2 Grandchildren XY l
Decide for yourself
Make a diagram to illustrate the text below, showing the genotypes and patterns of inheritance of hemophilia.
An example of sex-linked inheritance is the inheritance of a recessive semi-lethal gene that causes blood to clot in air - hemophilia. X This disease appears almost exclusively in boys. In hemophilia, the formation of a factor that accelerates blood clotting is impaired. The recessive gene that controls the synthesis of this factor is located in a certain region U-chromosome and does not have an allele in
-chromosome. After solving the problem, answer the question: “Why are women with hemophilia extremely rare?”Write down the results of the cross that can be obtained in the following cases:
a) the father is a hemophiliac, the mother is a carrier of the hemophilia gene;
b) the father is healthy, the mother is a carrier of the hemophilia gene;
c) the father is hemophiliac, the mother does not carry the hemophilia gene.
Previously, we emphasized that nucleotides have an important feature for the formation of life on Earth - in the presence of one polynucleotide chain in a solution, the process of formation of a second (parallel) chain spontaneously occurs based on the complementary connection of related nucleotides. The same number of nucleotides in both chains and their chemical affinity are an indispensable condition for the implementation of this type of reaction. However, during protein synthesis, when information from mRNA is implemented into the protein structure, there can be no talk of observing the principle of complementarity. This is due to the fact that in mRNA and in the synthesized protein not only the number of monomers is different, but also, what is especially important, there is no structural similarity between them (nucleotides on the one hand, amino acids on the other). It is clear that in this case there is a need to create a new principle for accurately translating information from a polynucleotide into the structure of a polypeptide. In evolution, such a principle was created and its basis was the genetic code.
The genetic code is a system for recording hereditary information in nucleic acid molecules, based on a certain alternation of nucleotide sequences in DNA or RNA, forming codons corresponding to amino acids in a protein.
The genetic code has several properties.
Tripletity.
Degeneracy or redundancy.
Unambiguity.
Polarity.
Non-overlapping.
Compactness.
Versatility.
It should be noted that some authors also propose other properties of the code related to the chemical characteristics of the nucleotides included in the code or the frequency of occurrence of individual amino acids in the body’s proteins, etc. However, these properties follow from those listed above, so we will consider them there.
A. Tripletity. The genetic code, like many complexly organized systems, has the smallest structural and smallest functional unit. Triplet – the smallest structural unit genetic code. It consists of three nucleotides. A codon is the smallest functional unit of the genetic code. Typically, triplets of mRNA are called codons. In the genetic code, a codon performs several functions. Firstly, its main function is that it encodes a single amino acid. Secondly, the codon may not code for an amino acid, but, in this case, it performs another function (see below). As can be seen from the definition, a triplet is a concept that characterizes elementary structural unit genetic code (three nucleotides). Codon – characterizes elementary semantic unit genome - three nucleotides determine the attachment of one amino acid to the polypeptide chain.
The elementary structural unit was first deciphered theoretically, and then its existence was confirmed experimentally. Indeed, 20 amino acids cannot be encoded with one or two nucleotides because there are only 4 of the latter. Three out of four nucleotides give 4 3 = 64 variants, which more than covers the number of amino acids available in living organisms (see Table 1).
The 64 nucleotide combinations presented in table have two features. Firstly, of the 64 triplet variants, only 61 are codons and encode any amino acid, they are called sense codons. Three triplets do not encode
Table 1.
Messenger RNA codons and corresponding amino acids
|
FOUNDATION OF KODONOV |
|||||
|
Nonsense |
Nonsense |
||||
|
Nonsense | |||||
|
Meth |
|||||
|
Shaft |
|||||
amino acids a are stop signals indicating the end of translation. There are three such triplets - UAA, UAG, UGA, they are also called “meaningless” (nonsense codons). As a result of a mutation, which is associated with the replacement of one nucleotide in a triplet with another, a nonsense codon can arise from a sense codon. This type of mutation is called nonsense mutation. If such a stop signal is formed inside the gene (in its information part), then during protein synthesis in this place the process will be constantly interrupted - only the first (before the stop signal) part of the protein will be synthesized. A person with this pathology will experience a lack of protein and will experience symptoms associated with this deficiency. For example, this kind of mutation was identified in the gene encoding the hemoglobin beta chain. A shortened inactive hemoglobin chain is synthesized, which is quickly destroyed. As a result, a hemoglobin molecule devoid of a beta chain is formed. It is clear that such a molecule is unlikely to fully fulfill its duties. A serious disease occurs that develops as hemolytic anemia (beta-zero thalassemia, from the Greek word “Thalas” - Mediterranean Sea, where this disease was first discovered).
