Question
The confirmation of DNA as the genetic material, together with the elucidation of its structure by Crick and Watson, had a profound effect on modern biology.
(a) Outline the reason that DNA replication is described as semi-conservative.
(b) Describe how genetic variation is brought about by sexual reproduction.
(c) Explain how isolation leads to speciation.
▶️Answer/Explanation
(a) a. replication is forming new DNA/strands from existing DNA/strands;
b. nucleotides added to exposed bases/to exposed strand/to bases on the template strand;
c. complementary base pairing (between free nucleotides and template/existing
strands);
d. both strands/lagging and leading strands of the (parent) DNA act as templates/OWTTE;
e. one newly synthesized strand and one original/conserved strand (in daughter molecules);
(b) a. meiosis produces haploid (nuclei)/gametes
OR
meiosis breaks up combinations of genes/alleles / segregation of alleles during meiosis;
b. crossing over/crossover/chiasmata formation;
c. alleles/DNA/genetic material/genes exchanged between homologous chromosomes (by crossing over)
OR
recombination / recombinants / chromosome with new allele combination (by crossing over);
d. random orientation of bivalents/pairs of homologous chromosomes (in metaphase I)
OR
random/independent assortment of chromosomes;
e. different combinations of chromosomes (possible because of random orientation);
f. alleles/genes/DNA from two different parents brought together during fertilization;
g. random fusion of gametes/female gamete could be fertilized by any of the male gametes
OR
random mating / any female can potentially mate with any male;
(c) a. speciation is a new species forming from an existing species/splitting of a
species;
b. speciation depends on reproductive isolation/no interbreeding between populations;
c. gene pools of populations are separate/no gene flow between populations;
d. temporal isolation if two populations breed at different times/valid example;
e. behavioral isolation if breeding prevented by differences in behaviour/valid example;
f. geographical isolation if populations live in different areas / are separated by a physical barrier/valid example;
g. differences in mutations/genetic drift between isolated populations;
h. differences between environments cause different selection pressures/natural selection;
i. allele frequencies of isolated populations diverge;
j. evolutionary divergence/different evolutionary paths/different adaptation;
k. populations have become separate species when they are too different for successful interbreeding/for interbreeding to produce fertile offspring
OR
polyploidy can cause (instant) reproductive isolation;
Question
a.i. Distinguish between the structure of amylose and the structure of amylopectin. [1]
a.ii. Suggest the reason for cellulose passing undigested through the human gut. [1]
b. Draw an annotated diagram to show how a peptide bond is formed. [3]
c. State two structural features that differ between RNA and DNA. [2]
▶️Answer/Explanation
Markscheme
amylose unbranched/helical while amylopectin branched / vice versa
enzymes required to digest cellulose not present in the human gut
OR
undigested cellulose provides bulk/fibre
a. correct structure of two amino acids
b. H2O lost
c. C from COOH of one links to N of NH2/NH3+ from the other
d. correct labelling of the peptide bond
e.g.
a. number of strands
OR
(usually) only one strand in RNA/two strands in DNA
b. base composition
OR
uracil only in RNA / thymine only in DNA
c. type of pentose
OR
ribose only in RNA / deoxyribose only in DNA
Question
The diagram shows a nucleosome from the nucleus of a eukaryotic cell.
(a) Identify the protein labelled in the diagram. [1]
(b) Outline how nucleosomes affect the transcription of DNA. [1]
The image shows the regulation of the gene responsible for producing lactase.
(c) Identify:
(i) X, the enzyme which copies a DNA sequence [1]
(ii) Y, non-coding DNA at the start of a gene. [1]
(d) Explain the role of lactose in the expression of the gene for lactase production. [3]
(e) State one reason that identical twins may show different methylation patterns as they grow older. [1]
Answer/Explanation
a histone; 1
b a. (nucleosomes can) promote AND inhibit transcription of genes/expression of genes;
b. (nucleosomes can) prevent transcription by (tight) condensation/supercoiling/packing of DNA;
c. (nucleosomes can) allow/prevent binding of RNA polymerase/transcription factors;
d. tagging/acetylation/methylation of nucleosomes/histones can promote/inhibit transcription;
e. movement of histones/nucleosomes (along DNA) can affect which genes are transcribed;
c i RNA polymerase;
c ii promoter;
d a. lactose binds to repressor protein;
b. repressor protein (with lactose bound) cannot block/bind to the promoter/Y;
c. RNA polymerase/X binds to the promotor/transcribes the gene;
d. lactase produced (if lactose present)/lactase production inhibited if lactose absent;
e (different) environment/illness/disease/diet;