IB Biology HL Theme D: Continuity and change questions

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.

Answer: A. Transcription uses a gene’s DNA sequence to synthesize mRNA. If transcription of gene X is switched off, new mRNA from that gene is not produced; translation uses mRNA to synthesize a polypeptide.
B: confuses transcription with translation by using DNA directly as the template for polypeptide synthesis.
C: confuses gene expression with DNA replication; switching off transcription does not itself prevent DNA copying.
D: confuses transcriptional control with translational control; existing mRNA can still be translated.
Remember: transcription makes mRNA; translation makes a polypeptide from mRNA.

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.

Answer: D. Nondisjunction is the failure of chromosomes or chromatids to separate during meiosis. This can produce a gamete with two copies of chromosome 21; fusion with a gamete containing one copy gives a zygote with three copies, associated with Down syndrome. A: crossing over exchanges chromosome segments but does not produce an extra copy of chromosome 21. B: a mutation changes DNA sequence, not the number of chromosome 21 copies. C: normal separation gives each gamete one copy of chromosome 21. Remember: nondisjunction can change the number of chromosomes in a gamete.

Answer: C. Cytosine methylation in a promoter represses transcription of the downstream gene. In contrast, methylation of histone amino acids in nucleosomes can either repress or activate transcription. A: Extends the two possible effects of histone methylation to promoter cytosine methylation. B: Reverses the effect of promoter cytosine methylation, although its histone statement is correct. D: Treats histone methylation as invariably repressive, excluding its possible activating effect. Remember: the effect of methylation depends on whether the tag is on promoter DNA or histones.

Answer: A. Water entering cells causes them to swell, and cells without cell walls can burst in a hypotonic medium. The greatest increase in volume is in P, at 130% of the initial volume, so this medium gives the greatest risk of bursting.

B: recognizes swelling in Q but overlooks that its volume increase is smaller than in P.
C: treats unchanged volume as swelling; it indicates no net change in cell volume.
D: reverses the direction of water movement: the decrease in volume indicates shrinkage, not swelling.

Remember: water entry swells cells without walls; water loss shrinks them.

Answer: C. Asexual reproduction produces genetically identical offspring, so an adaptation suited to the existing environment can be retained as the population increases.

A: confuses fertilization with cloning; fusion of gametes combines alleles from two gametes.
B: assigns meiosis to asexual reproduction; meiosis occurs in sexual life cycles and breaks up parental allele combinations.
D: reverses the role of meiosis, which breaks up rather than preserves parental allele combinations.
Remember: asexual reproduction retains successful combinations, while sexual reproduction generates new combinations that can contribute to variation.

Answer: B. Each zygote combines one allele from a pollen gamete with one allele from an ovule gamete. Two of the four equally likely grid outcomes are Yy, so the fraction is 2/4.

A: counts only one Yy cell, missing the second combination of these alleles.
C: counts the three cells containing at least one Y, rather than only the heterozygous cells.
D: treats every fertilization as producing a heterozygote, overlooking the YY and yy cells.
Remember: each gamete contributes one allele to the zygote.

Answer: A. Sweat released onto the skin can evaporate, removing heat from the body and helping lower its temperature.

B: Vasodilation increases heat transfer to the skin but does not provide the water for evaporation.
C: Shivering contractions generate heat rather than directly increasing evaporative heat loss.
D: Hair erection helps retain an insulating layer of air, reducing heat loss.

Remember: sweating promotes evaporative cooling, whereas vasodilation promotes heat transfer to the skin.

Answer: B. Sexual reproduction combines alleles from the parents, creating new combinations of alleles already present. The alleles themselves need not be new for offspring to show a new combination.

A: confuses a new combination of alleles with a new allele, which mutation can produce.
C: treats a change in allele frequency through natural selection as the source of the offspring’s combination.
D: selects transmission from one parent, rather than the combination of alleles from two parents.

Remember: mutation generates new alleles; sexual reproduction generates new combinations of alleles.

Answer: A. All four consecutive intervals are identified as forest, spanning four million years. Fewer than 10 groups are shared at each transition, so species-group composition changed while the ecosystem type remained forest.
B: Confuses the same number of groups in each interval with the same groups; the overlap values are below 10.
C: Uses species-group turnover to infer ecosystem-type change, although every interval is identified as forest.
D: Stops at the most recent two intervals and overlooks the forest classifications in the earlier intervals.
Remember: continuity of ecosystem type does not require identical species composition.

Answer: A. Increased water temperature is a cause of coral bleaching.

B: Confuses the effect of warming with ocean acidification, which is caused by increased carbon dioxide. C: Selects suppressed calcification, which is associated with increased carbon dioxide rather than warming. D: Gives an ecosystem-level consequence of coral loss, not the effect indicated here. Remember: warming causes bleaching; increased carbon dioxide causes ocean acidification and suppresses calcification.