Textbook - BIOLOGICAL CHEMISTRY - Hubsky Y.I. - 2000

Chapter IV. MOLECULAR MECHANISMS OF HEREDITY AND REALIZATION OF GENETIC INFORMATION

CHAPTER 22. REGULATION OF GENE EXPRESSION. GENETIC RECOMBINATIONS

22.3. MOLECULAR MECHANISMS OF MUTATIONS. DNA REPAIR

Mutations are changes in hereditary properties resulting from quantitative and qualitative alterations in the Organism's genotype. During METABOLISM/36.html">DNA Replication, mutations are transmitted from Cell to Cell and from generation to generation.

Along with genetic recombinations (see above), mutations form The basis of the hereditary Variability of Living organisms. Mutations can be caused by certain natural factors—spontaneous mutations, or artificial factors—induced mutations.

According to the nature of changes in The Structure of the organism's genetic apparatus, mutations are divided into:

(1) genomic mutations, which involve Changes in the number of the complete set of Chromosomes or individual chromosomes within the diploid set; such mutations cause the most widespread and severe forms of human Chromosomal Disorders;

(2) chromosomal mutations, which are associated with structural changes in specific chromosomes (Chromosomal aberrations) resulting from the translocation, loss, or duplication of individual fragments of chromosomal DNA. The following types of chromosomal mutations are distinguished:

Transpositions — The transfer of a DNA fragment to another region of the same chromosome;

translocations — the transfer of a segment of one chromosome to another, non-homologous chromosome;

inversions — A change in The sequence of genes (nitrogenous bases) in a specific region of a chromosome to the opposite sequence;

deletions — the loss of specific chromosomal regions (DNA fragments);

duplications — the doubling of specific chromosomal regions.

(3) Gene (point) mutations, which are alterations in The Genome structure involving disruptions in the sequence of nitrogenous bases (NUCLEOTIDES) that make up the Introduction/19.html">Primary Structure of DNA. Gene Mutations are divided into the following types:

a) nucleotide substitutions — the most common gene mutations, which include such subtypes as:

transitions — the replacement of one purine base by another purine, or a pyrimidine by a pyrimidine;

transversions — the replacement of one type of nitrogenous base by another, i.e., a purine by a pyrimidine or vice versa;

b) loss (deletions) of one or more nitrogenous bases (and corresponding nucleotides) in the DNA chain;

c) insertions (incorporation) of additional nitrogenous bases (one or more) into the DNA chain.

Gene mutations, unless repaired by specialized cellular enzyme systems, lead to the termination of Synthesis of the protein encoded by the respective gene, or to The formation of a protein with an altered, "incorrect" primary structure.

Class="center">Agents that cause mutations (mutagens)

Mutations (most frequently gene mutations) arise As a result of Damage caused by adverse environmental chemical, physical, and biological factors acting on the genome, or errors in the functioning of DNA polymerases during the DNA replication stage.

The most common mutagens are:

(1) nitrogenous base analogues — compounds that substitute for normal nitrogenous bases in the polydeoxyribonucleotide chain. The most common substances of this class are 5-bromouracil and 2-aminopurine;

(2) Chemical Mutagens — compounds that alter the Covalent Structure of normal nitrogenous bases; the most common chemical mutagens include:

а) deaminating agents — nitrous acid (HNO₂) and substances that can be converted into nitrites during metabolism, particularly Organic compounds such as nitrosamines. Nitrous acid causes the deamination of cytosine (yielding uracil), adenine, and guanine (yielding hypoxanthine and xanthine, respectively). The substitution of a single nucleotide in the DNA chain alters a specific codon (missense mutation) and leads to the synthesis of a protein with an altered Amino Acid Sequence. Such mutations are believed to give rise to abnormal Hemoglobin variants with altered primary structure in their β-chains;

б) alkylating agents — compounds that cause methylation (or alkylation in general) of standard nitrogenous bases. Alkylating agents include alkyl sulfonates (such as dimethyl sulfonate and ethyl methanesulfonate), nitrogen and sulfur bis(β-chloroethyl)amines (mustards), alkylnitrosamines, etc.; many of these compounds possess antitumor (anti-blastomic) activity and are used in clinical and experimental oncology;

(3) ultraviolet (UV) and ionizing radiation — physical factors whose high mutagenic activity is attributed to free-radical destruction of DNA nitrogenous bases, producing analogs with altered chemical structures.

