Continuity and changeIB Biology HL: Topic test

20 questions, 54 marks

IB Biology HL

Continuity and change topic test

Total 54 marks

Name

Class

Date

1
A molecular biologist sets up a PCR reaction to amplify a 500 base pair region of a gene from a small blood sample. The reaction mixture contains the DNA template, two short primers complementary to the ends of the target region, free nucleotides, and Taq polymerase, and is cycled repeatedly between 95 °C, 55 °C and 72 °C. After 30 cycles, the amplified DNA is loaded into wells of an agarose gel alongside DNA of known fragment sizes, and an electric current is applied.
(a)
What is the purpose of heating the reaction mixture to 95 °C in each cycle?
[1 mark]
ATo denature the double-stranded DNA, breaking hydrogen bonds and separating the two strands
BTo activate Taq polymerase for the first time only
CTo allow the primers to anneal to the template
DTo synthesise new DNA strands
(b)
In which direction does DNA polymerase add nucleotides to a growing DNA strand during each cycle?
[1 mark]
AFrom the 3' end of the new strand towards its 5' end
BFrom the 5' end of an incoming nucleotide, added to the 3' end of the existing strand
CIn either direction depending on the strand
DRandomly, wherever a gap exists
(c)
Explain how gel electrophoresis separates the amplified DNA fragments according to size, once the electric current is applied.
[2 marks]
Total for question 1: 4 marks
2
A short section of the coding (template) strand of a gene reads 3'-TAC GGA CTT-5'. Transcription and translation of this section produces the tripeptide methionine-proline-glutamate. A patient's version of the same gene has a single base substitution at the fourth base of the template strand, counting from the left, changing this section to 3'-TAC TGA CTT-5', which is translated instead to methionine-threonine-glutamate.
(a)
What is the mRNA base sequence transcribed from the original (unmutated) template strand shown, written 5' to 3'?
[1 mark]
A5'-UAC GGA CUU-3'
B5'-AAG UCC GUA-3'
C5'-AUG CCU GAA-3'
D5'-TAC GGA CTT-3'
(b)
Which term describes the change from proline to threonine caused by the substitution?
[1 mark]
AA silent mutation, since the sequence is a single-nucleotide polymorphism
BA frameshift mutation, since a nucleotide was inserted
CA nonsense mutation, since a stop codon is introduced
DA missense mutation, since the substitution changes the amino acid coded by the mRNA codon
(c)
Explain, using the concept of degeneracy of the genetic code, why some base substitutions do not change the amino acid sequence of a polypeptide, even though this particular substitution did change proline to threonine.
[2 marks]
Total for question 2: 4 marks
3
A sample of cells from a growing plant root tip is stained and examined under a microscope. Of 400 cells counted, 360 are in interphase, 24 show condensed chromosomes aligned individually without a nuclear membrane, 8 show chromosomes separating to opposite poles, and 8 show two separate groups of chromosomes at opposite poles of the cell with a new cell wall forming between them. Researchers also note that a mutation has occurred in a tumour suppressor gene in one of these root cells.
(a)
Using the description, calculate the mitotic index of this sample, and identify which phase of mitosis is represented by the 8 cells showing chromosomes separating to opposite poles.
[3 marks]
(b)
Explain how a mutation in a tumour suppressor gene in a root cell could lead to uncontrolled cell division, and explain why this represents a loss rather than a gain of gene function.
[4 marks]
Total for question 3: 7 marks
4
In a large, randomly mating population of a beetle species, wing colour is controlled by a single autosomal gene with two alleles: allele W (dark wings) is dominant to allele w (pale wings). A survey of 1000 beetles finds that 160 have the pale-winged phenotype, and the rest are dark-winged. Pale wings make the beetle much more visible to bird predators after a nearby forest fire blackens the local tree bark, and over the following 10 generations the proportion of pale-winged beetles falls sharply.
(a)
Using the Hardy–Weinberg equation, calculate the frequency of the w allele in the original population of 1000 beetles, and use this to calculate the frequency of heterozygous (Ww) beetles in that population.
[6 marks]
(b)
Explain, in terms of natural selection and allele frequency, why the proportion of pale-winged beetles falls sharply over the 10 generations following the forest fire.
[6 marks]
Total for question 4: 12 marks
5
