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Nucleic Acid vs Nucleotide
Name: Janet C.
Status: Educator
Age: 40s
Location: N/A
Country: N/A
Date: 2002
Question:
Members of this group of chemical compounds are found in
all living cells. They consist of a molecule of sugar, either ribose or
deoxyribose, an inorganic phosphate, and an organic base. ATP is an
important one. What is this chemical group?
I need to be able to illustrate clearly why NUCLEIC ACID is not the
correct answer to this question but NUCLEOTIDE is.
Can you help?
Replies:
Nucleic acids are polymers made up of nucleotides. To quote Alberts et al.,
in Molecular Biology of the Cell, "Nucleotides serve as building blocks for
the construction of nucleic acids..."
Paul Mahoney, PhD
A nucleoside (with an s) consists of a nitrogenous base covalently attached
to a (ribose or deoxyribose) sugar but without the phosphate group.
A nucleotide (with a t) consists of a nitrogenous base, a sugar, and a
phosphate group. So, a nucleotide is a "nucleoside mono-phosphate."
A nucleic acid contains a chain of nucleotides covalently linked together to
form a sugar-phosphate backbone with protruding nitrogenous bases. In RNA
(ribonucleic acid), the sugar groups are ribose, whereas in DNA
(deoxyribonucleic acid), deoxyribose sugars are present instead of ribose.
DNA contains two such chains spiraling round each other in the famous double
helix shape, with sugar and phosphate chains forming the "uprights" of the
twisted ladder and bases pointing to the middle like the rungs of the
ladder. The two chains in the double helix are held together along their
length by hydrogen bonds that form between the bases on one chain and the
bases on the other. These are the base pairs that we have heard so much
about during the Human Genome Project.
Note that nucleotides have only one phosphate group, whereas ATP has three,
so ATP is not strictly speaking a nucleotide. However, its mono-phosphate
equivalent, AMP or adenosine mono-phosphate, is a nucleotide. ATP can be a
precursor, though, by losing its two terminal phosphate groups. AMP is also
called adenosine 5'-phosphoric acid or adenylic acid, according to my trusty
old biochemistry text, Lehninger. AMP contains a ribose group as its sugar,
while dAMP, or deoxy-AMP, contains a deoxyribose as its sugar.
Besides AMP, other nucleotides present in RNA are GMP (guanylic acid, or
guanosine 5'-phosphoric acid), CMP (cytidylic acid, or cytidine 5
'-phosphoric acid) and UMP (uridylic acid, or uridine 5'-phosphoric acid).
Besides dAMP, other nucleotides present in DNA are dGMP, dCMP and dTMP
(deoxythymidylic acid, or deoxythymidine 5'-phosphoric acid).
As you can see from these names - cytidylic acid and so on, the nucleotides
are acidic, and it makes sense that a chain of them would also be acidic.
These chains were called nucleic acids presumably because they are acidic
substances found in the nucleus of the cell.
If you want to play games with semantics you could insist that nucleotides
are acidic molecules found in the nucleus (in addition to the cytoplasm) so
they must be "nucleic acids." You could also claim, by the same logic, that
amino acids (the building blocks of proteins) are cytoplasmic acids. But don
't mess with the jargon like this. The term nucleic acid is understood to
mean a polynucleotide chain - either DNA or RNA.
Prooreading note to the editor: Avoid using "smart quotes" or curly quotes
for the 5' and 3' (five prime and three prime) designations of these
molecules, e.g., cytidine 5'-phosphoric acid. These are primes and not
apostrophes. Regular quotation marks on other words, of course, can follow
whatever style you prefer.
Sarina Kopinsky, MSc, CGC, HED
This is more semantic than substantive at a certain level. Arms and Camp
lumps the energy carrying nucleotides under the heading Nucleic acids. In
this case the individual nucleotides are under this group. This has an
obvious weakness which you are aware in that the individual nucleotides are
NOT "nucleic acids". Nucleic acid structure was elucidated in the early
50's and are considered "linear polymers of nucleotides whose phosphates
bridge the 3 prime and 5 prime positions of successive sugar residues". ATP
is clearly NOT a polymer but it IS a nucleotide. At physiological pH
nucleic acids are polyanions and nucleotides although a proton donor are NOT
POLYanions.
Peter Faletra Ph.D.
Assistant Director
Science Education
Office of Science
Department of Energy
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Update: June 2012
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