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Name: Jordan
Status: Student
Grade:  9-12
Location: IN
Country: United States
Date: Fall 2010


Question:
My teacher's biology book says that nucleic acid isn't actually acidic. Why is it called an acid?



Replies:
DNA is acidic because it contains phosphoric acid, H3P04.

Ron Baker, Ph.D.


Jordan,

A little history might help explain this. The term 'nucleic acid' was coined at time when the structure of biological molecules was unknown -- proteins were known to exist, but were very mysterious, and real knowledge of the nature of the chemicals of heredity was still 50 years away.

The term nucleic acid came to popularity in the late 19th century. In the late 1890's, scientists were able to recover a material from various biological sources (such as salmon sperm, yeast, plants) that was not protein or carbohydrate or lipid, but they weren't sure of its structure. Experiments with bacteria showed that this material plays a role in heredity. Furthermore, this material could be broken down ninto known bases such as purine or thymine. So they assumed it was an acid that combined with those bases. Originally it was called nuclein, but later, in 1899, a German chemist named Altmann coined the term "nucleic acid", and the name stuck. It was called an 'acid' because it combines with a base, and it was weakly acidic on its own, and although it was insoluble natively, it could be solubilized in water by forming a metal salt (such as with copper). It should be noted that this was around the same time as modern definitions of acids (e.g. Arrhenius, Bronsted, and Lewis) were being developed. As other science-question sites have noted, there are acidic (e.g. 7 pKa) groups on nucleic acids, but that's not the whole story.

Much later in the 20th century (and still ongoing today), scientists elucidated the structure, chemistry, and function of nucleic acids, such as DNA, RNA, and synthetic nucleic acids like PNA (you can read more about all of these)

Can you send me the passage from the biology book? I'm curious how they phrase it. It's possible the book is correct, and possible it's mistaken, depending on how it's phrased. There are many definitions of acids, and in this case the issue is complicated by the fact that the term 'nucleic acid' was coined around 1899 before definitions were well established.

Thanks,
Burr


Acids taste sour. We usually call our as acidic taste.

Based on the dissociation property of hydrogen, acids can be grouped into strong acids (like sulfuric acid, hydrochloric acid, nitric acid etc) and weak acids (generally organic acids & others like phosphoric acids). Some of the weak acids have a weak sour taste.

Nucleic acids are composed of along chain of monomers called as nucleotides. Each nucleotide is made up of a nitrogenous base (either adenine or guanine or cytosine or thymine or uracil) a five carbon sugar and a phosphate group (PO4). These three compounds are linearly arranged:

NITROGENOUS BASE-SUGAR-PHOSPHORIC ACID

Note that a single nucleotide has a base at one end and an acid at another end – hence neutral!

But when these single units combine to form polymers (this is how they normally exist in cells) the bases (hydrogen acceptors) are “buried” and are involved in hydrogen bonding in DNA double helical structure. The sugar (neutral) and phosphoric acid group form the backbone. The acid phosphoric group gives “DNA” the negative charge. The polymeric form is acidic and may not be having a sour/acidic taste (I have not tasted DNA so far!).

Dr. Kumarasamy



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