Re: Are there ionic bonds within DNA?

Hi Rhett,

In general, almost all of the bonds that hold together a single strand of DNA are covalent bonds. As you know, a DNA molecule has three principal components; there are the nitrogenous bases, the deoxyribose sugars, and the phosphate groups. These three components are connected to one another by covalent bonds, and the atoms that comprise each component are connected to one another by covalent bonds. So I am afraid that your idea that there are ionic bonds between the actual atoms is not correct.

Now, you also know that the DNA molecule is usually double stranded, and that the two strands are held togehter by hydrogen bonds. What you might not also know is that there are hydrophobic stacking forces between the bases in a given strand of DNA that give some structure to the individual DNA strand and stabilize the double helical structure. The nitrogenous bases tend to have saturated ring structures, like the saturated aromatic ring of benzene, and these rings are somewhat hydrophobic -- they tend to "stick together" and exclude water. So there are hydrophobic bonds that stabilize the DNA molecule along with the hydrogen bonds. Since these hydrophobic bonds permit some stable structures to form in single stranded DNA molecules, I can't say that all of the bonds that hold together a single strand of DNA are covalent.

An ionic bond is a bond that is formed between ions of opposite charge. You might not know that the entire DNA molecule is negatively charged (this is why it is called Deoxyribo Nucleic Acid). If you look at the phosphate groups in the DNA molecule in your textbook, there should be a negative charge associated with each group. These negative charges allow the DNA molecule to be "salted out" during DNA purification by using positively charged ions (divalent cations) such as Mg2+, Mn2+, Ca2+, etc. So, the DNA molecule can form ionic bonds with positively charged molecules and positvely charged ions, but not with other DNA molecules.

You should be able to find descriptions of the various types of bonding forces at work in a DNA molecule in the DNA Structure chapter of a college-level Biochemistry textbook. I'm not sure what textbook you are using in your High School biology classes, so I can't say what you will find in there. However, you should be able to find good descriptions of the differences between covalent and ionic bonds in your High School Chemistry textbook. For a good college-level Biochemistry text, I recommend Biochemistry by Lubert Stryer.

Keep asking questions!