Researchers use the CRISPR-Cas9 system to investigate the function of a gene, gene K, suspected to be involved in wound healing in mice. They design a guide RNA sequence complementary to a specific site within gene K, which directs the Cas9 enzyme to cut the DNA at that site in mouse embryos. Small insertions and deletions occur during the cell's repair of this cut, disrupting the gene's reading frame in the resulting mice, and these 'knockout' mice heal wounds significantly more slowly than normal mice.
(a)
What is the purpose of the guide RNA in this CRISPR-Cas9 system?
[1 mark]
ATo direct the Cas9 enzyme to a specific target sequence in the DNA by complementary base pairing
BTo provide the energy needed for the Cas9 enzyme to cut DNA
CTo replace the damaged DNA with a corrected sequence
DTo translate the edited gene into a functional protein
(b)
Why do small insertions or deletions at the cut site disrupt the reading frame and inactivate gene K?
[1 mark]
AThey change the total length of the chromosome, preventing replication
BThey add or remove a number of bases that is not a multiple of three, shifting how the sequence is read into codons downstream of the cut
CThey prevent RNA polymerase from binding to the promoter
DThey convert the gene into a pseudogene with no promoter
(c)
Explain why the knockout mice's slower wound healing provides evidence for a specific function of gene K, and outline one limitation of drawing this conclusion from a single knockout experiment.
[2 marks]
Total for question 5: 4 marks
6
A person exercises vigorously in a hot environment. Blood glucose concentration is monitored and stays close to 5 mmol dm⁻³ throughout, despite muscles using glucose rapidly. Skin temperature is also measured: as core body temperature rises during exercise, blood vessels in the skin dilate and sweat production increases markedly, and skin temperature falls back towards its resting value within 15 minutes of stopping exercise.
(a)
Which hormone is mainly responsible for raising blood glucose concentration back towards 5 mmol dm⁻³ if it falls during exercise, by stimulating the breakdown of glycogen in the liver?
[1 mark]
AInsulin
BThyroxine
CGlucagon
DAntidiuretic hormone
(b)
Why does dilation of skin blood vessels help to lower core body temperature during exercise?
[1 mark]
AIt reduces blood flow to the skin, conserving heat
BIt increases blood flow near the skin surface, increasing heat loss to the surrounding air by radiation and convection
CIt increases the rate of shivering, generating heat
DIt reduces sweat gland activity
(c)
Explain, in terms of negative feedback, how skin temperature is returned to its resting value within 15 minutes of stopping exercise.
[2 marks]
Total for question 6: 4 marks
7
Discs of potato tissue of equal size are placed in sucrose solutions of different concentrations for two hours, then reweighed. In 0.0 mol dm⁻³ sucrose (pure water), discs gain 8% in mass; in 0.4 mol dm⁻³ sucrose, discs show no change in mass; in 1.0 mol dm⁻³ sucrose, discs lose 12% in mass. Separately, a biology class studies fertilisation in a flowering plant species, observing pollen grains germinating on a stigma and growing pollen tubes down through the style towards ovules in the ovary.
(a)
Using the results described, explain what happens to the water potential of the potato cells as sucrose concentration increases from 0.0 to 1.0 mol dm⁻³, and identify the approximate solute concentration of the potato cells' cytoplasm.
[3 marks]
(b)
Explain the roles of pollination and the growth of the pollen tube in bringing about fertilisation in this flowering plant species.
[4 marks]
Total for question 7: 7 marks
8
A boreal (taiga) forest ecosystem has accumulated carbon in its trees and soil for thousands of years, acting as a net carbon sink. Over recent decades, the region has experienced warmer temperatures and decreased winter snowfall, leading to more frequent droughts. Researchers observe increased forest browning, more frequent and intense wildfires, and a growing area of the forest converting to shrubland after fires, rather than regenerating as forest.
(a)
Explain how warmer temperatures and decreased snowfall have led to the increased frequency and intensity of wildfires described, and explain why this represents a positive feedback cycle for climate change.
[6 marks]
(b)
Explain why the conversion of burned forest area to shrubland, rather than regeneration as forest, represents a tipping point for the stability of this ecosystem, and outline why this matters for the ecosystem's role as a carbon sink.
[6 marks]
Total for question 8: 12 marks

End of questions