Date: March 2004
What is the sugar phosphate supporting structure that holds up
the double helix of DNA?
Answer: Look at a diagram of a DNA molecule's sugar phosphate backbone, from
The phosphate group is shown in the pink circle: one phosphorus atom (P),
covalently bonded to four oxygen atoms (O). Below the phosphate group on the
diagram is a deoxyribose sugar molecule, shaded in pink. Above the phosphate
group is another pink shaded area; it represents another deoxyribose sugar
(not shown in detail).
The sugar phosphate backbone is a long chain of
sugar-phosphate-sugar-phosphate-sugar-etc., with the nitrogenous bases
sticking out of the backbone. Two such chains are twisted around each other
to form the famous double helix structure of DNA.
The deoxyribose sugar molecule is a 5-carbon structure. Four of the carbon
atoms and one oxygen atom form a ring. The 5th carbon atom is outside the
ring, like a tail. The carbons are distinguished from each other with a
numbering system (not shown on the illustration): they are called 1'
(one-prime) through 5' (five-prime).
Let's examine the carbon atoms in the deoxyribose, one at a time. As you may
know, a carbon atom will always have 4 covalent bonds.
The 1' carbon is attached by a covalent chemical bond to the nitrogenous
base, shown in maroon. (Its other three bonds are to a hydrogen atom, to the
2' carbon in the ring, and to the oxygen atom in the ring.)
The 2' carbon is attached to the 1' and 3' carbon in the ring, and to two
hydrogen atoms, H. (If this were a ribose sugar, this 2' carbon would have
an oxygen atom attached - hence the name deoxyribose, lacking an oxygen in
The 3' carbon is attached to the next phosphate group in the sugar-phosphate
backbone, by a covalent bond to one of the oxygen atoms in that phosphate.
(Its other bonds are to the 2' and 4' carbons in the ring, and a hydrogen
The 4' carbon is bonded to the 3' carbon and the O (oxygen) atom in the
ring, as well as the 5' C tail outside the ring, and a hydrogen atom, H.
The 5' carbon is attached to the 4' carbon and the O of the phosphate group,
and two H atoms.
As you see, the DNA strand has direction. One strand of the double helix is
in the 5' to 3' direction, and the opposite strand is anti-parallel, running
from 3' to 5'.
Note that all the chemical bonds described above are strong covalent bonds -
like glue. The hydrogen bonds that hold the two strands of DNA together in
the double helix are much weaker - perhaps like yellow sticky notes. They
can easily come apart and back together again. They are caused by specific
base pairing between the bases on the one DNA strand and the complementary
bases on the opposite strand.
Sarina Kopinsky, MSc, H.Dip.Ed.
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Update: June 2012