Hydrogen Bonds - Hydrogen bonds are weak bonds between polar molecules. An especially important hydrogen bond occurs between the oxygen of one water molecule and the hydrogen of another.
Ionic and covalent bonds form between the atoms of a molecule. Hydrogen bonds, however, form between molecules, not between atoms, and are made possible because of the polarity of certain molecules. To illustrate hydrogen bonds, we will look at the water molecule again, but remember that other polar molecules will act in a similar manner.
How does polarity affect the way a water molecule reacts with other molecules? The hydrogen atoms at each side have a slightly positive charge, so they will form attractions with atoms in other molecules having negative charges, such as oxygen, nitrogen, or fluorine. The oxygen atom in the center has a slightly negative charge, so it will form attractions with atoms or molecules having positive charges. This weak electrostatic attraction is called the hydrogen bond. For example, in water a hydrogen atom in one molecule (the relatively positive end) is attracted to the oxygen atom (the relatively negative portion) in another molecule, forming a hydrogen bond.
Hydrogen bonds are very important in biology, in part because they are so easy to break. For example, hydrogen bonds are responsible for holding certain portions of the DNA molecule together, and their weak hold allows the molecule to separate very easily in order to replicate itself. Hydrogen bonds also play a role in the activities of enzymes in chemical reactions. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 45-46)
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