▶️ Answer/Explanation
(a)
- Helicase is an enzyme that unwinds the DNA double helix by breaking the hydrogen bonds between the complementary bases. This separates the two strands, creating a replication fork where DNA replication can begin.
- Ligase is an enzyme that joins DNA fragments by forming covalent bonds between the sugar and phosphate groups of adjacent nucleotides. It is especially important on the lagging strand, where it joins Okazaki fragments after RNA primers are removed and replaced by DNA, making the strand continuous.
(b)
- Natural selection is a process where organisms with beneficial traits survive and reproduce more successfully than others.
- Variation in traits arises from mutations, meiosis, and sexual reproduction.
- Individuals with traits better suited to their environment are more likely to survive and pass on those traits to the next generation. Over time, this causes gradual changes in the population.
- If populations become reproductively isolated (due to geographic, temporal, or behavioral barriers), they may evolve independently.
- Over many generations, these genetic differences can become so significant that the populations can no longer interbreed, even if brought back together. This results in speciation, the formation of a new species.
(c)
- Ecosystems are sustainable when nutrients are recycled through processes like decomposition.
- Decomposers break down dead organisms, releasing inorganic nutrients (e.g., nitrogen, carbon) back into the soil and atmosphere for reuse.
- While energy cannot be recycled, it is continuously supplied by sunlight and captured by autotrophs through photosynthesis.
- Autotrophs provide energy to consumers through food chains.
- Gases like oxygen (from photosynthesis) and carbon dioxide (from respiration) are cycled within the ecosystem.
- Population sizes are regulated by food availability, predation, and competition, maintaining a balance in the ecosystem.
- Water is cycled through processes like precipitation and evaporation, ensuring a steady supply.
Markscheme:
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 ✔
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 ✔
▶️ Answer/Explanation
- A gene pool is the complete set of alleles (forms of a gene) present in an interbreeding population.
- During speciation, the gene pool of a species splits into two or more separate gene pools due to reproductive isolation.
- This isolation can be caused by geographic barriers (like mountains or rivers), behavioral differences (such as mating calls), or temporal separation (different breeding seasons). Once isolated, gene flow stops between the groups.
- Over time, natural selection acts differently on each population depending on their environment, causing them to diverge genetically.
- Random mutations also add new alleles, further increasing differences. These processes cause changes in allele frequencies in each gene pool.
- When the genetic differences become large enough that the populations can no longer interbreed and produce fertile offspring, speciation has occurred.
Markscheme:
a) 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;
▶️ Answer/Explanation
(a)
- Reproductive isolation prevents interbreeding between populations of the same species.
- Can be sympatric (within the same area) or allopatric (caused by physical separation).
- Temporal isolation – occurs when populations reproduce at different times of the year.
- Behavioral isolation – due to differences in courtship behaviors or mating calls.
- Geographic isolation – caused by physical barriers like mountains, rivers, or deserts, which prevent contact between groups.
- Polyploidy – a change in chromosome number (common in plants) can make individuals reproductively incompatible with the original population.
(b)
- Spermatogonia (2n) – undifferentiated germ cells that divide by mitosis to produce more spermatogonia and primary spermatocytes.
- Primary spermatocytes (2n) – undergo meiosis I to form secondary spermatocytes.
- Secondary spermatocytes (1n) – undergo meiosis II to form spermatids.
- Spermatids (1n) – immature sperm cells that differentiate into spermatozoa (sperm).
- Sertoli cells – also known as nurse cells, they support and nourish developing sperm cells and remove waste.
- Leydig (interstitial) cells – found outside the tubules, they produce testosterone, which is essential for spermatogenesis.
(c)
- The anterior pituitary gland releases FSH, which stimulates follicle development in the ovary.
- Follicles secrete estrogen, which:
– Stimulates thickening of the endometrium (uterine lining).
– Increases FSH receptor expression on follicle cells (positive feedback). - Rising estrogen levels stimulate a surge in LH (positive feedback).
- LH surge causes ovulation – the release of an egg from the follicle.
- LH also converts the follicle into the corpus luteum, which secretes progesterone.
- Progesterone:
– Maintains and thickens the endometrium.
– Inhibits FSH and LH secretion (negative feedback) to prevent another ovulation. - If no fertilization occurs:
– Progesterone levels fall, leading to menstruation (shedding of uterine lining).
– Inhibition on FSH is removed, so the cycle restarts.
Markscheme:
a. Reproductive Isolation:
- Can be sympatric or allopatric.
- Temporal isolation occurs when members of different populations reproduce at different times.
- Behavioural isolation occurs due to differences in courtship behaviours.
- Geographic isolation occurs when a population is separated by a river, mountain, or other barrier to contact. (An example of a geographic barrier is required.)
- Polyploidy can also lead to reproductive isolation.
b. Cell Types in Seminiferous Tubules:
- Spermatogonia (2n) are undifferentiated germ cells.
- Spermatogonia mature and divide by mitosis into primary spermatocytes (2n).
- Primary spermatocytes divide by meiosis I into secondary spermatocytes (1n).
- Secondary spermatocytes divide by meiosis II into spermatids (1n).
- Spermatids differentiate/mature into spermatozoa/sperm.
- Sertoli/nurse cells provide nourishment/support to developing cells.
- Leydig/interstitial cells produce testosterone.
c. Hormonal Roles in the Menstrual Cycle:
- Anterior pituitary secretes FSH, which stimulates follicle development in the ovary.
- Follicles secrete estrogen.
- Estrogen stimulates more FSH receptors on follicle cells, increasing responsiveness to FSH.
- Increased estrogen results in positive feedback on the anterior pituitary.
- Estrogen stimulates LH secretion.
- Estrogen promotes development of the endometrium/uterine lining.
- LH levels surge, causing ovulation.
- LH results in negative feedback on follicle cells/estrogen production.
- LH causes the follicle to develop into the corpus luteum, which produces progesterone.
- Progesterone thickens the uterine lining.
- High progesterone results in negative feedback on the pituitary, preventing FSH/LH secretion.
- Progesterone levels drop, allowing FSH secretion to restart the cycle.
- Falling progesterone leads to menstruation/degradation of the uterine lining.
(Award [5 max] if no reference to feedback mechanisms is made.)