A base pair is two chemical bases bonded to one another forming a "rung of the DNA ladder. Each strand has a backbone made of alternating sugar deoxyribose and phosphate groups. Attached to each sugar is one of four bases--adenine A , cytosine C , guanine G , or thymine T. The two strands are held together by hydrogen bonds between the bases, with adenine forming a base pair with thymine, and cytosine forming a base pair with guanine.
Oxygen and nitrogen are electronegative atoms found in nitrogenous bases. They are represented in models by the color conventions: red for oxygen , and blue for nitrogen. Electronegative O and N atoms with free lone pairs are potential hydrogen bond acceptors.
Hydrogen atoms attached to very electronegative atoms like O and N have strong partial positive charge and are potential hydrogen bond donors. The dotted line in the image below represents the non-covalent attractive force between a hydrogen bond donor H atom with little 'ownership' of its valence electrons and a hydrogen bond acceptor electronegative atom with at least one lone pair of electrons. Many of the oxygen, nitrogen, and hydrogen atoms in the nitrogenous bases are very effective hydrogen bond donors and acceptors, as illustrated in the image below.
Remember: Hydrogen bond donors are only those H atoms bound to an electronegative atom such as N or O. Hydrogen bond acceptors are electronegative atoms with at least one lone pair of electrons.
Also notice that potential hydrogen bond donors and acceptors close to the sugar R group are ignored in the image above. This is because those parts of the nitrogenous base close to the sugar-phosphate backbone will be unavailable for hydrogen bonding with the other base in the pair. Let's examine a single guanine residue to identify potential hydrogen bond donors and acceptors. Guanine will be highlighted in yellow , and the attached sugar and phosphate in the backbone will blink purple.
Keeping in mind the point of sugar attachment, we can identify guanine's hydrogen bond donors and acceptors that are available to interact with a paired nitrogenous base.
Locate these parts of the molecule yourself, then click the button below to see the relevant atoms blink yellow. Which of the following statements best describes the hydrogen bonding potential in guanine? Guanine has 3 H-bond donors. However, if we pair a purine and a pyrimidine together, they fit perfectly inside the DNA helix and are close enough to form hydrogen bonds.
Hydrogen bonds can form when a hydrogen atom is approximately two angstroms away from an electronegative atom, such as oxygen or nitrogen. Adenine has one hydrogen atom close to an oxygen and thymine.
And thymine has one hydrogen close to a nitrogen and adenine. This leads to the formation of two hydrogen bonds. Adenine cannot form hydrogen bonds with cytosine, because cytosine has a hydrogen atom where the oxygen and thymine would be. And the hydrogen atom that is present in thymine is absent in cytosine. A similar phenomenon occurs in the guanine cytosine base pair where an oxygen in guanine, and an oxygen and a nitrogen in cytosine are each positioned across from a hydrogen, leading to the formation of three hydrogen bonds, which does not happen in guanine thymine base pairing.
The high specificity of base pairing, along with the help of DNA replication enzymes, is why adenine always pairs with thymine and guanine always pairs with cytosine. Base pairings cause the nitrogenous bases to be inaccessible to other molecules until the hydrogen bonds separate. However, specific enzymes can easily break these hydrogen bonds to carry out necessary cell processes, such as DNA replication and transcription. Correct base pairing is essential for the faithful replication of DNA.
These analogs are effective antiviral and anticancer agents against diseases such as hepatitis, herpes, and leukemia. Acyclovir, also known as Acycloguanosine, is a base analog of guanine and is commonly used in the treatment of the herpes simplex virus. To learn more about our GDPR policies click here. If you want more info regarding data storage, please contact gdpr jove. Your access has now expired. Provide feedback to your librarian. Bioinformatics, Big Data, and Cancer.
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