Name: chargaff’s rules and dna pairing
Uploaded: 2020-09-24T15:06:53.000Z
Duration: 2 min 27 s
Channel: JoVE

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from-molecules-to-organisms-structures-and-processes

From Molecules To Organisms: Structures And Processes

HS-LS1-1

DNA Base Pairing

L'ADN ressemble à une échelle tordue. Et les échelons de l'échelle d'ADN sont des paires complémentaires de bases azotées. Selon les règles d'appariement des bases, l'adénine, une purine, se couple avec la thymine, une pyrimidine, avec deux liaisons hydrogène.Et la guanine, une purine, apparaît avec la cytosine, une pyrimidine, avec trois liaisons hydrogène. Mais pourquoi les purines sont-elles toujours associées aux pyrimidines? En raison de contraintes stériques imposées par des restrictions spatiales par le squelette sucre-phosphate de l'ADN, seul un espace de 10, 85 angströms est disponible pour la base paires dans une double hélice d'ADN.Les purines ont une structure à double anneau. Par conséquent, deux purines ensemble seront trop grand pour tenir dans cet espace. D'un autre côté, si nous mettons deux pyrimidines ensemble, qui ne contiennent qu'un seul anneau, la distance entre eux sera trop grand pour former des liaisons hydrogène, qui mesurent environ deux angströms.Cependant, si nous associons une purine et une pyrimidine ensemble, ils s'intègrent parfaitement dans l'hélice d'ADN et sont suffisamment proches pour former des liaisons hydrogène. Les liaisons hydrogène peuvent se former lorsqu'un atome d'hydrogène est approximativement à deux angströms d'un atome électronégatif, comme l'oxygène ou l'azote. Adénine a un atome d'hydrogène proche d'un oxygène et de la thymine.Et la thymine a un hydrogène proche à un azote et à l'adénine. Cela conduit à la formation de deux liaisons hydrogène. L'adénine ne peut pas former de liaisons hydrogène avec la cytosine, parce que la cytosine a un atome d'hydrogène où l'oxygène et la thymine le serait.Et l'atome d'hydrogène présent dans la thymine est absent de la cytosine. Un phénomène similaire se produit dans la base de paire cytosine guanine où un oxygène dans la guanine, et un oxygène et un azote dans la cytosine sont chacun positionnés en face d'un hydrogène, conduisant à la formation de trois liaisons hydrogène, qui ne se produit pas dans l'appariement de base guanine thymine. La grande spécificité de l'appariement de bases, avec l'aide d'enzymes de réplication de l'ADN, c'est pourquoi l'adénine s'associe toujours à la thymine et la guanine s'apparie toujours à la cytosine.

appariement des bases de l'adn

Les règles d'Erwin Chargaff sur l'équivalence de l'ADN ont ouvert la voie à la découverte de l'appariement des bases dans l'ADN. Les règles de Chargaff stipulent que dans une molécule d'ADN double brin, <ol> <li>la quantité d'adénine (A) est égale à la quantité de thymine (T) ;</li> <li>la quantité de guanine (G) est égale à la quantité de cytosine (C) ; et</li> <li>la somme des purines, A et G, est égale à la somme des pyrimidines, C et T (c'est-à-dire A+G = C+T).</li> </ol> 
 Des travaux ultérieurs de Watson et Crick ont révélé que dans l'ADN double brin, A forme toujours deux liaisons hydrogène avec T et G forme toujours trois liaisons hydrogène avec C.  Cet appariement de bases maintient une largeur constante de la double hélice d'ADN, car les paires A-T et C-G sont de 10,85 & 197; de longueur et s'insère parfaitement entre les deux squelettes sucre-phosphate.
 Les appariements de bases rendent les bases azotées inaccessibles aux autres molécules jusqu'à ce que les liaisons hydrogène se séparent. Cependant, des enzymes spécifiques peuvent facilement rompre ces liaisons hydrogène pour effectuer les processus cellulaires nécessaires, tels que la réplication et la transcription de l'ADN. Comme une paire G-C a plus de liaisons hydrogène qu'une  paire A-T, l'ADN avec un pourcentage élevé de paires G-C aura besoin d'une énergie plus élevée pour la séparation de deux brins d'ADN qu'un avec un pourcentage similaire de paires A-T.  
 Les analogues de base comme médicament
 Un appariement correct des bases est essentiel pour la réplication fidèle de l'ADN. Les analogues de bases sont des molécules qui peuvent remplacer les bases d'ADN standard lors de la réplication de l'ADN. Ces analogues sont des agents antiviraux et anticancéreux efficaces contre des maladies telles que l'hépatite, l'herpès et la leucémie. L'acyclovir, également connu sous le nom d'acycloguanosine, est une base analogue de la guanine et est couramment utilisé dans le traitement du virus de l'herpès simplex.  La partie guanine de l'acyclovir s'apparie avec l'adénine comme d'habitude lors de la réplication de l'ADN ; cependant, comme il n'a pas d’extrémité 3’ du nucléotide, l'ADN polymérase ne peut pas continuer à former des paires de bases et la réplication se termine.

Appariement des bases de l'ADN

Les règles d’Erwin Chargaff sur l’équivalence de l’ADN ont ouvert la voie à la découverte de l’appariement des bases dans l’ADN. Les règles de Chargaff stipulent que dans un ...

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chargaff’s rules and dna pairing

Chargaff’s rules and DNA base pairing explain how DNA strands match up. In double-stranded DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Chargaff also found that A equals T, G equals C, and the total amount of purines (A and G) equals the total amount of pyrimidines (C and T).

Watson and Crick later showed why these matches matter. A forms two hydrogen bonds with T, while G forms three hydrogen bonds with C. These pairings keep the DNA double helix a constant width because both A-T and C-G pairs fit neatly between the two sugar-phosphate backbones.

Base pairing also keeps the nitrogenous bases hidden from other molecules until the hydrogen bonds separate. Specific enzymes can break these bonds when the cell needs to copy DNA or make RNA during transcription. DNA with more G-C pairs usually needs more energy to separate than DNA with more A-T pairs, because G-C pairs have more hydrogen bonds.

Correct pairing is also important in medicine. Base analogs are molecules that can replace normal DNA bases during replication, and they can act as antiviral or anticancer drugs against diseases such as hepatitis, herpes, and leukemia. Acyclovir, also called acycloguanosine, is a guanine analog used to treat herpes simplex virus. It pairs with adenine, but it lacks a 3' end, so DNA polymerase cannot keep building the strand and replication stops.

Chargaff’s Rules and DNA Pairing

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dna base pairing

Chargaff’s rules and DNA base pairing explain how DNA strands match up. In double-stranded DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Chargaff also found that A equals T, G equals C, and the total amount of purines (A and G) equals the total amount of pyrimidines (C and T). Watson and Crick later showed why these matches matter. A forms two hydrogen bonds with T, while G forms three hydrogen bonds with C. These pairings keep the DNA double helix a...

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Learn how Chargaff’s rules, hydrogen bonding, and base analogs like acyclovir help explain DNA pairing, replication, and antiviral treatment.

Chargaff’s Rules and DNA Pairing | JoVE K12

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tRNA Activation

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HS-LS1-4

Chargaff’s Rules and DNA Pairing

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