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.
Question
The biological insights of Mendel and Darwin in the 19th century remain important to this day.
Discuss the role of genes and chromosomes in determining individual and shared character features of the members of a species.
Outline the process of speciation.
Describe, using one example, how homologous structures provide evidence for evolution.
▶️Answer/Explanation
Markscheme
Genes
a. mutation changes genes/causes genetic differences
b. genes can have more than one allele/multiple alleles
OR
alleles are different forms/versions of a gene
c. different alleles «of a gene» give different characters
OR
variation in alleles between individuals
d. eye colour/other example of «alleles of» a gene affecting a character
e. alleles may be dominant or recessive
OR
dominant alleles determine trait even if recessive allele is present
f. both alleles influence the characteristic with codominance
OR
reference to polygenic inheritance
g. all members of a species are genetically similar/have shared genes
OR
certain genes expressed in all members of a species
h. reference to epigenetics/methylation/acetylation / not all genes are expressed «in an individual»
i. genes are inherited from parents/passed on to offspring/passed from generation to generation
Chromosomes
j. same locus/same position of genes
OR
same sequence of genes/same genes on each chromosome «in a species»
k. same number of chromosomes «in a species»/all humans have 46 chromosomes/differences in chromosome number between species
l. some individuals have an extra chromosome/Down syndrome/other example of aneuploidy
OR
polyploidy divides a species/creates a new species
m. X and Y/sex chromosomes determine the sex/gender of an individual
n. meiosis/independent assortment/fertilization/sexual reproduction give new combinations «of chromosomes/genes»
a. speciation is the splitting of a species «into two species»
b. reproductive isolation/lack of interbreeding
c. isolation due to geography/«reproductive» behavior/«reproductive» timing
d. polyploidy can cause isolation
e. gene pools separated
f. differences in/disruptive selection cause traits/gene pools to change/diverge
g. gradualism / speciation/changes accumulating over long periods
h. punctuated equilibrium / speciation/changes over a short time period
a. similar structure but different function «in homologous structures»
b. pentadactyl limbs/limb with five digits/toes / other example
c. similar bone structure/example of similarity of bones «in pentadactyl limbs» but different uses/functions
d. two examples of use of pentadactyl limb by a vertebrate group
e. suggests a common ancestor «and evolutionary divergence»
f. process called adaptive radiation
Question
List two causes of variation within a gene pool.
Describe how variation contributes to evolution by natural selection.
Outline what is required for speciation to occur.
▶️Answer/Explanation
Markscheme
a. sexual reproduction / random fertilization / meiosis
b. mutation
No mark for crossing over unqualified.
Reject natural selection/evolution as causes of variation.
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”.
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
Evolution causes gene pools to change over time and new species to be formed.
(a) Outline how adaptive radiation provides evidence for evolution. [3]
(b) Describe polyploidy and how it can lead to speciation. [5]
(c) Explain how a newly discovered plant species would be classified and named. [7]
▶️Answer/Explanation
a a. diversification/ different species produced from a common/shared ancestor;
b. homologous features have similarities of structure
c. despite different functions;
d. (different) adaptation to different environments/different selective pressures;
e. pentadactyl limbs/Darwin’s finches/other example of adaptive radiation described correctly;
b a. polyploidy is having more than two (complete) sets of chromosomes/3n/4n/other specific example of polyploidy;
b. can be due to errors in meiosis/production of diploid gametes;
c. can be due to DNA replication without mitosis/cytokinesis;
d. polyploidy causes reproductive isolation;
e. diploids crossed with tetraploids produce infertile (triploid) offspring / triploid offspring are infertile;
f. tetraploids are therefore a new species/failure to interbreed/reproductive isolation leads to speciation;
g. (many) examples in the onion family/Allium/other valid example of speciation by polyploidy;
h. infertile interspecific hybrids can become fertile by becoming polyploid;
c Naming:
a. binomial nomenclature/(plant is) given a binomial/double name;
b. first name is the genus and second name is the species/genus initial upper case and species lower case;
c. names (of plant species) are international/are universally understood/are published in journals;
Classification:
d. study the characteristics/structure/reproduction/chemical properties/DNA (of the plant);
e. put/classify (the plant) in a group/genus with other similar species;
f. natural classification corresponds with evolution/natural classification is based on many features
g. analogous features/features due to convergent evolution should not be used;
h. hierarchy of groups/taxa (in traditional classification/3 or more taxa in correct sequence (kingdom-phylum-class);
i. two or more of bryophyta, filicinophyta, coniferophyta and angiospermophyta named;
j. a clade is a group of organisms evolved from a common ancestor;
k. base sequences/amino acid sequences used to group organisms into clades/deduce evolutionary relationships;
l. cladograms show the relationships between clades/likely evolutionary divergence of clades;
m. each branch point/node represents where species are formed via divergent evolution;
n. species are now classified into a sequence of clades (rather than a rigid hierarchy of taxa);