IB Biology HL Theme D: Continuity and change questions

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Question 1

Multiple choiceMedium1 mark

At a replication fork moving to the right, the upper parental template runs 3′→5′ from left to right, and the lower template runs 5′→3′. Which description of new-strand synthesis is correct?

Show answer and explanation

Answer: D. DNA polymerase extends a new strand at its 3′ end, so synthesis proceeds 5′→3′. The upper template allows continuous synthesis toward the fork from one primer. The lower template is copied away from the fork in Okazaki fragments, requiring repeated primers. A: reverses which template supports leading-strand synthesis toward the fork. B: reverses the direction of DNA synthesis; new DNA is made 5′→3′. C: reverses primer frequency: continuous synthesis uses one primer, whereas discontinuous synthesis requires repeated primers. Remember: the lagging strand is made discontinuously as Okazaki fragments.

Question 2

Paper 2Medium16 marks

A researcher follows one replication fork in Escherichia coli. Figure 1 shows the two parental template strands and newly synthesized strands A and B. The arrow indicates the direction in which the replication fork moves.

(a)

Using Figure 1, state which of A and B is synthesized continuously and which is synthesized discontinuously. Name the short DNA pieces on B.

[3]

(b)

Explain, using the strand orientations in Figure 1, why A requires one RNA primer whereas B requires repeated RNA primers.

[3]

(c)

A newly synthesized lagging-strand region is 2.4 kb long. The mean length of an Okazaki fragment is 200 nucleotides. For this calculation, use 1 kb = 1000 nucleotides. Estimate the number of Okazaki fragments and the number of RNA primers required.

[2]

(d)

In four separate bacterial preparations, the replication fork has already formed and exactly one enzyme is selectively inactivated. All other replication components remain active.

Preparation	Observation
P	The existing leading strand continues to extend, but no new RNA primers are detected on the lagging-strand template.
Q	RNA primers are present, but no DNA is added to their 3′ ends.
R	DNA has been extended from the primers, but RNA primer sections remain in the daughter strands.
S	RNA primers have been removed and replaced with DNA, but nicks remain between adjacent lagging-strand fragments.

For each preparation, identify the inactive enzyme and explain how its normal function accounts for the observation.

[8]

Question 3

Multiple choiceMedium1 mark

The diagram shows the base sequences in two cell lineages. Which interpretation correctly identifies the offspring that inherited the germ-cell mutation and the fate of the somatic-cell mutation?

Show answer and explanation

Answer: D. A mutation in a germ cell can pass through a gamete to the offspring, as shown by the mutated sequence in offspring P. The somatic-cell mutation is present in the tumour cells, while the separate germ-cell lineage and offspring Q retain the reference sequence.

A: swaps the offspring labels and incorrectly assigns the somatic mutation to offspring P.
B: selects Q rather than P as the offspring with the inherited germ-cell mutation.
C: reverses the somatic-cell pathway, placing its mutation in offspring Q rather than the tumour cells.
Remember: mutations in germ cells can be inherited; mutations in somatic cells can be associated with cancer but are not passed to offspring.

Question 4

Paper 2Medium16 marks

A gene in a population encodes part of a polypeptide. The table shows the coding-strand DNA sequence from a reference individual and three variants found in other individuals. The mRNA codons and their amino acids relevant to these sequences are also shown.

Sequence	Coding-strand DNA, 5′→3′
Reference	GAA CCT TGG
P	GAG CCT TGG
Q	GAA CAT TGG
R	GAA CCT TGC

mRNA codon	Amino acid
GAA	glutamic acid
GAG	glutamic acid
CCU	proline
CAU	histidine
UGG	tryptophan
UGC	cysteine

(a)

For P, Q and R, state the codon and nucleotide substitution. Explain why these differences are SNPs.

[4]

(b)

For each variant, state the amino acid encoded at the changed codon and whether this amino acid differs from that in the reference sequence.

[6]

(c)

Explain why the substitution in P does not change the amino acid.

[3]

(d)

A student concludes that variants Q and R must produce a different phenotype from the reference individuals. Evaluate this conclusion using the information given.

[3]

Question 5

Paper 2Medium16 marks

Figure 1 shows possible chromosome outcomes in a eukaryotic species with a diploid chromosome number of 2n = 4. The parent nucleus has completed DNA replication before either pathway begins. Chromosomes of the same length are homologous; solid and dashed lines distinguish the two homologues in each pair. The central dot on each chromosome represents a centromere. Crossing-over is not shown.

(a)

State the number of chromosomes and the number of chromatids in the replicated parent nucleus in Figure 1.

