Question

In a DNA double helix, why doesn't an A or T form two hydrogen bonds (out of the three possible) with G or C?

Answer 1

Answer:

Chemical structures of the five nitrogenous bases are shown below. The red N atom in each molecule is the point of attachment for a sugar molecule (ribose or deoxyribose).

Adenine and guanine are purine bases found in both DNA and RNA.

Cytosine is a pyrimidine base found in both DNA and RNA.

Thymine and uracil are pyrimidine bases found in only DNA or RNA, respectively

These five nitrogenous bases are all planar molecules, meaning that they are fairly flat and rigid.

Examine the structure of one nitrogenous base molecule by clicking on the button below (wait a few seconds for it to load in the space at right). You may click and drag the molecule to rotate it in three dimensions.

The green bond represents the point of attachment for a sugar (ribose or deoxyribose).

Nitrogenous Base

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Answer 2

The reason for an A or T not forming two hydrogen bonds (out of the three possible) with G or C is as follows -

• Adenine and Guanine are purines and Cytosine and Thymine are pyrimidines. The mentioned hydrogen bonding is not possible due to distance constraints.
• For the hydrogen bonding to occur, the distance between hydrogen acceptor and hydrogen donor should be less than 2.5 Angstroms.
• Hydrogen donors are electropositive atoms while hydrogen acceptors are electronegative atoms.
• Thus, the participating elements in the hydrogen bonds are oxygen, nitrogen, and hydrogen in DNA molecules.
• DNA molecule is an important molecule found in cells. It controls all the processes of the cell and transmits information to the new daughter cells.
• The information is decoded via transcription into mRNA which is further translated into protein, required for numerous processes in the cell.

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