DNA Detectives
Which base pairs with adenine during DNA replication?
A. Uracil
B. Guanine
C. Thymine
D. Cytosine
C. Thymine
Identify the technique used to determine the order of DNA bases.
A. DNA profiling
B. DNA sequencing
C. Gel electrophoresis
D. Polymerase chain reaction (1 mark)
B. DNA sequencing
Define genetic drift. (1 mark)
Genetic drift refers to changes in allele frequencies within a population that occur due to chance.
Explain how energy enters the biotic components of an ecosystem. (2 marks)
Energy enters the biotic components of an ecosystem via photosynthesis, where solar energy from the sun is used to convert carbon dioxide and water into chemical energy in the form of glucose.
Recognises that energy enters the biotic components of an ecosystem via photosynthesis. [1 mark]
Explains photosynthesis. [1 mark]
Original exam question
Distinguish between exons and introns. (2 marks)
STUDENT RESPONSE — AWARD A MARK
Exons are coding regions of DNA.
a) 0/2
b) 1/2
c) 2/2
b) 1/2
Official QCAA marking criteria:
Describes exons or introns — 1 mark
Distinguishes between exons and introns — 1 mark
Why this mark?
The student correctly describes exons, but does not explain how introns differ.
How to achieve full marks
Exons are coding sections of DNA, whereas introns are non-coding sections that are removed during RNA splicing.
Distinguish between exons and introns. (2 marks)
Exons are sections of DNA (or mRNA) that code for proteins; whereas introns are non-coding sections that get spliced out.
1 mark — describes exons or introns.
1 mark — distinguishes between exons and introns.
Describe the structure of plasmids and their role in making recombinant DNA. (2 marks)
Plasmids are small, circular DNA molecules found in bacteria. They are used as vectors to carry new genes into cells and allow the new gene to be replicated.
1 mark — describes the structure of plasmids.
1 mark — describes the role of plasmids in recombinant DNA technology.
Explain why genetic drift is usually high in populations affected by bottlenecks. (2 marks)
A population bottleneck is a random event that drastically reduces the size of a population. Because the survivors of a bottleneck event usually do so by chance (as opposed to by having advantageous alleles), the allele frequencies of the surviving population are usually very different from the parent population due to sampling error.
1 mark — recognises that a bottleneck reduces population size.
1 mark — explains why genetic drift is higher in populations affected by bottlenecks.
Distinguish between gross primary productivity (GPP) and net primary productivity (NPP). (2 marks)
Gross primary productivity (GPP) is the total amount of solar energy converted into chemical energy by photosynthesis, whereas net primary productivity (NPP) is the total energy stored as biomass and made available to consumers.
Describes gross or net primary productivity. [1 mark]
Distinguishes between gross and net primary productivity. [1 mark]
Original exam question
Identify two enzymes involved in DNA replication and describe their functions. (4 marks)
STUDENT RESPONSE — AWARD A MARK
Helicase and DNA polymerase are two enzymes involved in DNA replication. Helicase separates the DNA strands.
a) 0/4
b) 1/4
c) 2/4
d) 3/4
e) 4/4
d) 3/4
Official QCAA marking criteria
Identifies the first enzyme — 1 mark
Describes its function — 1 mark
Identifies a second enzyme — 1 mark
Describes the second enzyme's function — 1 mark
Why this mark?
Both enzymes are correctly identified, and the function of helicase is described. The function of DNA polymerase is missing.
How to achieve full marks
Add: DNA polymerase forms new complementary DNA strands by adding nucleotides according to base-pairing rules.
Explain how synthesis of the leading strand is different from the lagging strand. (2 marks)
Synthesis of the leading strand is continuous, whereas synthesis of the lagging strand occurs in segments, creating Okazaki fragments which must later be joined.
1 mark — explains that leading-strand synthesis is continuous.
1 mark — explains that lagging-strand synthesis is non-continuous.
An experiment investigated how calcium chloride (CaCl₂) concentration affects plasmid transformation efficiency. Error bars show standard error. Which concentration had the greatest variation in transformation efficiency?
A. 0.05 M
B. 0.15 M
C. 0.20 M
D. 0.30 M
(1 mark)
A. 0.05 M
1 mark — selects option A.
The largest error bars indicate the greatest variation.
Scientists analysed data on three populations (A, B and C) in a region affected by habitat fragmentation. The graphs show how the rate of gene flow and genetic differences between the populations changed over time.
Infer which populations underwent allopatric speciation. Justify your response. (2 marks)
Populations A and B. Allopatric speciation occurs when populations become geographically isolated, preventing gene flow from occurring. The graph for populations A and B shows that gene flow stopped suddenly at approximately 20 million years.
1 mark — infers populations A and B.
1 mark — provides appropriate justification.
Infer which ecosystem has the greatest species evenness. Justify your response using the provided biotic data. (2 marks)
The biotic data suggests that ecosystem 2 has the greatest species evenness. Species richness is similar for both ecosystems, but SDI is much higher in ecosystem 2 (0.82 vs 0.38). Because SDI considers both richness and evenness, it can be inferred that the higher SDI is due to greater evenness.
Infers ecosystem 2. [1 mark]
Provides appropriate reasoning. [1 mark]
Original exam question
Explain why genetic drift is usually high in populations affected by bottlenecks. (2 marks)
STUDENT RESPONSE — AWARD A MARK
A bottleneck drastically reduces the population size, so genetic drift is high.
What mark would you award?
a) 0/2
b) 1/2
c) 2/2
b) 1/2
Official QCAA marking criteria
Recognises that a bottleneck reduces population size — 1 mark
Explains why genetic drift is higher after a bottleneck — 1 mark
Why this mark?
The response identifies the reduction in population size but does not explain the role of chance in changing allele frequencies.
