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 ✔
Question
Describe the changes that occur in gene pools during speciation. [5]
▶️Answer/Explanation
Ans
a gene pool is all genes/alleles in an (interbreeding) population;
b gene pool splits/divides/separated during speciation;
c due to reproductive isolation (of groups within a species);
d temporal/behavioral/geographic isolation (can cause reproductive isolation);
e divergence of gene pools;
f allele frequencies change;
g natural selection different (in the isolated groups so there is divergence);
h different (random) mutations occur (in the isolated populations so there is divergence);
i speciation has occurred when differences between populations prevent interbreeding;
Question
Outline how reproductive isolation can occur in an animal population.
Describe the different cell types in the seminiferous tubules that are involved in the process of spermatogenesis.
Explain the roles of specific hormones in the menstrual cycle, including positive and negative feedback mechanisms.
▶️Answer/Explanation
Markscheme
a. can be sympatric or allopatric
b. temporal isolation by members of difference populations reproducing at different times OWTTE
c. behavioural isolation by difference in courtship behaviours OWTTE
d. geographic isolation by a population being separated by river/mountain/barrier to contact
An example of a geographic barrier is required.
e. polyploidy
a. spermatogonia «2n» are undifferentiated germ cells OWTTE
b. spermatogonia mature and divide «by mitosis» into primary spermatocytes «2n»
c. primary spermatocytes divide by meiosis I into secondary spermatocytes «1n»
d. secondary spermatocytes divide by meiosis II into spermatids «1n»
e. spermatids differentiate/mature into spermatozoa/sperm
f. Sertoli/nurse cells provide nourishment/support to these developing cells
g. Leydig/interstitial cells produce testosterone
a. anterior pituitary/hypophysis secretes FSH which stimulates ovary for follicles to develop
b. follicles secrete estrogen
c. estrogen stimulates more FSH receptors on follicle cells so respond more to FSH
d. increased estrogen results in positive feedback on «anterior» pituitary
e. estrogen stimulates LH secretion
f. estrogen promotes development of endometrium/uterine lining
g. LH levels increase and cause ovulation
h. LH results in negative feedback on follicle cells/estrogen production
i. LH causes follicle to develop into corpus luteum
OR
follicle cells produce more progesterone
j. progesterone thickens the uterus lining
k. high progesterone results in negative feedback on pituitary/prevents FSH/LH secretion
l. progesterone levels drop and allow FSH secretion
m. falling progesterone leads to menstruation/degradation of uterine lining
Award [5 max] if no reference to feedback is made.