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Date	May 2019	Marks available	7	Reference code	19M.2.HL.TZ1.7
Level	Higher level	Paper	Paper 2	Time zone	Time zone 1
Command term	Explain	Question number	7	Adapted from	N/A

Question

Outline the roles of helicase and ligase in DNA replication.

[4]

Explain how natural selection can lead to speciation.

[7]

Outline the features of ecosystems that make them sustainable.

[4]

Markscheme

helicase:

a. unwinds/uncoils the DNA «double helix» ✔

b. breaks hydrogen bonds «between bases» ✔

c. separates the «two» strands/unzips the DNA/creates replication fork ✔

ligase:

d. seals nicks/forms a continuous «sugar-phosphate» backbone/strand ✔

e. makes sugar-phosphate bonds/covalent bonds between adjacent nucleotides ✔

f. after «RNA» primers are removed/where an «RNA» primer was replaced by DNA ✔

g. «helps to» join Okazaki fragments ✔

a. variation is required for natural selection/evolution/variation in species/populations ✔

b. mutation/meiosis/sexual reproduction is a source of variation ✔

c. competition/more offspring than the environment can support ✔

d. adaptations make individuals suited to their environment/way of life ✔

e. survival of better adapted «individuals)/survival of fittest/converse ✔

f. inheritance of traits/passing on genes of better adapted «individuals»
OR
reproduction/more reproduction of better adapted/fittest «individuals» ✔

g. speciation is formation of a new species/splitting of a species/one population becoming a separate species ✔

h. reproductive isolation of separated populations ✔

i. geographic isolation «of populations can lead to speciation» ✔

j. temporal/behavioral isolation «of populations can lead to speciation» ✔

k. disruptive selection/differences in selection «between populations can lead to speciation» ✔

l. gradual divergence of populations due to natural selection/due to differences in environment ✔

m. changes in the gene pools «of separated populations»/separation of gene pools ✔

n. interbreeding becomes impossible/no fertile offspring «so speciation has happened» ✔

a. recycling of nutrients/elements/components/materials ✔

b. carbon/nitrogen/another example of recycled nutrient/element ✔

c. decomposers/saprotrophs break down organic matter/release «inorganic» nutrients ✔

d. energy supplied by the sun
OR
energy cannot be recycled «so ongoing supply is needed»
OR
energy is lost from ecosystems as heat ✔

e. energy flow along food chains/through food web/through trophic levels ✔

f. photosynthesis/autotrophs make foods/trap energy
OR
autotrophs supply the food that supports primary consumers ✔

g. oxygen «for aerobic respiration» released by autotrophs/photosynthesis/plants ✔

h. carbon dioxide «for photosynthesis» released by respiration ✔

i. populations limited by food supply/predator-prey/interactions/competition
OR
populations regulated by negative feedback
OR
fewer/less of each successive trophic level «along the food chain»/OWTTE ✔

j. supplies of water from rainfall/precipitation/rivers/water cycle ✔

Examiners report

This was generally well answered, with most candidates knowing at least something of the roles of these two enzymes. Most candidates knew that ligase connects Okazaki fragments but some claimed that it creates hydrogen bonds between nucleotides on template and the new strand. Many candidates did not distinguish between unwinding of DNA and separating the strands. Two details that should be more widely known are that helicase separates the two strands of a DNA molecule by encouraging the breakage of hydrogen bonds between bases and that ligase seals nicks by making sugar phosphate bonds.

Most candidates think they understand evolution by natural selection but many do not. Here the focus was on speciation - the splitting of a species into two or more species. Often answers described the evolution of one species over time, rather than speciation itself. An idea central to natural selection that was frequently missing from an answer is adaptation or fitness. Often traits were referred to as ‘favourable’ and therefore likely to lead to survival and reproduction but there is a circularity of argument there. Survival depends on traits fitting the environment, hence being an adaptation to it. The mostly common ideas seen in answers were differential survival and reproduction, due to differences in traits. A common fault was to confuse individuals and species and to refer to a whole species surviving and reproducing more successfully than another species.

There were some vague answers to this question but also some impressive ones that explained ecological processes including nutrient recycling, energy flow and regulation of population sizes.

Syllabus sections

Core » Topic 5: Evolution and biodiversity » 5.2 Natural selection

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Additional higher level » Topic 10: Genetics and evolution » 10.3 Gene pools and speciation

Additional higher level » Topic 10: Genetics and evolution

Core » Topic 5: Evolution and biodiversity

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