Overview
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are important information-carrying molecules. In all living cells, DNA holds genetic information and RNA transfers genetic information from DNA to the ribosomes.
Nucleic acids are essential biomolecules that carry the genetic information. This genetic information holds instructions for making proteins.
These nucleic acids include:
- DNA (Deoxyribonucleic Acid) - Contains the instructions (genes) for determining which proteins should be made
- RNA (Ribonucleic Acid) - Transfers those instructions (gene) from the DNA to ribosomes where the proteins are made. As we saw in the proteins topic, these proteins range from enzymes to antibodies.
Why did scientists once doubt that DNA carried the genetic code?
Why did scientists once doubt that DNA carried the genetic code?
As DNA is made from only 4 different nucleotides, it seemed too simple to hold all the genetic information, which is incredibly complex. At the time, scientists thought that proteins, made from 20 different amino acids, were more likely to hold this information given their complexity. But the information is stored in the linear sequence of bases. Each position can be any of 4 bases, so a sequence of just 10 bases has 4¹⁰ (over a million) possible combinations!
Nucleotide Structure
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
Both DNA and RNA are polymers of nucleotides. Each nucleotide is formed from a pentose, a nitrogen-containing organic base and a phosphate group.
Both DNA and RNA are polymers of nucleotides.
Each nucleotide (the monomer) is composed of 3 things:
- Pentose sugar:
- RNA = Ribose (contains an OH group at the 2′ carbon, see image below)
- DNA = Deoxyribose (lacks OH group, has H instead, see image below)
- Phosphate group
- An organic base: Adenine (A), Cytosine (C), Guanine (G) or Thymine (T) in DNA/Uracil (U) in RNA.
Ribose vs Deoxyribose
Phosphodiester Bond
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
A condensation reaction between two nucleotides forms a phosphodiester bond.
A single nucleotide (mononucleotide, which is a monomer) consists of 1 pentose sugar, 1 phosphate group and 1 organic base (as shown in the figure above)
Nucleotides can join together in a condensation reaction which forms a phosphodiester bond (a type of covalent bond) to form a polynucleotide.
As the name suggests, a phosphodiester bond contains 2 (di) ester bonds linking adjacent sugars via a phosphate group.
The diagram below shows an example of 2 nucleotides joining as part of a condensation reaction. As this is a condensation reaction you can see that water is lost.
The bond formed is a 3′–5′ phosphodiester linkage, between the 3′ hydroxyl group of one nucleotide and the 5′ phosphate group of the next.
Why do we call them 3′ and 5′?
Why do we call them 3′ and 5′?
The carbons are numbered 1′ to 5′. This refers to the specific carbon atoms in the pentose sugar (ribose or deoxyribose) of a nucleotide. See the image of ribose and deoxyribose to see the numbering on the carbons.
DNA Structure & Function
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine. A DNA molecule is a double helix with two polynucleotide chains held together by hydrogen bonds between specific complementary base pairs.
You will explore how DNA is organised and packaged in a cell in Unit 4: DNA, Genes and Chromosomes. For now you should understand the structure & function of it. Use the diagram below to understand these facts below.
-
DNA is made up of 2 long strands of polynucleotides arranged in a double helix
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The two strands run antiparallel to each other
- One goes in the 5′ to 3′ direction, and the other runs in the opposite direction (3′ to 5′)
-
There is hydrogen bonding between these complementary bases:
- Adenine pairs with thymine (2 hydrogen bonds)
- Cytosine pairs with guanine (3 hydrogen bonds)
-
Phosphodiester bonds hold together the nucleotides and this forms the sugar-phosphate backbone. Think of it like a skeleton, holding together the DNA nucleotides.
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The stability of the DNA structure is maintained by the bonding:
- Hydrogen bonding between bases
- Phosphodiester backbone (commonly referred to as sugar-phosphate backbone) held together by phosphodiester bonds helps to protect the inner bases.
Is a DNA molecule with a higher proportion of C-G base pairs more or less stable than one with more A-T base pairs?
Is a DNA molecule with a higher proportion of C-G base pairs more or less stable than one with more A-T base pairs?
More stable. Each C-G base pair is held together by 3 hydrogen bonds, compared with 2 for an A-T base pair (see image below). So a DNA molecule with more C-G pairs has more hydrogen bonds holding its two strands together.
- DNA is negatively charged due to the presence of phosphate groups (phosphate groups have a negative charge)
Image: DNA Nucleotides by OpenStax College, licensed under CC BY 3.0, via Wikimedia Commons.
Maths Skill: Calculating Base Frequencies
Because bases pair A with T and C with G, in double-stranded DNA:
- the amount of A = T, and the amount of C = G
- so A + G = 50% (and T + C = 50%)
Worked example: a DNA sample contains 30% adenine. What percentage is guanine?
- A = T, so thymine = 30%
- A + T = 60%, so C + G = 100 − 60 = 40%
- C = G, so guanine = 40 ÷ 2 = 20%
Try it: a DNA molecule contains 18% cytosine. What percentage is adenine?
Try it: a DNA molecule contains 18% cytosine. What percentage is adenine?
32%. C = G, so guanine = 18% and C + G = 36%. That leaves A + T = 100 − 36 = 64%, and A = T, so adenine = 64 ÷ 2 = 32%.
