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44-412: The term homodimer is used when the two subunits are identical (e.g. A–A) and heterodimer when they are not (e.g. A–B). The reverse of dimerization is often called dissociation . When two oppositely-charged ions associate into dimers, they are referred to as Bjerrum pairs , after Danish chemist Niels Bjerrum . Anhydrous carboxylic acids form dimers by hydrogen bonding of the acidic hydrogen and

88-401: A covalent bond between an electronegative atom and a hydrogen atom is broken by heterolytic fission , which gives a proton (H ) and a negative ion . Dissociation is the opposite of association or recombination . For reversible dissociations in a chemical equilibrium the dissociation constant K d is the ratio of dissociated to undissociated compound where the brackets denote

132-534: A cyclobutane ring when joined together and cause a distortion in the DNA. This distortion prevents DNA replication and transcription mechanisms beyond the dimerization site. While up to 100 such reactions per second may transpire in a skin cell exposed to sunlight resulting in DNA damage , they are typically rectified promptly through DNA repair , such as through photolyase reactivation or nucleotide excision repair , with

176-435: A solution , such as water , means the separation of the anions and cations . The salt can be recovered by evaporation of the solvent. An electrolyte refers to a substance that contains free ions and can be used as an electrically conductive medium. Most of the solute does not dissociate in a weak electrolyte, whereas in a strong electrolyte a higher ratio of solute dissociates to form free ions. A weak electrolyte

220-479: A 6–4 photoproduct under further light exposure. Mutagenesis, the process of mutation formation, is significantly influenced by translesion polymerases which often introduce mutations at sites of pyrimidine dimers. This occurrence is noted both in prokaryotes , through the SOS response to mutagenesis, and in eukaryotes . Despite thymine-thymine CPDs being the most common lesions induced by UV, translesion polymerases show

264-478: A more explicit description is provided by the Brønsted–Lowry acid–base theory , which specifies that the proton H+ does not exist as such in solution but is instead accepted by (bonded to) a water molecule to form the hydronium ion H 3 O . The reaction can therefore be written as and better described as an ionization or formation of ions (for the case when HA has no net charge). The equilibrium constant

308-522: A primary factor in melanoma development in humans. Pyrimidine dimers encompass several types, each with distinct structures and implications for DNA integrity. Cyclobutane pyrimidine dimer (CPD) is a dimer which features a four-membered ring formed by the fusion of two double-bonded carbons from adjacent pyrimidines. CPDs disrupt the formation of the base pair during DNA replication , potentially leading to mutations . The 6–4 photoproduct (6–4 pyrimidine– pyrimidone , or 6–4 pyrimidine–pyrimidinone)

352-441: A result of UV damage. Individuals with XP are also at a much higher risk of cancer than others, with a greater than 5,000 fold increased risk of developing skin cancers. Some common features and symptoms of XP include skin discoloration, and the formation of multiple tumors proceeding UV exposure. A few organisms have other ways to perform repairs: Another type of repair mechanism that is conserved in humans and other non-mammals

396-552: A tendency to incorporate adenines , resulting in the accurate replication of thymine dimers more often than not. Conversely, cytosines that are part of CPDs are susceptible to deamination , leading to a cytosine to thymine transition, thereby contributing to the mutation process. Pyrimidine dimers introduce local conformational changes in the DNA structure , which allow recognition of the lesion by repair enzymes. In most organisms (excluding placental mammals such as humans) they can be repaired by photoreactivation. Photoreactivation

440-414: A weak electrolyte. Weak bases and weak acids are generally weak electrolytes. In an aqueous solution there will be some CH 3 COOH and some CH 3 COO and H . A strong electrolyte is a solute that exists in solution completely or nearly completely as ions. Again, the strength of an electrolyte is defined as the percentage of solute that is ions, rather than molecules. The higher the percentage,

484-600: Is (1 – α ) + 2 α , which is equivalent to 1 + α . Thus, substituting the mole fractions with actual values in term of α and simplifying; K p = p T ( 4 α 2 ) ( 1 + α ) ( 1 − α ) = p T ( 4 α 2 ) 1 − α 2 {\displaystyle K_{p}={\frac {p_{T}(4\alpha ^{2})}{(1+\alpha )(1-\alpha )}}={\frac {p_{T}(4\alpha ^{2})}{1-\alpha ^{2}}}} This equation

