DNA (A-level only) — AQA A-Level Chemistry
Test yourself on DNA (A-level only) with AQA A-Level practice questions.
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DNA (A-level only) explained
You are not asked to memorise these shapes, you are asked to read them off the booklet and use them, so practise with it open.
Read the full explanation
The phosphate ion is PO₄³⁻, drawn with phosphorus bonded to four oxygens and carrying the negative charge. 2-deoxyribose is a pentose: a five-membered ring of four carbons and one oxygen, with an OH on carbon one, an OH on carbon three and a CH₂OH side group on carbon four. The missing OH on carbon two is what deoxy means, and it is the only difference from the ribose of RNA. Adenine and guanine are purines, built from two fused rings; cytosine and thymine are pyrimidines, with one ring. Every base carries an N-H that becomes the link to the sugar, and guanine keeps a ring nitrogen with a spare lone pair that cisplatin later attacks.
A nucleotide is made up from a phosphate ion bonded to 2-deoxyribose which is in turn bonded to one of the four bases adenine, cytosine, guanine and thymine.
The unit is built in two condensation steps, and both eliminate water. The phosphate joins through the CH₂OH group on carbon five of the sugar, giving a phosphate ester link. The base joins at the far end, at carbon one, through a nitrogen atom that carries a hydrogen in the booklet structure; that N-H and the sugar's OH condense together. So the sugar sits in the middle, phosphate hangs off one end and the base off the other, and phosphate is never bonded straight to base. Each unit carries exactly one base, which is why there are four of them in DNA and why they differ in nothing else. Adenine and guanine are the two-ring purines; cytosine and thymine are the single-ring pyrimidines.
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. 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.
This results in a sugar- phosphate-sugar-phosphate polymer chain with bases attached to the sugars in the chain.
Every second unit along the chain is a sugar and every other one a phosphate, so the backbone repeats and carries no information at all. Everything that distinguishes one gene from another is the order of the bases hanging off those sugars. After forming two ester links a phosphate still keeps one ionisable OH, and at the pH inside a cell that group loses its proton, so the backbone is a chain of negative charges. It is therefore strongly attracted to water, which is why the backbone faces outwards while the flat, far less polar bases point inwards. That is a neat piece of structure-and-property reasoning, and it also puts the bases exactly where they can hydrogen bond to a partner strand.
DNA exists as two complementary strands arranged in the form of a double helix.
The pairing is fixed: adenine always hydrogen bonds to thymine, and cytosine always to guanine. Each hydrogen bond runs from an N-H on one base to a lone pair on an oxygen or a nitrogen of its partner, and there are two such bonds in an adenine-thymine pair but three in a cytosine-guanine pair. A two-ring purine always pairs with a one-ring pyrimidine, so every rung is the same width and the helix keeps a constant diameter. Complementary means the sequence of one strand fixes the sequence of the other, which is exactly what makes copying possible: pull the strands apart and each one is a template. Because it has three hydrogen bonds per pair rather than two, a region rich in cytosine and guanine takes more energy to separate.
Your focus
- Use the booklet structure of guanine to pick out the ring nitrogen that keeps an unused lone pair, and distinguish it from the nitrogen that bonds to the sugar.
- Distinguish 2-deoxyribose from ribose by the missing OH group on carbon two, and say why that difference names the sugar.
- Classify adenine and guanine as purines and cytosine and thymine as pyrimidines, using the number of rings in the booklet structures.
Show all 20 objectives
- Identify, on a booklet structure, the OH group and the nitrogen atom that react when a nucleotide is assembled.
- Construct the structure of a named nucleotide from the phosphate ion, 2-deoxyribose and a base given in the Data Booklet.
- Identify the two condensation reactions in that assembly and state the small molecule eliminated at each.
- Predict which atom of a base forms the bond to the sugar and explain why it is nitrogen rather than carbon.
- Compare the four DNA nucleotides and state the only part of the structure that differs between them.
- Draw the link formed when two nucleotides join, showing one phosphate bridging the two sugars.
- Explain why the backbone is a condensation polymer and name the molecule eliminated.
- Compare the strength of the backbone bonds with the hydrogen bonds between strands and relate that difference to replication.
- Deduce how many water molecules are eliminated when a stated number of nucleotides polymerise.
- Sketch three repeat units of the backbone, placing each base on a sugar.
- Explain why the backbone carries no genetic information while the base sequence does.
- Relate the negative charge on the phosphate groups to the position of the backbone on the outside of the helix.
- Identify an incorrect DNA diagram by checking what each base is attached to.
- State the two base pairs and give the number of hydrogen bonds in each.
