A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: DNA Structure and Gene Function in Prokaryotes and Eukaryotes — mark scheme explained
The short answer
The structure of DNA and its organization within cells are fundamental to understanding genetic information, variation, and relationships between organisms. This section delves into the differences between prokaryotic and eukaryotic cells, focusing on how DNA is organized and functions in these different cell types.
The question
Describe the key differences between DNA structure in prokaryotic and eukaryotic cells.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For descriptive questions, marks are typically awarded for providing accurate and detailed information. For explanatory questions, marks are given for clear reasoning and understanding of the underlying principles. For comparative questions, marks are allocated for identifying both similarities and differences. Definitions should be concise and to the point.
What the command words demand
- Describe
- Provide a detailed account of the characteristics or processes involved.
- Explain
- Give reasons for why something happens or how it works, including relevant principles and relationships.
- Compare
- Identify similarities and differences between two or more concepts or structures.
- Define
- State the meaning of a term or concept clearly and concisely.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, provide detailed answers, and check your work.
- Identify the characteristics of prokaryotic DNA.2 marksProkaryotic DNA is short, circular, and not associated with proteins.
- Identify the characteristics of eukaryotic DNA.2 marksEukaryotic DNA is long, linear, and associated with proteins called histones.
- Summarize the differences.0 marksProkaryotic DNA is simpler and more compact, while eukaryotic DNA is longer and requires complex packaging into chromosomes.
Final answer: Prokaryotic DNA is short, circular, and not associated with proteins. Eukaryotic DNA is long, linear, and associated with histones.
Work through every step correctly and you earn all 4 marks.
Another worked example
Explain the role of histones in eukaryotic cells.
- Define histones.1 markHistones are proteins that help to compact DNA into a structure called chromatin.
- Describe the function of chromatin.1 markChromatin allows the long DNA molecules to fit within the nucleus and is further condensed into chromosomes during cell division.
- Summarize the role of histones.1 markHistones play a crucial role in organizing and compacting eukaryotic DNA, allowing it to fit within the nucleus and facilitating processes like replication and transcription.
Final answer: Histones are proteins that help to compact DNA into chromatin. Chromatin allows long DNA molecules to fit within the nucleus and is further condensed into chromosomes during cell division.
Work through every step correctly and you earn all 3 marks.
Common mistakes
Confusing prokaryotic DNA with eukaryotic DNA in terms of structure.
Why it happens: Students may mix up the characteristics of prokaryotic and eukaryotic DNA, leading to incorrect descriptions.
Fix: Always remember that prokaryotic DNA is short, circular, and not associated with proteins, while eukaryotic DNA is long, linear, and associated with histones.
Forgetting the role of histones in chromatin formation.
Why it happens: Students might overlook the importance of histones in organizing eukaryotic DNA into chromatin.
Fix: Histones are essential for compacting DNA into chromatin, which allows it to fit within the nucleus and facilitates processes like replication and transcription.
Misunderstanding the concept of a locus.
Why it happens: Students may not fully grasp that a gene is located at a specific position on a chromosome, known as a locus.
Fix: A locus is the fixed position of a gene on a particular DNA molecule. This helps to identify and locate genes within the genome.
Confusing exons with introns in gene structure.
Why it happens: Students might mix up the roles of exons and introns, leading to incorrect descriptions of gene processing.
Fix: Exons are coding sequences that are retained in mature mRNA, while introns are non-coding sequences that are removed during mRNA processing.
Failing to explain the properties of the genetic code clearly.
Why it happens: Students may struggle to articulate the universal, non-overlapping, and degenerate nature of the genetic code.
Fix: Practice explaining each property: universal (same codons in all organisms), non-overlapping (each triplet codes for one amino acid), and degenerate (multiple triplets can code for the same amino acid).
Overlooking the presence of organelle DNA in eukaryotic cells.
Why it happens: Students might focus only on nuclear DNA and forget about mitochondrial and chloroplast DNA.
Fix: Remember that eukaryotic cells also contain short, circular DNA in mitochondria and chloroplasts, which is essential for their function.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Compare DNA Structure | State how prokaryotic and eukaryotic DNA differ in length, shape, and protein association. | 4 |
| Explain Protein Function | Explain how histones organise and compact DNA within eukaryotic cells. | 3 |
| Define Gene Structure | State what a gene codes for and identify its location on a chromosome. | 3 |
| Genetic Code Properties | Describe features of the genetic code and give a worked example of degeneracy. | 4 |
| Total across these question types | 14 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.