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528-399: Is sucrose , a dimer of fructose and glucose, which follows the same reaction equation as presented above. Amino acids can also form dimers, which are called dipeptides . An example is glycylglycine , consisting of two glycine molecules joined by a peptide bond . Other examples include aspartame and carnosine . Many molecules and ions are described as dimers, even when the monomer

572-476: Is translesion synthesis. Typically, the lesion associated with the pyrimidine dimer blocks cellular machinery from synthesizing past the damaged site. However, in translesion synthesis, the CPD is bypassed by translesion polymerases, and replication and or transcription machinery can continue past the lesion. One specific translesion DNA polymerase, DNA polymerase η, is deficient in individuals with XPD. Direct DNA damage

616-428: Is a repair process in which photolyase enzymes reverse CPDs using photochemical reactions. In addition, some photolyases can also repair 6-4 photoproducts of UV induced DNA damage. Photolyase enzymes utilize flavin adenine dinucleotide (FAD) as a cofactor in the repair process. The UV dose that reduces a population of wild-type yeast cells to 37% survival is equivalent (assuming a Poisson distribution of hits) to

660-484: Is a simple relationship between this parameter and the van 't Hoff factor i {\displaystyle i} . If the solute substance dissociates into n {\displaystyle n} ions, then For instance, for the following dissociation As n = 2 {\displaystyle n=2} , we would have that i = 1 + α {\displaystyle i=1+\alpha } . The dissociation of salts by solvation in

704-448: Is a substance whose solute exists in solution mostly in the form of molecules (which are said to be "undissociated"), with only a small fraction in the form of ions. Simply because a substance does not readily dissolve does not make it a weak electrolyte. Acetic acid ( CH 3 COOH ) and ammonium ( NH + 4 ) are good examples. Acetic acid is extremely soluble in water, but most of the compound dissolves into molecules, rendering it

748-415: Is an alternate dimer configuration consisting of a single covalent bond linking the carbon at the 6 (C6) position of one pyrimidine ring and carbon at the 4 (C4) position of the adjoining base's ring. This type of conversion occurs at one third the frequency of CPDs and has a higher mutagenic risk. A third type of molecular lesion is a Dewar pyrimidinone, resulting from the reversible isomerization of

792-487: Is elusive. Diborane (B 2 H 6 ) is an dimer of borane , which is elusive and rarely observed. Almost all compounds of the type R2BH exist as dimers. Trialkylaluminium compounds can exist as either monomers or dimers, depending on the steric bulk of the groups attached. For example, trimethylaluminium exists as a dimer, but trimesitylaluminium adopts a monomeric structure. Cyclopentadienylchromium tricarbonyl dimer exists in measureable equilibrium quantities with

836-423: Is in accordance with Le Chatelier's principle . K p will remain constant with temperature. The addition of pressure to the system will increase the value of p T , so α must decrease to keep K p constant. In fact, increasing the pressure of the equilibrium favours a shift to the left favouring the formation of dinitrogen tetroxide (as on this side of the equilibrium there is less pressure since pressure

880-411: Is proportional to number of moles) hence decreasing the extent of dissociation α . The reaction of an acid in water solvent is often described as a dissociation where HA is a proton acid such as acetic acid, CH 3 COOH. The double arrow means that this is an equilibrium process, with dissociation and recombination occurring at the same time. This implies that the acid dissociation constant However

924-444: Is reduced by sunscreen, which also reduces the risk of developing a sunburn. When the sunscreen is at the surface of the skin, it filters the UV rays, which attenuates the intensity. Even when the sunscreen molecules have penetrated into the skin, they protect against direct DNA damage, because the UV light is absorbed by the sunscreen and not by the DNA. Sunscreen primarily works by absorbing

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968-451: Is repaired. Protein dimers arise from the interaction between two proteins which can interact further to form larger and more complex oligomers . For example, tubulin is formed by the dimerization of α-tubulin and β-tubulin and this dimer can then polymerize further to make microtubules . For symmetric proteins, the larger protein complex can be broken down into smaller identical protein subunits , which then dimerize to decrease

1012-414: Is then where [ H 2 O ] {\displaystyle {\ce {[H_2O]}}} is not included because in dilute solution the solvent is essentially a pure liquid with a thermodynamic activity of one. K a is variously named a dissociation constant , an acid ionization constant , an acidity constant or an ionization constant . It serves as an indicator of