How to achieve full marks
A bottleneck drastically reduces population size. Because survivors are often determined by chance, their allele frequencies may differ substantially from those of the original population.
Identify two enzymes involved in DNA replication and describe their functions. (4 marks)
Enzyme 1: Helicase Function: Helicase unwinds the double helix and separates the strands.
Enzyme 2: DNA polymerase Function: DNA polymerase forms new complementary strands by adding nucleotides using the base pairing rules.
1 mark — identifies an enzyme involved in DNA replication.
1 mark — describes its function.
1 mark — identifies a second enzyme.
1 mark — describes the second enzyme's function.
Explain two ways the structure of DNA allows helicase and DNA polymerase to carry out their roles. (2 marks)
The weak nature of hydrogen bonds makes it easy for helicase to separate strands. Complementary base pairing allows DNA polymerase to be guided by the template strand.
Marking criteria
1 mark — explains one way DNA structure aids replication.
1 mark — explains a second way DNA structure aids replication.
Use your knowledge of convergent and divergent evolution to explain why molecular sequence data is often more accurate than anatomical data for determining the relatedness of different species. (2 marks)
Both types of evolution may result in organisms with similar structures, but in divergent evolution, structural similarity is due to the short period of time since divergence (i.e. organisms are closely related), whereas in convergent evolution, similarity is due to similar environments. This means that some organisms have similar features without being closely related, so physical features aren’t always a reliable indicator.
1 mark — distinguishes between divergent and convergent evolution.
1 mark — explains the difference in accuracy.
Compare the abiotic data for the two ecosystems to determine which ecosystem is healthier. (3 marks)
Similarity: Both ecosystems are within the healthy range for dissolved oxygen and turbidity. Difference: Only ecosystem 2 is in the healthy range for pH. Significance: This suggests that ecosystem 2 is healthier than ecosystem 1.
Identifies a similarity. [1 mark]
Identifies a difference. [1 mark]
Identifies the significance. [1 mark]
Original exam question
Explain how transcription factors allow for cell differentiation, using an example. (3 marks)
STUDENT RESPONSE — AWARD A MARK
Transcription factors bind to specific DNA sequences and regulate which genes are expressed in different cell types. For example, the SRY transcription factor activates genes involved in testis development.
a) 0/3
b) 1/3
c) 2/3
d) 3/3
d) 3/3
Official QCAA marking criteria
Recognises binding to specific DNA sequences — 1 mark
Explains differential gene expression between cell types — 1 mark
Provides an appropriate example — 1 mark
Why this mark?
This is deliberately a full-mark response! It includes the binding mechanism, links this to differential gene expression and gives an appropriate example.
How to achieve full marks
No improvement is required for the listed criteria. Students should recognise that a concise response can still earn full marks.
Explain how transcription factors allow for cell differentiation, using an example. (3 marks)
Transcription factors are proteins that turn specific genes on or off by binding to nearby sections of DNA. This allows different genes to be expressed in different cell types. For example, the SRY gene on the Y chromosome promotes the development of testes by producing a protein that regulates the expression of genes required by Sertoli cells.
1 mark — recognises that transcription factors bind to specific DNA sequences.
1 mark — explains that different genes can be expressed in different cell types.
1 mark — provides an example.
Explain how errors during DNA replication can lead to point and frameshift mutations. (2 marks)
Mutations occur when DNA polymerase makes an error by inserting, removing or adding an incorrect nucleotide. Substitution reactions involve A, T, C or G being swapped for a different nucleotide, and result in point mutations. Because of the way that codons work, these mutations will only affect one amino acid. Additions or deletions cause frameshift mutations, where all codons after the mutation will be affected.
Marking criteria
1 mark — explains how replication errors cause point mutations.
1 mark — explains how replication errors cause frameshift mutations.
Use the principles of natural selection to provide a possible explanation for the trend in body length over geological time. (3 marks)
Larger body size provided a selective advantage to marine organisms. Organisms with larger bodies were more likely to survive until reproductive age, when they could pass on the advantageous alleles to their offspring. This caused the allele/s for increased body size to become more common in the population over time.
1 mark — explains the observed trend using phenotypic variation.
1 mark — explains differential survival and reproduction.
1 mark — explains change over time.
Calculate the average annual population growth rate for fur seals between 2000 and 2025. (3 marks)
Number of seal pups in 2025 = (230 × 250) ÷ 2 = 28 750
Growth rate = (28 750 − 15 000) ÷ 15 000 × 100 = 92%
Average annual population growth rate = 92% ÷ 25 years = 3.7% per year.
Calculates the number of seal pups in 2025. [1 mark]
Determines the growth rate. [1 mark]
Determines the average annual population growth rate. [1 mark]
Original exam question
Use your knowledge of convergent and divergent evolution to explain why molecular sequence data is often more accurate than anatomical data for determining the relatedness of different species. (2 marks)
STUDENT RESPONSE — AWARD A MARK
Molecular data is more accurate because DNA shows how closely related species are, while anatomical structures can look similar.
What mark would you award?
a) 0/2
b) 1/2
c) 2/2
a) 0/2
Official QCAA marking criteria
Distinguishes between convergent and divergent evolution — 1 mark
Explains the difference in accuracy — 1 mark
Why this mark?
The student makes a broad claim but does not distinguish convergent from divergent evolution or explain why similar anatomical structures can be misleading. This is a suggested strict application of the published criteria.
How to achieve full marks
Convergent evolution can produce similar anatomical features in distantly related species because they experience similar selection pressures. In divergent evolution, related species may become anatomically different. Therefore, anatomical similarity may not reliably reflect ancestry, whereas molecular sequence comparisons provide evidence of evolutionary relatedness.