The mechanism of action of stop codons differs from the mechanism of action of sense codons. This follows from the fact that for all codons encoding amino acids, corresponding tRNAs have been found. No tRNAs were found for nonsense codons. Consequently, tRNA does not take part in the process of stopping protein synthesis.
CodonAUG (sometimes GUG in bacteria) not only encode the amino acids methionine and valine, but are alsobroadcast initiator .
b. Degeneracy or redundancy.
61 of the 64 triplets encode 20 amino acids. This three-fold excess of the number of triplets over the number of amino acids suggests that two coding options can be used in the transfer of information. Firstly, not all 64 codons can be involved in encoding 20 amino acids, but only 20 and, secondly, amino acids can be encoded by several codons. Research has shown that nature used the latter option.
His preference is obvious. If out of 64 variant triplets only 20 were involved in encoding amino acids, then 44 triplets (out of 64) would remain non-coding, i.e. meaningless (nonsense codons). Previously, we pointed out how dangerous it is for the life of a cell to transform a coding triplet as a result of mutation into a nonsense codon - this significantly disrupts the normal functioning of RNA polymerase, ultimately leading to the development of diseases. Currently, three codons in our genome are nonsense, but now imagine what would happen if the number of nonsense codons increased by about 15 times. It is clear that in such a situation the transition of normal codons to nonsense codons will be immeasurably higher.
A code in which one amino acid is encoded by several triplets is called degenerate or redundant. Almost every amino acid has several codons. Thus, the amino acid leucine can be encoded by six triplets - UUA, UUG, TSUU, TsUC, TsUA, TsUG. Valine is encoded by four triplets, phenylalanine by two and only tryptophan and methionine encoded by one codon. The property that is associated with recording the same information with different symbols is called degeneracy.
The number of codons designated for one amino acid correlates well with the frequency of occurrence of the amino acid in proteins.
And this is most likely not accidental. The higher the frequency of occurrence of an amino acid in a protein, the more often the codon of this amino acid is represented in the genome, the higher the likelihood of its damage by mutagenic factors. Therefore, it is clear that a mutated codon has a greater chance of encoding the same amino acid if it is highly degenerate. From this perspective, the degeneracy of the genetic code is a mechanism that protects the human genome from damage.
It should be noted that the term degeneracy is used in molecular genetics in another sense. Thus, the bulk of the information in a codon is contained in the first two nucleotides; the base in the third position of the codon turns out to be of little importance. This phenomenon is called “degeneracy of the third base.” The latter feature minimizes the effect of mutations. For example, it is known that the main function of red blood cells is to transport oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs. This function is performed by the respiratory pigment - hemoglobin, which fills the entire cytoplasm of the erythrocyte. It consists of a protein part - globin, which is encoded by the corresponding gene. In addition to protein, the hemoglobin molecule contains heme, which contains iron. Mutations in globin genes lead to the appearance of different variants of hemoglobins. Most often, mutations are associated with replacing one nucleotide with another and the appearance of a new codon in the gene 400 , which may encode a new amino acid in the hemoglobin polypeptide chain. In a triplet, as a result of mutation, any nucleotide can be replaced - the first, second or third. Several hundred mutations are known that affect the integrity of the globin genes. Near 100 of which are associated with the replacement of single nucleotides in a gene and the corresponding amino acid replacement in a polypeptide. Of these only
replacements lead to instability of hemoglobin and various kinds of diseases from mild to very severe. 