1056-405: The cytoplasmic kinase domains that are responsible for further signal transduction . Dissociation (chemistry) Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts , or complexes ) separate or split into other things such as atoms, ions, or radicals , usually in a reversible manner. For instance, when an acid dissolves in water,

1100-429: The halogens fluorine , chlorine , bromine and iodine . Some metals form a proportion of dimers in their vapour phase: dilithium ( Li 2 ), disodium ( Na 2 ), dipotassium ( K 2 ), dirubidium ( Rb 2 ) and dicaesium ( Cs 2 ). Such elemental dimers are homonuclear diatomic molecules . In the context of polymers , "dimer" also refers to the degree of polymerization 2, regardless of

1144-487: The nucleotide chain near their carbon–carbon double bonds, the photo-coupled dimers are fluorescent . Such dimerization , which can also occur in double-stranded RNA (dsRNA) involving uracil or cytosine , leads to the creation of cyclobutane pyrimidine dimers (CPDs) and 6–4 photoproducts . These pre- mutagenic lesions modify the DNA helix structure, resulting in abnormal non-canonical base pairing and, consequently, adjacent thymines or cytosines in DNA will form

1188-810: The partial pressure . Hence, through the definition of partial pressure and using p T to represent the total pressure and x to represent the mole fraction ; K p = p T 2 ( x NO 2 ) 2 p T ⋅ x N 2 O 4 = p T ( x NO 2 ) 2 x N 2 O 4 {\displaystyle K_{p}={\frac {p_{T}^{2}{\bigl (}x\,{\ce {NO2}}{\bigr )}^{2}}{p_{T}\cdot x\,{\ce {N2O4}}}}={\frac {p_{T}{\bigl (}x\,{\ce {NO2}}{\bigr )}^{2}}{x\,{\ce {N2O4}}}}} The total number of moles at equilibrium

1232-470: The UV dose that causes an average of one lethal hit to each of the cells of the population. The number of pyrimidine dimers induced per haploid genome at this dose was measured as 27,000. A mutant yeast strain defective in the three pathways by which pyrimidine dimers were known to be repaired in yeast was also tested for UV sensitivity. It was found in this case that only one or, at most, two unrepaired pyrimidine dimers per haploid genome are lethal to

1276-410: The UV light from the sun through the use of organic compounds, such as oxybenzone or avobenzone. These compounds are able to absorb UV energy from the sun and transition into higher-energy states. Eventually, these molecules return to lower energy states, and in doing so, the initial energy from the UV light can be transformed into heat. This process of absorption works to reduce the risk of DNA damage and

1320-402: The acid strength: stronger acids have a higher K a value (and a lower p K a value). Fragmentation of a molecule can take place by a process of heterolysis or homolysis . Receptors are proteins that bind small ligands . The dissociation constant K d is used as indicator of the affinity of the ligand to the receptor. The higher the affinity of the ligand for the receptor

1364-493: The carbonyl oxygen. For example, acetic acid forms a dimer in the gas phase, where the monomer units are held together by hydrogen bonds . Many OH-containing molecules form dimers, e.g. the water dimer . Excimers and exciplexes are excited structures with a short lifetime. For example, noble gases do not form stable dimers, but they do form the excimers Ar 2 *, Kr 2 * and Xe 2 * under high pressure and electrical stimulation. Molecular dimers are often formed by

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1408-404: The cell. These findings thus indicate that the repair of thymine dimers in wild-type yeast is highly efficient. Nucleotide excision repair , sometimes termed "dark reactivation", is a more general mechanism for repair of lesions and is the most common form of DNA repair for pyrimidine dimers in humans. This process works by using cellular machinery to locate the dimerized nucleotides and excise

1452-571: The equilibrium concentrations of the species. The dissociation degree α {\displaystyle \alpha } is the fraction of original solute molecules that have dissociated. It is usually indicated by the Greek symbol α. More accurately, degree of dissociation refers to the amount of solute dissociated into ions or radicals per mole. In case of very strong acids and bases, degree of dissociation will be close to 1. Less powerful acids and bases will have lesser degree of dissociation. There

