Question
Cell biologists play an important role in research into disease, fertility, evolution and many other areas of science.
Describe the origin of eukaryotic cells according to the endosymbiotic theory.
Cell biologists play an important role in research into disease, fertility, evolution and many
other areas of science.
Compare and contrast the processes of spermatogenesis and oogenesis.
Cell biologists play an important role in research into disease, fertility, evolution and many
other areas of science.
Outline the evidence for evolution provided by selective breeding.
▶️Answer/Explanation
Markscheme
a. mitochondria and chloroplasts are similar to prokaryotes
b. «host» cell took in another cell by endocytosis/by engulfing «in a vesicle»
Allow “taking in” in place of “engulfing”
c. but did not digest the cell/kept the «ingested» cell alive
OR
symbiotic/mutualistic relationship «between engulfed and host cell»
d. chloroplasts and mitochondria were once independent/free-living «organisms»
e. DNA «loop» in chloroplast/mitochondrion
f. division/binary fission of chloroplast/mitochondrion
g. double membrane around chloroplast/mitochondrion
h. 70s ribosomes «in chloroplast/mitochondrion»
Award up to [2] for evidence from mpe to mph
[Max 4 Marks]
a. both result in haploid cells/gametes
b. both involve mitosis at the start/in the «germinal» epithelium
c. both have cell growth «before meiosis»
d. both involve «two divisions of» meiosis
e. both involve differentiation to produce a gamete
f. both are stimulated by hormones
OR
spermatogenesis stimulated by testosterone and oogenesis stimulated by FSH
A table is not required but both statements in one row of the table must either be explicitly stated or clearly implied to award the mark
[Max 8 Marks]
a. crop plants/domesticated animals/livestock produced by selective breeding
b. specific example of a domesticated animal/crop plant and the wild species from which it
was developed
OR
specific example of a domesticated animal/crop plant and the features in it which have been improved «compared with the wild species»
For example dogs have been developed from wolves
c. artificial selection/crossing selected varieties/eliminating undesirable varieties
d. «selective breeding/artificial selection can cause» significant/rapid change over time/from the original wild species
e. «changes due to selective breeding/artificial selection» shows natural selection can cause change/evolution «in a species»
[Max 3 Marks]
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
Outline the structure and functions of nucleosomes.
Explain how DNA is used to pass on genetic information to offspring accurately but also produce variation in species.
Accurate transmission of base sequences to offspring depends on successful gamete production. Describe how spermatogenesis occurs in humans.
▶️Answer/Explanation
Markscheme
Remember, up to TWO “quality of construction” marks per essay.
a. found in eukaryotes;
b. consists of DNA wrapped around proteins/histones;
c. histones are in an octamer/group of eight;
d. are held together by another histone/protein;
e. in linker region;
f. help to supercoil chromosomes / to facilitate DNA packing;
g. (function is to) regulate transcription / gene expression;
Remember, up to TWO “quality of construction” marks per essay.
a. DNA is replicated/copied semi-conservatively/from a template;
b. mutations can be a source of variation / resulting protein has new or different functions;
c. mutations/changes in the DNA may not result in changes in the amino acid for which the triplet codes;
d. genetic code is redundant;
e. genes occur as paired alleles which can be different;
f. crossing-over occurs;
g. recombines linked alleles producing new combinations;
h. random orientation of bivalents / homologous chromosomes (in metaphase I);
i. large genetic variation in (haploid) gametes / 2n / 223;
j. random recombination of alleles during fertilization (leads to variation);
k. different phenotypes among members of the same population;
l. natural selection may lead to enhanced survival of recombinants;
Remember, up to TWO “quality of construction” marks per essay.
a. germinal cells / spermatogonia undergo mitosis to keep a supply of germinal cells present;
b. some germinal cells / spermatogonia grow larger to become primary spermatocytes;
c. primary spermatocytes go through meiosis I;
d. to form secondary spermatocytes;
e. these secondary spermatocytes go through meiosis II;
f. to produce spermatids;
g. spermatids differentiate/grow a tail and reduce their cytoplasm
h. spermatids associated with nurse cells (Sertoli cells);
i. sperm detach from Sertoli cells and enter lumen of the seminiferous tubule;
j. testosterone stimulates sperm production;
Question
Draw a labelled diagram of the human adult male reproductive system.
