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Name: Brianna
Status: Student
Age: 13
Location: N/A
Country: N/A
Date: March 2004


Question:
What is the sugar phosphate supporting structure that holds up the double helix of DNA?



Replies:
Answer: Look at a diagram of a DNA molecule's sugar phosphate backbone, from

http://www.pbs.org/wgbh/nova/photo51/images/jour-16structure.gif.

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 this position.)

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 atom.)

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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