300 (approximately 64%) substitution mutations do not affect hemoglobin function and do not lead to pathology. One of the reasons for this is the above-mentioned “degeneracy of the third base,” when a replacement of the third nucleotide in a triplet encoding serine, leucine, proline, arginine and some other amino acids leads to the appearance of a synonymous codon encoding the same amino acid. Such a mutation will not manifest itself phenotypically. In contrast, any replacement of the first or second nucleotide in a triplet in 100% of cases leads to the appearance of a new hemoglobin variant. But even in this case, there may not be severe phenotypic disorders. The reason for this is the replacement of an amino acid in hemoglobin with another one similar to the first one in physicochemical properties. For example, if an amino acid with hydrophilic properties is replaced by another amino acid, but with the same properties. Hemoglobin consists of the iron porphyrin group of heme (oxygen and carbon dioxide molecules are attached to it) and protein - globin. Adult hemoglobin (HbA) contains two identical -chains and two -chain contains 141 amino acid residues, -chain - 146, - And -chains differ in many amino acid residues. The amino acid sequence of each globin chain is encoded by its own gene. Gene encoding -the chain is located in the short arm of chromosome 16, -gene - in the short arm of chromosome 11. Substitution in the gene encoding -the hemoglobin chain of the first or second nucleotide almost always leads to the appearance of new amino acids in the protein, disruption of hemoglobin functions and serious consequences for the patient. For example, replacing “C” in one of the triplets CAU (histidine) with “Y” will lead to the appearance of a new triplet UAU, encoding another amino acid - tyrosine. Phenotypically this will manifest itself in a severe disease.. A similar substitution in position 63 -chain of histidine polypeptide to tyrosine will lead to destabilization of hemoglobin. The disease methemoglobinemia develops. Replacement, as a result of mutation, of glutamic acid with valine in the 6th position -chain is the cause of the most severe disease - sickle cell anemia. Let's not continue the sad list. Let us only note that when replacing the first two nucleotides, an amino acid may appear physical and chemical properties similar to the previous one. Thus, replacement of the 2nd nucleotide in one of the triplets encoding glutamic acid (GAA) in -chain with “U” leads to the appearance of a new triplet (GUA), encoding valine, and replacing the first nucleotide with “A” forms the triplet AAA, encoding the amino acid lysine. Glutamic acid and lysine are similar in physicochemical properties - they are both hydrophilic. Valine is a hydrophobic amino acid. Therefore, replacing hydrophilic glutamic acid with hydrophobic valine significantly changes the properties of hemoglobin, which ultimately leads to the development of sickle cell anemia, while replacing hydrophilic glutamic acid with hydrophilic lysine changes the function of hemoglobin to a lesser extent - patients develop a mild form of anemia. As a result of the replacement of the third base, the new triplet can encode the same amino acids as the previous one. For example, if in the CAC triplet uracil was replaced by cytosine and a CAC triplet appeared, then practically no phenotypic changes will be detected in humans. This is understandable, because both triplets code for the same amino acid – histidine.
In conclusion, it is appropriate to emphasize that the degeneracy of the genetic code and the degeneracy of the third base from a general biological point of view are protective mechanisms that are inherent in evolution in the unique structure of DNA and RNA.
V. Unambiguity.
Each triplet (except nonsense) encodes only one amino acid. Thus, in the direction codon - amino acid the genetic code is unambiguous, in the direction amino acid - codon it is ambiguous (degenerate).
Unambiguous
Amino acid codon
Degenerate
And in this case, the need for unambiguity in the genetic code is obvious. In another option, when translating the same codon, different amino acids would be inserted into the protein chain and, as a result, proteins with different primary structures and different functions would be formed. Cell metabolism would switch to the “one gene – several polypeptides” mode of operation. It is clear that in such a situation the regulatory function of genes would be completely lost.
g. Polarity
Reading information from DNA and mRNA occurs only in one direction. Polarity is important for defining higher order structures (secondary, tertiary, etc.). Earlier we talked about how lower-order structures determine higher-order structures. Tertiary structure and structures more high order in proteins, they are formed immediately as soon as the synthesized RNA chain leaves the DNA molecule or the polypeptide chain leaves the ribosome. While the free end of an RNA or polypeptide acquires a tertiary structure, the other end of the chain continues to be synthesized on DNA (if RNA is transcribed) or a ribosome (if a polypeptide is transcribed).
Therefore, the unidirectional process of reading information (during the synthesis of RNA and protein) is essential not only for determining the sequence of nucleotides or amino acids in the synthesized substance, but for the strict determination of secondary, tertiary, etc. structures.
d. Non-overlapping.
The code may be overlapping or non-overlapping. Most organisms have a non-overlapping code. Overlapping code is found in some phages.
