Question
Antibiotics such as penicillin are secreted by fungi and other microorganisms in soil. By secreting them, a microorganism can inhibit the growth of a competitor. In research published in 2014, nearly 3000 antibiotic resistance genes were discovered in soil microorganisms, giving resistance to 18 different antibiotics. The types of antibiotic resistance gene varied between soil types.
(a) Explain how natural selection could increase the prevalence of an antibiotic resistance gene in a species of soil bacterium.
(b) (i) There are viruses in soils that are pathogens of animals. Outline a reason for antibiotics in soil not eliminating these viruses.
(ii) Explain the reasons for antibiotics secreted into soil not harming insects or other animals in the soil.
Answer/Explanation
Answer:
(a)
a. if antibiotic is in the environment/soil there is selection (pressure);
b. bacteria without resistance (gene) die / converse;
c. bacteria with resistance (gene) reproduce
OR
bacteria exchange/obtain resistance genes using plasmids/by conjugation;
d. offspring inherit (the gene for) resistance/resistance passed on (to offspring);
(b)
(i)
a. viruses lack metabolism/plasma membranes/cell walls / viruses are not living organisms;
b. antibiotics kill/target bacteria/prokaryotes but not viruses;
c. viral pathogens of animals use animal metabolism (which is not affected by antibiotics);
(ii)
a. antibiotics (only) affect/kill/block processes in bacteria/prokaryotes / do not affect eukaryotes;
b. metabolism/protein synthesis/ribosomes/cell walls not targeted in insects/animals/eukaryotes;
c. no cell walls in animals (so antibiotics cannot attack cell walls);
d. antibiotics are secreted (by microbes/fungi) to prevent competition (with other microbes);
Question
a. List two causes of variation within a gene pool.[2]
b. Describe how variation contributes to evolution by natural selection.[3]
c. Outline what is required for speciation to occur.[3]
▶️Answer/Explanation
Ans:
a. a. sexual reproduction / random fertilization / meiosis
b. mutation
No mark for crossing over unqualified.
Reject natural selection/evolution as causes of variation.
b. a. (variation is) different phenotypes/differences between individuals in a population/species
b. struggle/competition for survival
c. some individuals have advantageous characteristics/are better adapted/have greater chance of survival/reproduction (than others)
d. favourable alleles/genetic variations passed on/inherited by offspring/next generation Reject “pass on phenotypes”.
c. a. divided species/gene pool / part of species/gene pool becomes separated / species splits into separate populations
b. reproductive isolation / lack of interbreeding
Mark point \(b\) refers to a lack of interbreeding between separated populations in a species, not the lack of interbreeding after speciation.
c. may be due temporal/behavioural/geographic isolation
d. different natural selection/different selective pressures
Question
- (a) Outline the roles of helicase and ligase in DNA replication. [4]
- (b) Explain how natural selection can lead to speciation. [7]
- (c) Outline the features of ecosystems that make them sustainable
Answer/Explanation
Ans:
a
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 ✔
b
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» ✔
c
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 ✔
energy flow along food chains/through food web/through trophic levels ✔ 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 ✔