Thymine and adenine form a base pair in the DNA molecule. These two bases can form a connection between two strands of DNA via two hydrogen bonds. Which of the following diagrams shows the correct representation of the hydrogen bonding (denoted by dashed lines) between thymine and adenine base pairs? (In each diagram, thymine is shown at the left and adenine is shown at the right. The bases are attached to the backbone portion of the DNA strands.)

Hydrogen bonding occurs between a hydrogen atom that is covalently bonded to a more electronegative atom, and another electronegative atom. Hydrogen and carbon have similar electronegativity, so this type of hydrogen typically does not participate in hydrogen bonding.

In the last option, the hydrogen involved in hydrogen bonding are covalently bonded to nitrogen atoms. Since nitrogen is electronegative, these hydrogen atoms will have a partial positive charge due to unequal sharing of electrons. They will be attracted to electronegative atoms such as nitrogen and oxygen, thus forming a hydrogen bond.