RNA Structure & Function
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
The components of an RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil. Ribosomes are formed from RNA and proteins. An RNA molecule is a relatively short polynucleotide chain.
- RNA has ribose as its pentose sugar
- RNA is a single-stranded polynucleotide molecule.
- It’s shorter than a DNA molecule
- 3 types of RNA
- Messenger RNA (mRNA): Made during transcription; carries genetic information from DNA to the ribosomes.
- Transfer RNA (tRNA): Involved in translation by bringing specific amino acids to the ribosome
- Ribosomal RNA (rRNA): Combines with proteins to form ribosomes.
- More about the different types of RNA and their role in protein synthesis is covered in DNA & Protein Synthesis
Differences between DNA & RNA
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases present | Adenine, Thymine, Cytosine, Guanine | Adenine, Uracil, Cytosine, Guanine |
| Number of strands | Double-stranded (double helix) | Single-stranded |
| Length | Very long | Shorter |
| Stability | More stable | Less stable |
| Function | Stores genetic information | Involved in protein synthesis |
| Types | One type | mRNA, tRNA, rRNA |
| Phosphodiester bonds | 3′–5′ phosphodiester bonds | 3′–5′ phosphodiester bonds |
How this topic is tested
This analysis is based on past paper data from 2017 to 2025. It is intended for interest only and is not predictive of what will appear in future papers.
- Tested in 1 of 9 years (2017–2025): 1 question parts worth 5 marks.
- 50th most-examined topic overall by marks, 7th in Unit 1.
- Not tested in 2023–2025.
Marks by year
Most-tested spec points
- DNA Structure: tested in 1 part (5 marks)
Tips from examiner reports
What students commonly get wrong
- Name the bases in fullWatch out: write the bases' names in full: adenine, thymine, cytosine, guanine and uracil. Answers that gave only the letters (A, T, C, G, U) weren't credited. 2022 P1 Q7.1 · 2023 P1 Q10.2 · 2019 P1 Q6.2
- Phosphodiester bonds vs hydrogen bondsWatch out: phosphodiester bonds join the sugar of one nucleotide to the phosphate of the next, forming the sugar-phosphate backbone. They don't form between bases: the bases are paired by hydrogen bonds. 2025 P1 Q2.2 · 2023 P1 Q10.2 · 2020 P1 Q10.1
- DNA is a double helix; RNA is single-strandedWatch out: say DNA is a double helix, not just a "helix". RNA, including mRNA and tRNA, is single-stranded and contains uracil, not thymine. 2022 P1 Q7.1 · 2024 P1 Q6.1
Exam Question Practice
A few exam-style questions on this topic to try. Have a go at each one before opening the answer.
During transcription, RNA polymerase joins free RNA nucleotides together to make a strand of mRNA.
Describe how the nucleotides are joined.
(2 marks)
Hint
Think about the name of the bond formed between adjacent nucleotides and the type of reaction that forms it.
Mark Scheme
- A phosphodiester bond forms between adjacent nucleotides (1 mark)
- By a condensation reaction (1 mark)
Also accepted / rejected
Tips
- Name the phosphodiester bond and say it forms between the phosphate of one nucleotide and the ribose of the next, not between bases or between RNA and DNA
- Don’t confuse phosphodiester bonds with hydrogen bonds
- Answer the question asked. Describing the rest of transcription gains no marks
What earns marks
- Stating the bond is formed by condensation (the mark most students get)
Describe the structure of DNA.
(5 marks)
Hint
It’s a 5 mark question, so try to include 5 points about DNA structure. Think about the components that make DNA, and the two types of bonds that are present in DNA.
Mark Scheme
- Polymer of nucleotides (1 mark)
- Each nucleotide is made of deoxyribose, a phosphate (group) and an organic/nitrogenous base (1 mark)
- Phosphodiester bonds (between nucleotides) (1 mark)
- Double helix / 2 strands held together by hydrogen bonds (1 mark)
- (Hydrogen bonds/pairing) between adenine and thymine, and cytosine and guanine (1 mark)
Also accepted / rejected
- For point 1: accept “polynucleotide”
- For point 2: accept “phosphoric acid” for “phosphate”
DNA and tRNA are both nucleic acids.
Complete Table 1 to give three differences between a DNA molecule and a tRNA molecule.
Table 1
| DNA | tRNA | |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 |
(3 marks)
Hint
Compare the two molecules on sugar, bases, number of strands, size and shape.
Mark Scheme
Any three rows, each comparing the same feature (3 marks)
| DNA | tRNA | |
|---|---|---|
| 1. | Deoxyribose | Ribose |
| 2. | Double-stranded (double helix) | Single-stranded |
| 3. | Thymine | Uracil |
| 4. | Many nucleotides (long) | Few nucleotides (short) |
| 5. | Linear (double helix) | Clover-leaf shape |
| 6. | No amino acid binding site | Binds an amino acid |
| 7. | No exposed bases | Has an anticodon |
Also accepted / rejected
Not accepted
- Bases written as letters only (T and U)
- A row that doesn’t compare the same feature
Tips
- Make each row a direct comparison, e.g. deoxyribose v ribose, double-stranded v single-stranded
- Write thymine and uracil in full: letters alone score nothing
- Say “double helix”, not just “helix”
- If you give tRNA’s anticodon, the DNA side must be “no exposed bases”
Not accepted