A G:C base pair has three hydrogen bonds, because the exocyclic NH, at C2 on guanine lies opposite to, and can hydrogen bond with, a carbonyl at C2 on cytosine. Likewise, a hydrogen bond can form between N't of guanine and N3 of cytosine and between the carbonyl at C6 of guanine and the exocyclic NR, at C4 of cytosine. Watson-Crick base pairing requires that the bases are in their preferred tautomeric, states. Neither does T:A or C:G. In other words,.
In addition, the base pairs can stack neatly on top of each other between the two helical sugar-phosphate backbones. FIGURE A:C incompatibility, the structure shows the inability of adenine to form the proper hydrogen bonds with cytosine the base parr is therefore unstable.
The hydrogen bonds between complementary bases are a fundamental feature of the double helix, contributing to the thermodynamic stability of the helix and the specificity of base pairing.
Hydrogen bonding might not, at first glance, appear to contribute importantly to the stability of DMA for the following reason. An organic molecule in aqueous solution has all of its hydrogen bonding properties satisfied by water molecules that come on and off very rapidly. As a result, for every hydrogen bond that is made when a base pair forms, a hydrogen bond with water is broken that was there before the base pair formed. Thust the net energetic contribution of hydrogen bonds to the stability of the double helix would appear to be modest.
However, when polynucleotide strands are separate, water molecules are lined up on the bases. When strands come together in the double helix, the water molecules are displaced from the bases.
This creates disorder and increases entropy, thereby stabilizing the double helix. Hydrogen bonds are not the only force that stabilizes the double helix. A second important contribution comes from stacking interactions between the bases. The bases are flat, relatively water-insoluble molecules, and they tend to stack above each other roughly perpendicular to the direction of the helical axis.
Electron cloud interactions it— tr between bases in the helical stacks contribute significantly to the stability of the double helix. Hydrogen bonding is also important for the specificity of base pairing. Suppose we tried to pair an adenine with a cytosine. Then we would have a hydrogen bond acceptor Nl of adenine lying opposite a hydrogen bond acceptor N3 of cytosine with no room to put a water molecule in between to satisfy the two acceptors Figure , Likewise, two hydrogen bond donors, the NH; groups at C6 of adenine and C4 of cytosine, would lie opposite each other.
What is the identity of the nucleotide triphosphate displayed in the computer model? Adenosine triphosphate ATP. Deoxyadenosine triphosphate dATP. Guanosine triphosphate GTP. Deoxyguanosine triphosphate dGTP. Thymidine triphosphate TTP. Click the button below to examine the structure of deoxyadenine monosphosphate dAMP.
Notice the angle of the sugar and phosphate groups in relation to the planar nitrogenous base. In double-stranded DNA, two long molecules twist around one another in a double helix. These molecules are d eoxy n ucleic a cids DNA : polymers made up of nucleotides In a DNA double helix, the phosphate and sugar groups make up the outer 'backbones,' and the flat nitrogenous bases are pointed toward the middle of the helix.
Click the buttons below to examine a segment of a DNA double helix from many angles. The first button has colored the backbone sugar and phosphate groups purple to simplify the image. One key point to notice in the DNA double helix structure is that the planar nitrogenous bases from the two strands are pointing toward each other, in the middle of the helix. Pairs of nitrogenous bases are set in the same plane, and interact with each other via hydrogen bonding.
These pairs are often referred to as base pairs , abbreviated 'bp. Recall that electronegativity values generally increase toward the top and right of the periodic table, as illustrated in the image below. 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.
Therefore a purine has to make a hydrogen bond with another pyrimidine. Hence adenine makes hydrogen bonds with thymine and guanine makes hydrogen bonds with cytosine. The arrangements of atoms in the four kinds of nitrogenous bases is such that two hydrogen bonds are formed automatically when A and T are present on opposite DNA strands, and three are formed when G and C come together this way.
A-C or G-T pairs would not be able to form similar sets of hydro- gen bonds. Can adenine pair with itself? You see, cytosine can form three hydrogen bonds with guanine, and adenine can form two hydrogen bonds with thymine.
Or, more simply, C bonds with G and A bonds with T. It's called complementary base pairing because each base can only bond with a specific base partner. Why does thymine pair with adenine? Adenine and Thymine also have a favorable configuration for their bonds.
When one pairs Adenine with Cytosine, the various groups are in each others way. For them to bond with each other would be chemically unfavorable. What will pair with adenine? In DNA base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.
What is complementary base pairing? Complementary base pairing is the phenomenon where in DNA guanine always hydrogen bonds to cytosine and adenine always binds to thymine.
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