[2]

(b)

Using Figure 1, compare the number of nuclei formed and the chromosome number per nucleus after mitosis and meiosis.

[4]

(c)

Explain the role of DNA replication before mitosis and meiosis, referring to chromatids and their separation during these processes.

[4]

(d)

Explain two processes by which meiosis can generate genetically different nuclei. Explain why nuclear division should be completed before cytokinesis.

[6]

Question 6

Paper 2Medium3 marks

Two individual plants, A and B, are of the same species. An insect visits flowers on both plants. The table shows when pollen is available and when stigmas are receptive.

Plant	Pollen available	Stigmas receptive
A	Days 1–2	Days 4–5
B	Days 4–5	Days 1–2

(a)

State which plant’s pollen could be transferred to a receptive stigma on day 1.

[1]

(b)

Explain how the different maturation times and one feature of an insect-pollinated flower promote cross-pollination between the plants.

[2]

Question 7

Paper 2Medium16 marks

In a beetle species, allele A gives a dark body and allele a gives a pale body. Allele B gives long antennae and allele b gives short antennae. A and B are completely dominant to a and b, respectively.

A homozygous dark-bodied, long-antennaed beetle (AABB) is crossed with a homozygous pale-bodied, short-antennaed beetle (aabb). An F₁ beetle is testcrossed with an aabb beetle. The following offspring are recorded.

Offspring phenotype	Number
Dark body, long antennae	416
Pale body, short antennae	404
Dark body, short antennae	92
Pale body, long antennae	88

(a)

State the genotype of the F₁ beetle and the arrangement of its alleles on its homologous chromosomes.

[2]

(b)

Assuming the genes assort independently, determine the gamete types produced by the F₁ beetle and the expected offspring genotypes and phenotypes from the testcross. State the expected phenotypic ratio.

[6]

(c)

Using the observed offspring, identify the recombinant gamete types from the F₁ beetle and their corresponding offspring genotypes and phenotypes.

[4]

(d)

Identify the parental offspring phenotype and genotype classes. Compare the observed results with the expected outcome if the genes assort independently, and infer whether the genes are linked.

[4]

Question 8

Paper 2Medium16 marks

In a moth population, wing pattern is controlled by two alleles, B and b. The B allele gives dark wings and is dominant; moths with genotype bb have pale wings. Treat the counts below as the complete populations, so ignore sampling error.

Generation	Population information
1	1000 moths; 160 have pale wings
2	1000 moths; 400 BB, 400 Bb and 200 bb

Assume generation 1 is in genetic equilibrium. Allele counting in generation 2 gives p = 0.60 and q = 0.40. Records of the mating pairs that produced generation 2 show that moths preferentially mated with partners having the same wing pattern.

(a)

State what p and q represent for this gene and give their relationship.

[2]

(b)

Calculate p and q for generation 1, and the expected frequencies of BB, Bb and bb.

[5]

(c)

Calculate the observed genotype frequencies in generation 2. Determine whether they fit the Hardy–Weinberg equation for the allele frequencies given.

[4]

(d)

Explain how the mating pattern could account for the genotype frequencies in generation 2.

[2]

(e)

State three conditions, other than random mating, required for a population to remain in genetic equilibrium.

[3]

Question 9

Paper 2Medium16 marks

A kelp-forest survey recorded the following data. Sea otters feed on sea urchins, which graze on kelp. Kelp forests provide food and habitat for many species.

Year	Sea otters counted along a 10 km coastline	Sea urchins (individuals m⁻²)	Kelp cover (%)
2018	40	8	82
2021	29	15	63
2024	16	24	43

Managers also estimated annual replacement and harvest for two renewable resources. Annual replacement is the number of individuals entering the harvestable stock during the year through growth or recruitment.

Resource	Annual replacement	Annual harvest
Scots pine, a terrestrial plant	150 trees year⁻¹	180 trees year⁻¹
Atlantic cod, a marine fish	30 000 fish year⁻¹	24 000 fish year⁻¹

(a)

Describe the patterns in the sea otter, sea urchin and kelp data between 2018 and 2024.

[3]

(b)

Using the data and feeding relationships, explain how a decline in sea otters could affect ecosystem stability and support identifying sea otters as a keystone species.

[4]

(c)

Calculate the percentage of annual replacement harvested for each resource. Show your working.

[4]

(d)

Using your results from (c), assess whether harvesting meets the sustainability criterion for each resource in the year shown.

[3]

(e)

Explain why the results for one year cannot by themselves establish whether each harvest is sustainable over the long term.

[2]

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