The sugar–phosphate backbone at Higher Level: The same statement as SL. At HL each link also has a direction: it joins carbon 3′ of one sugar to carbon 5′ of the next, which gives each strand a 5′ end and a 3′ end. Learn the backbone here; the direction builds on it.
Practise this as you read
- Name the backbone bond: covalent, or phosphodiester.
- Explain why the backbone is strong: a continuous chain of covalent bonds.
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The big idea: A strand of DNA or RNA is nucleotides joined in a line. Each link is a covalent bond from the sugar of one nucleotide to the phosphate of the next.
The points to remember
- Nucleotides join in a line: the sugar of one nucleotide bonds to the phosphate of the next.
- The bond is covalent: atoms share electrons. Its full name is a phosphodiester bond.
- The same kind of bond links nucleotides in DNA and in RNA.
Remember it as: Sugar to phosphate, sugar to phosphate: covalent all the way.
In the drawing, every line between a pentagon and the next circle is one of these covalent bonds.
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The big idea: The covalent links make one continuous chain along each strand: the sugar–phosphate backbone. Because every link is covalent, the backbone is strong.
The points to remember
- Sugar, phosphate, sugar, phosphate: one continuous chain of covalently bonded atoms runs along each strand.
- That chain is the sugar–phosphate backbone.
- Every link is covalent, so the backbone is strong and does not break easily.
- The bases stick out to one side of the backbone, one on each sugar.
- DNA and RNA have the same kind of backbone; only the sugar differs (deoxyribose or ribose).
Covalent, not hydrogen: The bonds along the backbone are covalent. Hydrogen bonds join the bases of two strands, never the backbone.
Real example: the Neanderthal genome: In 2010 scientists read most of the Neanderthal genome from bones about 40 000 years old. Short pieces of DNA survived that long because their backbones are chains of covalent bonds.
How it is asked: A drawn strand with a letter on one bond, and one mark for naming the bond. A second short part can ask why the backbone is strong.
The diagram shows part of a nucleic acid. Identify the type of bond indicated by Z.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
And the follow-on part, two points for two marks:
Outline why the sugar–phosphate backbone of DNA is strong.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.