The essence of a non-overlapping code is that a nucleotide of one codon cannot simultaneously be a nucleotide of another codon. If the code were overlapping, then the sequence of seven nucleotides (GCUGCUG) could encode not two amino acids (alanine-alanine) (Fig. 33, A) as in the case of a non-overlapping code, but three (if there is one nucleotide in common) (Fig. . 33, B) or five (if two nucleotides are common) (see Fig. 33, C). In the last two cases, a mutation of any nucleotide would lead to a violation in the sequence of two, three, etc. amino acids.
However, it has been established that a mutation of one nucleotide always disrupts the inclusion of one amino acid in a polypeptide. This is a significant argument that the code is non-overlapping.
Let us explain this in Figure 34. Bold lines show triplets encoding amino acids in the case of non-overlapping and overlapping code. Experiments have clearly shown that the genetic code is non-overlapping. Without going into details of the experiment, we note that if you replace the third nucleotide in the sequence of nucleotides (see Fig. 34)U (marked with an asterisk) to some other thing:
1. With a non-overlapping code, the protein controlled by this sequence would have a substitution of one (first) amino acid (marked with asterisks).
2. With an overlapping code in option A, a substitution would occur in two (first and second) amino acids (marked with asterisks). Under option B, the replacement would affect three amino acids (marked with asterisks).
However, numerous experiments have shown that when one nucleotide in DNA is disrupted, the disruption in the protein always affects only one amino acid, which is typical for a non-overlapping code.
GZUGZUG GZUGZUG GZUGZUG
GCU GCU GCU UGC GCU GCU GCU UGC GCU GCU GCU
*** *** *** *** *** ***
Alanin - Alanin Ala - Cis - Ley Ala - Ley - Ley - Ala - Ley
A B C
Non-overlapping code Overlapping code
Rice. 34. A diagram explaining the presence of a non-overlapping code in the genome (explanation in the text).
The non-overlap of the genetic code is associated with another property - the reading of information begins from a certain point - the initiation signal. Such an initiation signal in mRNA is the codon encoding methionine AUG.
It should be noted that humans still have a small number of genes that deviate from general rule and overlap.
e. Compactness.
There is no punctuation between codons. In other words, triplets are not separated from each other, for example, by one meaningless nucleotide. The absence of “punctuation marks” in the genetic code has been proven in experiments.
and. Versatility.
The code is the same for all organisms living on Earth. Direct proof The universality of the genetic code was obtained by comparing DNA sequences with corresponding protein sequences. It turned out that all bacterial and eukaryotic genomes use the same sets of code values. There are exceptions, but not many.
The first exceptions to the universality of the genetic code were found in the mitochondria of some animal species. This concerned the terminator codon UGA, which reads the same as the codon UGG, encoding the amino acid tryptophan. Other rarer deviations from universality were also found.
MZ. The genetic code is a system for recording hereditary information in nucleic acid molecules, based on a certain alternation of nucleotide sequences in DNA or RNA that form codons,
corresponding to amino acids in protein.The genetic code has several properties.
After the discovery of the principle of molecular organization of such a substance as DNA in 1953, molecular biology began to develop. Further in the process of research, scientists found out how DNA is recombined, its composition and how our human genome is structured.
Every day, complex processes occur at the molecular level. How is the DNA molecule structured, what does it consist of? And what role do DNA molecules play in a cell? Let's talk in detail about all the processes occurring inside the double chain.
What is hereditary information?
So where did it all start? Back in 1868 they found it in the nuclei of bacteria. And in 1928, N. Koltsov put forward the theory that all genetic information about a living organism is encrypted in DNA. Then J. Watson and F. Crick found a model of the now well-known DNA helix in 1953, for which they deservedly received recognition and an award - the Nobel Prize.
What is DNA anyway? This substance consists of 2 united threads, or rather spirals. A section of such a chain with certain information is called a gene.
DNA stores all the information about what kind of proteins will be formed and in what order. The DNA macromolecule is a material carrier of incredibly voluminous information, which is recorded in a strict sequence of individual bricks - nucleotides. There are 4 nucleotides in total; they complement each other chemically and geometrically. This principle of complementation, or complementarity, in science will be described later. This rule plays a key role in the encoding and decoding of genetic information.
Since the DNA strand is incredibly long, there are no repetitions in this sequence. Every living creature has its own unique strand of DNA.
Functions of DNA
Functions include storage of hereditary information and its transmission to offspring. Without this function, the genome of a species could not be preserved and developed over thousands of years.
