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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the characteristics of prokaryotic DNA.
- S2
Prokaryotic DNA is short, circular, and not associated with proteins.
- S3
Step 2: Identify the characteristics of eukaryotic DNA.
- S4
Eukaryotic DNA is long, linear, and associated with proteins called histones.
- S5
Step 3: Summarize the differences.
- S6
Prokaryotic DNA is simpler and more compact, while eukaryotic DNA is longer and requires complex packaging into chromosomes.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the characteristics of prokaryotic DNA.
- S2
Prokaryotic DNA is short, circular, and not associated with proteins.
- S3
Step 2: Identify the characteristics of eukaryotic DNA.
- S4
Eukaryotic DNA is long, linear, and associated with proteins called histones.
- S5
Step 3: Summarize the differences.
- S6
Prokaryotic 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.
Common mistakes
- Confusing prokaryotic DNA with eukaryotic DNA in terms of structure. — 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. — 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. — 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. — 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. — 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. — Remember that eukaryotic cells also contain short, circular DNA in mitochondria and chloroplasts, which is essential for their function.
Where the marks go
- Full worked solution (all marking points)4 marks