- Draw the hydrogen bonds between a named base pair, showing the lone pair and the polar hydrogen involved.
- Deduce the base sequence of the partner strand from a sequence you are given.
- Explain why a region rich in cytosine and guanine needs more energy to separate than one rich in adenine and thymine.
DNA (A-level only) exam tips
Marking Points
- one mark for 2-deoxyribose drawn correctly, with no OH group on carbon two
- one mark for the phosphate ion drawn with phosphorus bonded to four oxygens and the negative charge shown
- one mark for identifying adenine and guanine as the two-ring purines and cytosine and thymine as the one-ring pyrimidines
- one mark for showing a nitrogen atom of the base as the atom that bonds to the sugar
- one mark for naming the three components as a phosphate ion, 2-deoxyribose and one of the four bases
- one mark for showing the phosphate bonded to the sugar through an oxygen atom
- one mark for showing the base bonded to the sugar through a nitrogen atom
- one mark for identifying both linkages as condensation reactions, with water eliminated at each
- one mark for identifying the polymerisation as condensation, with water eliminated at every link
- one mark for the link running from the phosphate of one unit to the sugar of the next
- one mark for stating that these backbone bonds are covalent
- one mark for contrasting the covalent backbone with the weaker hydrogen bonds joining the two strands
- one mark for describing the backbone as alternating sugar and phosphate units
- one mark for the bases being attached to the sugars rather than to the phosphates
- one mark for stating that the base sequence, not the backbone, carries the information
- one mark for linking the negative charge on the phosphates to the backbone facing the aqueous surroundings
- one mark for stating that adenine pairs with thymine and cytosine pairs with guanine
- one mark for naming hydrogen bonding as the interaction holding the two strands together
- one mark for two hydrogen bonds in an adenine-thymine pair and three in a cytosine-guanine pair
- one mark for identifying the atoms involved, a polar N-H hydrogen and a lone pair on nitrogen or oxygen
- one mark for deducing the complementary sequence from the strand given
Examiner Tips
- 💡Open the Data Booklet at the DNA structures before you start the question; the marks are for using them, not recalling them.
- 💡Mark the OH groups and the N-H you are going to react before you draw anything, so the bond you form is the one being credited.
- 💡Draw large. A displayed formula must show every bond and every atom, and an examiner cannot credit a link they cannot see.
- 💡Say condensation and show the water; the elimination is often a mark in its own right.
- 💡Draw the sugar first and in the middle, because students who start from the phosphate run out of room and blur the linkage.
- 💡A structure may be displayed, structural or skeletal provided it is unambiguous, so pick the style that shows the two new bonds most clearly.
- 💡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.
- 💡Sketch three repeat units rather than one; the alternation only becomes visible once the chain repeats.
- 💡Label the backbone and the bases separately on a diagram so the examiner can see which part you mean.
- 💡When a question asks what the chain results in, it wants the pattern, so name sugar and phosphate in the order they alternate.
- 💡Draw hydrogen bonds as dashed lines running from the hydrogen to a marked lone pair; a solid line reads as a covalent bond and scores nothing.
- 💡Count before you commit: two bonds for adenine-thymine, three for cytosine-guanine.
- 💡If asked which region needs more energy to separate, answer in terms of the number of hydrogen bonds per base pair.
Common Mistakes
- drawing ribose, with an OH on carbon two, when the booklet gives 2-deoxyribose
- copying uracil from a biology textbook in place of thymine, which is the base this specification lists
- drawing phosphoric acid rather than the phosphate ion the booklet shows
- redrawing a base from memory and losing a ring nitrogen, which makes the link to the sugar impossible to show
- treating the booklet structures as material to learn, then spending exam time reproducing them instead of reading them
- bonding the base straight to the phosphate, which leaves the sugar dangling off the side
- attaching the base through a carbon atom instead of through nitrogen
- showing two or three bases attached to one sugar
- forgetting that each link is a condensation, so water is never shown as a product
- drawing ribose, which belongs to RNA, in place of 2-deoxyribose
- 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
- drawing the bases hanging off the phosphate groups
- drawing an alternating base and sugar chain with the phosphates as side groups
- claiming the backbone differs between organisms
- describing the phosphate group as neutral at the pH inside a cell
- drawing hydrogen bonds from C-H groups, which are not polar enough to form them
- pairing adenine with guanine, so purine with purine, which would make that rung too wide
- saying the two strands are held together by covalent bonds
- giving three hydrogen bonds to adenine-thymine and two to cytosine-guanine, the wrong way round
- writing the partner strand by copying the sequence instead of swapping each base for its pair