Organisms that have undergone severe gene mutations are more likely to not survive or lose the ability to produce offspring. This is how natural protection against the degeneration of the species occurs. Another significant important function
— implementation of stored information. A cell cannot create a single vital protein without those instructions that are stored in a double chain.
Nucleic acid composition
- It is now known for certain what the nucleotides themselves—the building blocks of DNA—are made of. They contain 3 substances:
- Orthophosphoric acid.
- Nitrogenous base. Pyrimidine bases - which have only one ring. These include thymine and cytosine. Purine bases, which contain 2 rings. These are guanine and adenine.
Sucrose. DNA contains deoxyribose, RNA contains ribose.
The number of nucleotides is always equal to the number of nitrogenous bases. In special laboratories, the nucleotide is broken down and the nitrogenous base is isolated from it. This is how the individual properties of these nucleotides and possible mutations in them are studied.
Levels of organization of hereditary information
There are 3 levels of organization: genetic, chromosomal and genomic. All the information needed for the synthesis of a new protein is contained in a small section of the chain - the gene. That is, the gene is considered the lowest and simplest level of information encoding.
Genes, in turn, are assembled into chromosomes. Thanks to this organization of the carrier of hereditary material, groups of characteristics alternate according to certain laws and are transmitted from one generation to another. It should be noted that there are an incredible number of genes in the body, but the information is not lost even when it is recombined many times.
- There are several types of genes:
- According to their functional purpose, there are 2 types: structural and regulatory sequences;
Based on their influence on the processes occurring in the cell, they distinguish: supervital, lethal, conditionally lethal genes, as well as mutator and antimutator genes.
Genes are arranged along the chromosome in a linear order. In chromosomes, information is not focused randomly; there is a certain order. There is even a map that shows the positions, or loci, of genes. For example, it is known that chromosome No. 18 encrypts data about the color of a child's eyes.
What is the human genetic code?
The fact is that all the enormous potential human development laid down already during the period of conception. All hereditary information that is necessary for the development of the zygote and the growth of the child after birth is encrypted in genes. DNA sections are the most basic carriers of hereditary information.
Humans have 46 chromosomes, or 22 somatic pairs plus one sex-determining chromosome from each parent. This diploid set of chromosomes encodes the entire physical appearance of a person, his mental and physical abilities and susceptibility to diseases. Somatic chromosomes are outwardly indistinguishable, but they carry different information, since one of them is from the father, the other from the mother.
The male code differs from the female code in the last pair of chromosomes - XY. The female diploid set is the last pair, XX. Males receive one X chromosome from their biological mother, which is then passed on to their daughters. The sex Y chromosome is passed on to sons.
Human chromosomes vary greatly in size. For example, the smallest pair of chromosomes is No. 17. And the biggest pair is 1 and 3.
Diameter double helix in humans it is only 2 nm. The DNA is coiled so tightly that it fits inside the small nucleus of a cell, although it would be up to 2 meters long if untwisted. The length of the helix is hundreds of millions of nucleotides.
How is the genetic code transmitted?
So, what role do DNA molecules play in cell division? Genes - carriers of hereditary information - are located inside every cell of the body. To pass on their code to a daughter organism, many creatures divide their DNA into 2 identical helices. This is called replication. During the replication process, DNA unwinds and special “machines” complete each strand. After the genetic helix bifurcates, the nucleus and all organelles begin to divide, and then the entire cell.
But humans have a different process of gene transmission - sexual. The characteristics of the father and mother are mixed, the new genetic code contains information from both parents.
The storage and transmission of hereditary information is possible due to the complex organization of the DNA helix. After all, as we said, the structure of proteins is encrypted in genes. Once created at the time of conception, this code will copy itself throughout life. The karyotype (personal set of chromosomes) does not change during the renewal of organ cells. The transfer of information is carried out with the help of sex gametes - male and female.
Only viruses containing one strand of RNA are not capable of transmitting their information to their offspring. Therefore, they need human or animal cells to reproduce.
Implementation of hereditary information
Important processes constantly occur in the cell nucleus. All information recorded in chromosomes is used to build proteins from amino acids. But the DNA chain never leaves the nucleus, so it needs the help of another important compound: RNA. It is RNA that is able to penetrate the nuclear membrane and interact with the DNA chain.
