why/how does isopropyl alcohol precipitate RNA?
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RNA (and DNA) have charged inorganic phosphate (-PO3 -)groups which make it highly water soluble (hydrophilic). Each water molecule acts as a tiny magnet whose positive end attracts the negative ends of the RNA molecules. The RNA is dissolved in the water so in order to get it to precipitate, you have to get the water to "let go" of the RNA.
This is where salt and alcohol come in. You add salt to neutralize the charge on the RNA, since salts are made of positively charged (cationic) metals and negatively charged (anionic) non-metals. The positively charge metals attach to the RNA and neutralize the charge to make it non-polar, and thus water-insoluble to precipitate out of the solution.
The reason alcohol is used is because RNA has a higher affinity for water than it does salt-cations, but it has a higher affinity for salt-cations than alcohol molecules. Alcohol basically guarantees the salt cations will bind to the RNA in a solution.
This is where salt and alcohol come in. You add salt to neutralize the charge on the RNA, since salts are made of positively charged (cationic) metals and negatively charged (anionic) non-metals. The positively charge metals attach to the RNA and neutralize the charge to make it non-polar, and thus water-insoluble to precipitate out of the solution.
The reason alcohol is used is because RNA has a higher affinity for water than it does salt-cations, but it has a higher affinity for salt-cations than alcohol molecules. Alcohol basically guarantees the salt cations will bind to the RNA in a solution.