A common mutation observed under UV exposure is the formation of covalent bonds between adjacent thymine residues located on the same strand. Such thymine dimers impede the normal progression of DNA polymerases during replication, halting DNA Synthesis altogether.

Mechanisms of DNA Repair

All living organisms on Earth are subject to the constant action of physical Mutagenic Factors, notably UV radiation (with a wavelength of 200–400 nm), which constitutes a significant portion of the solar spectrum, and ionizing radiation originating from cosmic rays and radioactive isotopes of radium, plutonium, carbon, etc., present in inorganic environmental objects. According to experts, UV and ionizing radiation account for approximately 10 % of all DNA damage caused by non-biological factors. Another crucial component of environmental mutagenic pressure on the genome of living organisms is the action of numerous chemical compounds, particularly foreign substances—xenobiotics—that continuously affect nuclear genetic DNA. As with the aforementioned physical factors, The Mechanism of the damaging action of chemical mutagens largely depends on the generation of free radicals of oxygen and Water within The Cell, which induce changes in the covalent structure of DNA nitrogenous bases (Yu.I. Gubsky, 1993).

The impact of these Physical and Chemical factors on the genome leads to DNA instability, subjecting it to constant point mutations. The most frequent among these are the Cleavage of purine bases (DNA depurination), cytosine deamination, and depyrimidination. Consequently, specific molecular mechanisms have evolved in living organisms to counteract ongoing DNA damage and repair structural alterations that have already occurred.

1. Repair of UV-induced damage.

DNA damage caused by UV rays is most commonly observed in Bacterial Cells and unprotected human Skin. The restoration of normal DNA Structure disrupted by thymine dimer formation is carried out through the mechanisms indicated in Fig. 22.14 (stages I–IV):

Fig. 22.14. Scheme of DNA fragment repair involving a thymine dimer.

I. Cleavage ("cutting") of the DNA strand to the "left" (in the →5' direction) of the dimer, and Displacement of the free end containing the thymine dimer; the reaction is catalyzed by a specialized enzyme—UV-specific endonuclease.

II. Formation of a polydeoxyribonucleotide "patch" on the DNA region containing the dimer; the reaction is catalyzed by DNA polymerase (in prokaryotes, DNA polymerase I) and involves The addition of mononucleotides to the free 3'-end of the "cut" DNA strand in the 5'→3' direction.

III. Excision of the damaged DNA region containing the thymine dimer (in prokaryotes, via the 5'→3' exonuclease activity of DNA polymerase I).

IV. Ligation ("sealing") of the 3'-end of the newly synthesized "patch" to the 5'-end of the cut main DNA strand.

Impairment of the enzymatic repair process for UV-induced DNA damage leads to a severe hereditary human disorder known as xeroderma pigmentosum (Fig. 22.15).

Fig. 22.15. Xeroderma pigmentosum — a sublethal hereditary skin disease (A. Muntzing, 1967).

Xeroderma pigmentosum is inherited as an autosomal recessive trait; in this pathology, patients' skin is extremely sensitive to the damaging effects of sunlight, which can induce skin Cancer. The most common form of xeroderma pigmentosum is caused by a hereditary defect in the synthesis of UV-specific endonuclease, disrupting the entire DNA repair mechanism.

2. Repair of cytosine deamination.

Cytosine (2-oxy-4-aminopyrimidine) deamination leads to the formation of uracil (2,4-dioxypyrimidine)—a reaction that occurs spontaneously In aqueous solutions of cytosine. As a result of this process (C to U substitution), replication in the complementary DNA strand also causes the substitution of the corresponding purine base (adenine is incorporated instead of guanine). Ultimately, the daughter double-stranded DNA will contain an A-T pair instead of the expected G-C pair, resulting in a substitution-type mutation. The repair of such a mutation involves: 1967).

а) removal of the "incorrect" base from the DNA chain through the excision of uracil by uracil-DNA glycosylase. The enzyme hydrolytically cleaves the N-glycosidic bond between the nitrogenous base (uracil) and deoxyribose, generating an abasic pentose-phosphate backbone. Other DNA glycosylases operate via a similar mechanism, creating apyrimidinic and apurinic sites in the damaged DNA molecule;

б) endonuclease Cleavage of the 3',5'-phosphodiester bond to the "left" of the depyrimidinated pentose-phosphate residue;

c) insertion of the "correct" nitrogenous base (in this case, cytosine) in place of the removed base through the action of DNA polymerase;

d) sealing of the nick in the DNA strand by DNA ligase.