Through the interaction of DNA and 3 types of RNA, all encoded information is realized. At what level does the implementation of hereditary information occur? All interactions occur at the nucleotide level. Messenger RNA copies a section of the DNA chain and brings this copy to the ribosome. Here the synthesis of a new molecule from nucleotides begins.
In order for the mRNA to copy the necessary part of the chain, the helix unfolds and then, upon completion of the recoding process, is restored again. Moreover, this process can occur simultaneously on 2 sides of 1 chromosome.
Principle of complementarity
They consist of 4 nucleotides - adenine (A), guanine (G), cytosine (C), thymine (T). They are connected by hydrogen bonds according to the rule of complementarity. The work of E. Chargaff helped establish this rule, since the scientist noticed some patterns in the behavior of these substances. E. Chargaff discovered that the molar ratio of adenine to thymine is equal to one. And in the same way, the ratio of guanine to cytosine is always equal to one.
Based on his work, geneticists formed a rule for the interaction of nucleotides. The complementarity rule states that adenine combines only with thymine, and guanine only combines with cytosine. During the decoding of the helix and the synthesis of a new protein in the ribosome, this alternation rule helps to quickly find the necessary amino acid that is attached to the transfer RNA.
RNA and its types
What is hereditary information? nucleotides in a double strand of DNA. What is RNA? What is her job? RNA, or ribonucleic acid, helps extract information from DNA, decode it and, based on the principle of complementarity, create proteins necessary for cells.
There are 3 types of RNA in total. Each of them performs strictly its own function.
- Informational (mRNA), or also called matrix. It goes straight into the center of the cell, into the nucleus. Finds the necessary one in one of the chromosomes genetic material to build a protein and copies one side of the double chain. Copying occurs again according to the principle of complementarity.
- Transport is a small molecule that has nucleotide decoders on one side, and amino acids corresponding to the basic code on the other side. The task of tRNA is to deliver it to the “workshop,” that is, to the ribosome, where it synthesizes the necessary amino acid.
- rRNA is ribosomal. It controls the amount of protein that is produced. It consists of 2 parts - an amino acid and a peptide section.
The only difference in decoding is that RNA does not have thymine. Instead of thymine, uracil is present here. But then, during the process of protein synthesis, tRNA still correctly installs all the amino acids. If any failures occur in decoding information, then a mutation occurs.
Repair of damaged DNA molecule
The process of restoring a damaged double strand is called repair. During the repair process, damaged genes are removed.
Then the required sequence of elements is exactly reproduced and cut back into the same place on the chain from where it was removed. All this happens thanks to special chemicals- enzymes.
Why do mutations occur?
Why do some genes begin to mutate and cease to perform their function - storing vital hereditary information? This occurs due to an error in decoding. For example, if adenine is accidentally replaced with thymine.
There are also chromosomal and genomic mutations. Chromosomal mutations occur when sections of hereditary information are lost, duplicated, or even transferred and inserted into another chromosome.
Genomic mutations are the most serious. Their cause is a change in the number of chromosomes. That is, when instead of a pair - a diploid set, a triploid set is present in the karyotype.
The most famous example of a triploid mutation is Down syndrome, in which the personal set of chromosomes is 47. In such children, 3 chromosomes are formed in place of the 21st pair.
There is also a known mutation called polyploidy. But polyploidy occurs only in plants.
In the section on the question What is the genetic code called? List the main properties of the genetic code. given by the author Christina the best answer is The genetic code is a method of encoding the amino acid sequence of proteins using a sequence of nucleotides, characteristic of all living organisms. Properties
Triplet - a meaningful unit of code is a combination of three nucleotides (triplet, or codon).
Continuity - there is no punctuation between triplets, that is, the information is read continuously.
Non-overlap - the same nucleotide cannot simultaneously be part of two or more triplets (not observed for some overlapping genes of viruses, mitochondria and bacteria, which encode several frameshift proteins).
Unambiguity (specificity) - a certain codon corresponds to only one amino acid (however, the UGA codon in Euplotes crassus encodes two amino acids - cysteine and selenocysteine)
Degeneracy (redundancy) - several codons can correspond to the same amino acid.
Universality - the genetic code works the same in organisms of different levels of complexity - from viruses to humans (methods are based on this genetic engineering; there are a number of exceptions, shown in the table in the Variations in the Standard Genetic Code section below).
Noise immunity - mutations of nucleotide substitutions 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.