Compare the processes of spermatogenesis and oogenesis
Describe the consequences of the potential overproduction of offspring.
▶️Answer/Explanation
Markscheme
a. scrotum – shown around testes;
b. testes/testis/testicles – shown inside scrotum;
c. epididymis – shown adjacent to testis and connected to sperm duct;
d. sperm duct/vas deferens – double line connecting testis/epididymis to urethra;
e. seminal vesicle – sac shown branched off sperm duct (not off the urethra);
f. prostate gland – shown positioned where sperm duct connects with urethra;
g. urethra – shown as double line linking bladder to end of penis;
h. penis – with urethra passing through it;
Award [1] for each structure clearly drawn and labelled that conforms to the italicized guidelines given.
Remember, up to TWO “quality of construction” marks per essay.
a. both produce haploid cells / both produce (mature/male/female) gametes;
b. both have mitosis at start/in epithelium / both involve mitosis and meiosis;
c. both have cell growth before meiosis;
d. both involve differentiation (to produce a specialised gamete);
A table is not required but both statements in one row of the table above must either be explicitly stated or clearly implied for each mark awarded.
Remember, up to TWO “quality of construction” marks per essay.
a. more (offspring) than the environment can support / carrying capacity reached;
b. increased mortality/lower life expectancy/more deaths;
c. competition (for resources) / struggle for survival;
d. food/mates/nest sites/territory/other example of resource shortage / example of greater need;
e. variation between members of population / example of variation;
f. better adapted more likely to survive / converse; (reject Lamarckian statements such as those who adapt survive)
g. better adapted reproduce / pass on (favourable) genes/traits / converse;
h. natural selection / (survival of fittest) leads to evolution;
Question
Draw a labelled diagram of a mature human egg.
Outline a technique used for gene transfer.
Explain how evolution may happen in response to environmental change with evidence from examples.
▶️Answer/Explanation
Markscheme
Award [1] for each structure accurately drawn and correctly labelled.
a. haploid nucleus;
b. cytoplasm – with nucleus-to-membrane distance >4 times nucleus diameter;
c. centrioles – two must be shown but only one needs to be labelled;
d. cortical granules – needs to be drawn in vicinity of plasma membrane;
e. plasma membrane – shown as a single line and approximately circular overall;
f. polar cell / (first) polar body – needs to be drawn outside the egg cell;
g. zona pellucida / layer of gel (outside the cell membrane);
h. follicle cells / corona radiata (outside the cell membrane);
i. size shown as 100 μm/0.1mm; (accept 90 μm to 120 μm)
a. plasmid used for gene transfer/removed from bacteria;
b. plasmid is a small/extra circle of DNA;
c. restriction enzymes/endonucleases cut/cleave DNA (of plasmid);
d. each restriction enzyme cuts at specific base sequence/creates sticky ends;
e. same (restriction) enzyme used to cut DNA with (desired) gene;
f. DNA/gene can be added to the open plasmid/sticky ends join gene and plasmid;
g. (DNA) ligase used to splice/join together/seal nicks;
h. recombinant DNA/plasmids inserted into host cell/bacterium/yeast;
a. variation in population;
b. (variation is) due to mutation/sexual reproduction;
c. valid example of variation in a specific population;
d. more offspring are produced than can survive / populations over-populate;
e. competition / struggle for resources/survival;
f. example of competition/struggle for resources;
g. survival of fittest/best adapted (to the changed environment)/those with beneficial adaptations / converse;
h. example of changed environment and adaptation to it;
i. favourable genes/alleles passed on / best adapted reproduce (more) /converse;
j. example of reproduction of individuals better adapted to changed environment;
k. alleles for adaptations to the changed environment increase in the population;
l. example of genes/alleles for adaptations increasing in a population;
m. evolution by natural selection;
n. evolution is (cumulative) change in population/species over time / change in allele frequency;
Suitable examples are antibiotic resistance and the peppered moth but any genuine evidence-based example of adaptation to environmental change can be credited.
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);