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Date	May 2019	Marks available	7	Reference code	19M.2.SL.TZ1.7
Level	Standard level	Paper	Paper 2	Time zone	Time zone 1
Command term	Explain	Question number	7	Adapted from	N/A

Question

Outline the stages in the production of mRNA by transcription.

[4]

Describe the functions of proteins in cell membranes.

[4]

Explain how natural selection can lead to speciation.

[7]

Markscheme

a. DNA is unwound/strands are separated «by RNA polymerase» ✔

b. new nucleotides attached to template strand «by RNA polymerase» ✔

c. complementary base pairing/base pairing with an example
OR
adenine with thymine/uracil with adenine/cytosine with guanine/guanine with cytosine ✔

d. mRNA detaches from template ✔

e. DNA rewinds ✔

a. facilitated diffusion by channel proteins ✔

b. active transport by protein pumps
OR
protein pumps eg sodium-potassium ✔

c. cell recognition by glycoproteins/protein receptors ✔

d. communication/receptors for hormones/signal molecules ✔

e. cell adhesion ✔

f. allow up to one additional mark for AHL material ✔

a. natural selection is caused by selection pressures in the environment
OR
example of a selection pressure ✔

b. natural selection requires that variation exists within a species ✔

c. variation arises randomly due to mutation
OR
variation is enhanced by meiosis/sexual reproduction ✔

d. over-production of offspring promotes selection
OR
natural selection occurs when there is competition/overpopulation/predators/environmental changes/changes in selection pressures ✔

e. well adapted individuals/individuals with best variations survive to reproduce/survival of fittest ✔

f. «frequency of» genes/alleles conferring an advantage are selected for
OR
genes/alleles conferring a disadvantage are selected against ✔

g. genetic divergence/difference increases
OR
natural selection «genetically» isolates members of a species so eventually they can no longer produce fertile offspring ✔

h. genetic divergence» leads to reproductive isolation ✔

i. geographical/behavioural/ecological factors may lead to «reproductive» isolation ✔

j. prolonged «reproductive» isolation leads to speciation ✔

k. up to one additional mark for AHL information ✔

Examiners report

Almost all students had a clear idea of complementary base pairing and that A pairs with U in RNA. However, this question revealed weakness in the understanding of transcription and the ability to distinguish transcription from DNA replication and RNA translation. Often times candidates had hybrid answers containing information from at least two, but sometimes all three processes. Many candidates were able to gain a couple of the easier marks in 7a on transcription despite having a poor understanding of the process.

A wide variety of fairly well-developed answers. Often, they did not couple a type of protein with its function or missed marks by being too general. Saying “transport proteins” was not enough. Often the terms integral and peripheral proteins were mentioned without going beyond that. Channel proteins and facilitated diffusion was most commonly given. The mistaken idea that proteins offer structural support or strengthen the cell membrane was seen repeatedly.

There were some excellent explanations given in well-structured responses. Many candidates avoided talking about natural selection in genetics terms, losing marks in the process. Darwin’s finches, industrial melanism and the giraffe were common examples. Some students invented their own creative examples.

The idea that natural selection limits biodiversity appeared a few times. The flawed case of the peppered moth was given to exemplify speciation, however Darwin's finches were the most frequent example.

Syllabus sections

Core » Topic 5: Evolution and biodiversity » 5.2 Natural selection

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