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DNA is charged negative because of its phosphate backbone. Since charges need to be balanced (so that there are no charges building up somewhere), what is the positive charge which neutralizes this negative charge?

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It would be great if you state the context of the question to allow a more constructive response. Many ions such as Na+ and Mg2+ are positively charged. –  Bez Jul 31 at 20:29
    
There is no noteworthy context. I'm a physicist, just saw the structure formula of DNA and was wondering about this. –  Daniel R Jul 31 at 20:42
    
@Bez The question is short and it is ok to ask it out of curiosity. –  Chris Jul 31 at 20:59
    
Another point to be noted is that isoelectric pH of DNA is close to 7 (6.7) [Ref]. –  WYSIWYG Aug 1 at 6:21

3 Answers 3

up vote 8 down vote accepted

DNA in the body is not available as a free molecule, it is organized around DNA binding proteins, mostly the histone octamers. These proteins carry positive charges (mostly from lysine side chains) which interact with the phosphate backbone of the DNA. It is like in the figure below, the charges play an important role in the tight packaging. See here for more details.

enter image description here

The charge (or the temporary neutralization at some points of the DNA) is also important for the regulation of the DNA. Acetylation neutralizes the histone tail and loosens the DNA-protein structure (see here for more information).

Positively charged ions don't play a role in neutralizing the DNA charge in living cells (this is different for precipitations), as both are solubilized and split into ions. The concentration of positively charged ions like sodium and potassium are tightly controlled by the cells, as they are important for the function of transporters (for example the sodium-glucose transporters and others) or for keeping up membrane potentials. Moving the ions, which have to neutralize the DNA charge around would lead to a polarized nucleus.

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Your cells have a high concentration of sodium ions that are positive. Other important cations are potassium and calcium. Additionally, many amino acids are positively charged at physiological pH. DNA is not the only source of negative charge in your body, other amino acids are negative, and most cell surfaces are negative as well. The interactions between charged species play an important role in many biochemical processes, but at the end of the day, and for the whole organism, the charges are all pretty much balanced out.

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Could you please provide a reference regarding the statement "the charges are all pretty much balanced out"? I could be wrong but the resting membrane potential for example in neurone is -70mV so that doesn't sound like balanced! –  Bez Jul 31 at 20:35
    
That's charge of inside vs outside of cell. If your whole body had a significant net charge, you'd be constantly affected by static cling, or sparking static electricity all the time, or electrostatically repelled from other living things. Based on my observations, these things don't happen. –  user137 Jul 31 at 20:47
    
I agree! but at a single cell level cells can have a charge so perhaps it would be best if you edited your response slightly to clarify the last sentence. –  Bez Jul 31 at 21:25
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Made an edit to clarify the statement is about the whole animal. –  user137 Jul 31 at 22:01

The negative charge in DNA is localized to individual oxygen atoms in DNA. This negative charge is balanced by positive ions in the body.

In the case of a magnet or molecules with a dipole, negative and positive magnetic field lines align to produce an overall magnetic field. DNA has many small dipole moments but none across the entire molecule.

In the case of a neuron, an excess of negative charge inside the cell and positive charge outside the cell is maintained by the diffusion gradient of potassium and sodium across a membrane. An overall electric potential is not created with DNA.

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