1496-518: The genetic code required to make the functional protein. As the largest and most diverse family of receptors within the human genome, G protein-coupled receptors (GPCR) have been studied extensively, with recent studies supporting their ability to form dimers. GPCR dimers include both homodimers and heterodimers formed from related members of the GPCR family. While not all, some GPCRs require dimerization to function, such as GABA B -receptor, emphasizing

1540-418: The importance of dimers in biological systems. Much like for G protein-coupled receptors, dimerization is essential for receptor tyrosine kinases (RTK) to perform their function in signal transduction , affecting many different cellular processes. RTKs typically exist as monomers, but undergo a conformational change upon ligand binding, allowing them to dimerize with nearby RTKs. The dimerization activates

1584-547: The initial concentration of dinitrogen tetroxide is 1  mole per litre , this will decrease by α at equilibrium giving, by stoichiometry, α moles of NO 2 . The equilibrium constant (in terms of pressure) is given by the equation K p = p ( NO 2 ) 2 p N 2 O 4 {\displaystyle K_{p}={\frac {p{\bigl (}{\ce {NO2}}{\bigr )}^{2}}{p\,{\ce {N2O4}}}}} where p represents

1628-600: The latter being prevalent in humans. Conversely, certain bacteria utilize photolyase, powered by sunlight, to repair pyrimidine dimer-induced DNA damage. Unrepaired lesions may lead to erroneous nucleotide incorporation by polymerase machinery. Overwhelming DNA damage can precipitate mutations within an organism's genome , potentially culminating in cancer cell formation. Unrectified lesions may also interfere with polymerase function, induce transcription or replication errors , or halt replication. Notably, pyrimidine dimers contribute to sunburn and melanin production, and are

1672-442: The lesion. Once the CPD is removed, there is a gap in the DNA strand that must be filled. DNA machinery uses the undamaged complementary strand to synthesize nucleotides off of and consequently fill in the gap on the previously damaged strand. Xeroderma pigmentosum (XP) is a rare genetic disease in humans in which genes that encode for NER proteins are mutated and result in decreased ability to combat pyrimidine dimers that form as

1716-452: The lower the K d value (and the higher the p K d value). Pyrimidine dimers Pyrimidine dimers represent molecular lesions originating from thymine or cytosine bases within DNA , resulting from photochemical reactions . These lesions, commonly linked to direct DNA damage , are induced by ultraviolet light (UV), particularly UVC , result in the formation of covalent bonds between adjacent nitrogenous bases along

1760-414: The monometallic radical (C 5 H 5 )Cr(CO) 3 . Pyrimidine dimers (also known as thymine dimers) are formed by a photochemical reaction from pyrimidine DNA bases when exposed to ultraviolet light. This cross-linking causes DNA mutations , which can be carcinogenic , causing skin cancers . When pyrimidine dimers are present, they can block polymerases , decreasing DNA functionality until it

1804-536: The percentage of gas molecules which dissociate. Various relationships between K p and α exist depending on the stoichiometry of the equation. The example of dinitrogen tetroxide ( N 2 O 4 ) dissociating to nitrogen dioxide ( NO 2 ) will be taken. N 2 O 4 ↽ − − ⇀ 2 NO 2 {\displaystyle {\ce {N2O4 <=> 2NO2}}} If

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1848-467: The reaction of two identical compounds e.g.: 2A → A−A . In this example, monomer "A" is said to dimerize to give the dimer " A−A ". Dicyclopentadiene is an asymmetrical dimer of two cyclopentadiene molecules that have reacted in a Diels-Alder reaction to give the product. Upon heating, it "cracks" (undergoes a retro-Diels-Alder reaction) to give identical monomers: Many nonmetallic elements occur as dimers: hydrogen , nitrogen , oxygen , and

1892-407: The stoichiometry or condensation reactions . One case where this is applicable is with disaccharides . For example, cellobiose is a dimer of glucose , even though the formation reaction produces water : Here, the resulting dimer has a stoichiometry different from the initial pair of monomers. Disaccharides need not be composed of the same monosaccharides to be considered dimers. An example

1936-431: The stronger the electrolyte. Thus, even if a substance is not very soluble, but does dissociate completely into ions, the substance is defined as a strong electrolyte. Similar logic applies to a weak electrolyte. Strong acids and bases are good examples, such as HCl and H 2 SO 4 . These will all exist as ions in an aqueous medium. The degree of dissociation in gases is denoted by the symbol α , where α refers to

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