AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
A single strand of DNA (deoxyribonucleic acid) is a polymer of nucleotides…
Full specification wording
DNA (A-level only): A single strand of DNA (deoxyribonucleic acid) is a polymer of nucleotides linked by covalent bonds between the phosphate group of one nucleotide and the 2- deoxyribose of another nucleotide.
Each phosphate ends up esterified twice.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Each phosphate ends up esterified twice. It is already joined to the CH₂OH on carbon five of its own sugar; it then condenses with the OH on carbon three of the next sugar, eliminating a second molecule of water and leaving one phosphate bridging two sugars. Repeat that thousands of times and the result is a condensation polymer. Those bridging bonds are ordinary covalent bonds, so the backbone does not come apart during normal cell chemistry and the order of the bases is fixed. That is the contrast the examiners want: the backbone is covalent, while the two strands are held to each other only by hydrogen bonds, which are far weaker and are broken and remade every time a cell copies itself. A chain also has direction, finishing with a free phosphate at one end and a free OH at the other.
What earns the marks
- Identifying the polymerisation as condensation, with water eliminated at every link.
- The link running from the phosphate of one unit to the sugar of the next.
- Stating that these backbone bonds are covalent.
- Contrasting the covalent backbone with the weaker hydrogen bonds joining the two strands.
Common mistakes and how to put them right
Describing a single strand as held together by hydrogen bonds, which join one strand to the other and not one unit to the next.
Calling the polymerisation addition rather than condensation.
Joining sugar straight to sugar and leaving the phosphate hanging as a side group.
Saying the backbone bonds break during replication, when it is the hydrogen bonds that are broken.
Exam tips
- If you are given two nucleotides and asked for the link, circle the two OH groups that react before you draw anything.
- Use the word covalent explicitly; strong bond is not the same answer.
- Draw one repeat unit rather than the whole chain, and finish the bonds at each edge with a continuation line.
Try the lesson
The start of the DNA (A-level only): A single strand of DNA (deoxyribonucleic acid) is a polymer of nucleotides linked by covalent bonds between the phosphate group of one nucleotide and the 2- deoxyribose of another nucleotide. lesson, made from checked teaching notes for this specification point.
Have a go first
In a single strand of DNA, which two groups are joined by the covalent bond that links one nucleotide to the next?
Listen
Each phosphate ends up esterified twice. It is already joined to the CH₂OH on carbon five of its own sugar; it then condenses with the OH on carbon three of the next sugar, eliminating a second molecule of water and leaving one phosphate bridging two sugars. Repeat that thousands of times and the result is a condensation polymer. Those bridging bonds are ordinary covalent bonds, so the backbone does not come apart during normal cell chemistry and the order of the bases is fixed. That is the contrast the examiners want: the backbone is covalent, while the two strands are held to each other only by hydrogen bonds, which are far weaker and are broken and remade every time a cell copies itself. A chain also has direction, finishing with a free phosphate at one end and a free OH at the other. A strong answer earns marks for these points. identifying the polymerisation as condensation, with water eliminated at every link. the link running from the phosphate of one unit to the sugar of the next. stating that these backbone bonds are covalent. contrasting the covalent backbone with the weaker hydrogen bonds joining the two strands. Exam tip: If you are given two nucleotides and asked for the link, circle the two OH groups that react before you draw anything.
Key words
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condensation polymer
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phosphate bridge between sugars
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covalent backbone
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loss of water
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See every definition
- condensation polymer
- The result of repeating the link thousands of times, with water eliminated at every link.
- phosphate bridge between sugars
- One phosphate joined to the CH₂OH on carbon five of its own sugar and condensing with the OH on carbon three of the next sugar.
- covalent backbone
- The bridging bonds are ordinary covalent bonds, so the backbone does not come apart during normal cell chemistry.
- loss of water
- A second molecule of water is eliminated when the phosphate condenses with the OH on carbon three of the next sugar.
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Test your understanding of DNA (A-level only): A single strand of DNA (deoxyribonucleic acid) is a polymer of nucleotides linked by covalent bonds between the phosphate group of one nucleotide and the 2- deoxyribose of another nucleotide.
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