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Date	May 2021	Marks available	5	Reference code	21M.2.HL.TZ2.7
Level	Higher level	Paper	Paper 2	Time zone	Time zone 2
Command term	Describe	Question number	7	Adapted from	N/A

Question

Describe the structure of the DNA molecule.

[5]

Outline the role of three enzymes used in the replication of DNA.

[3]

Insulin is produced in β cells of the pancreas and not in other cells of the human body. Explain how differentiation of cells and regulation of gene expression allow proteins such as insulin to be produced in only certain types of body cell.

[7]

Markscheme

a. two stranded/double helix ✔

b. antiparallel / strands running in opposite directions
OR
one strand organized 5’ to 3’ and the other 3’ to 5’ ✔

c. sugar-phosphate backbone ✔

d. each strand formed by chains of nucleotides ✔

e. each nucleotide is formed by a phosphate, a deoxyribose and a base / annotated diagram of a nucleotide clearly indicated as a nucleotide ✔

f. the bases are adenine, guanine, cytosine and thymine ✔

g. strands held together by hydrogen bonds (between complementary base pairs)
OR
A pairs with T and C pairs with G ✔

Both helix and two strands needed for mp a. Double helix is sufficient for the mark.

Points can be awarded to annotated diagrams.

For mp c, the explicit label sugar phosphate backbone is required.

To award mp d from a diagram, at least three pairs of nucleotides should be shown.

For mp e, the diagram would need to be labelled as a nucleotide.

For mp e, expect deoxyribose not just sugar.

The written names of the bases are required for mp f.

Do not penalize twice for mp f and g for using letters.

If they only ever use the symbols A,T,C and G they are ineligible for mp f. If however, they say A pairs with T and C pairs with G, then they would get mp g. If they wrote adenine pairs with thymine and cytosine pairs with guanine, then they would obtain both mp f and mp g.

a. helicase to separate/unwind DNA strands ✔

b. gyrase / toposiomerase to relax the tension as bacterial DNA is being uncoiled / prevent supercoiling ✔

c. primase to synthesise primers ✔

d. polymerase (I) removes primers and replaces with nucleotide ✔

e. polymerase (III) adds nucleotides (in a 5' to 3' direction) ✔

f. ligase joins (Okazaki) fragments together ✔

Accept the enzyme name without ‘DNA’ included; e.g. ‘DNA ligase’ or ‘ligase’ can both be accepted.

a. insulin production is determined by a gene ✔

b. gene for insulin (is found in all cells), but only activated in (β cells of) pancreas ✔

c. stem cells differentiate into specialized cells/(into pancreatic β) ✔

d. during differentiation some genes are turned on and others off ✔

e. insulin is a hormone that regulates the amount of glucose/sugar in blood ✔

f. pancreatic β cells have sensors that detect glucose level in blood ✔

g. an increase in glucose will increase transcription of mRNA of insulin ✔

h. the site of transcription of insulin is in the pancreatic β cells ✔

i. gene transcription is regulated by proteins that bind to specific base sequence in DNA/ enhancers/silencers/promoter proximal elements ✔

j. regulatory sequences/proteins are specific to the gene they regulate / insulin regulator proteins are only found in in the pancreatic β cells ✔

k. (DNA) methylation (usually) inhibits gene expression / (histone) acetylation promotes gene expression / tightness of coiling of DNA around histones affects gene expression ✔

Accept sugar as equivalent to glucose.

Examiners report

A surprising number of students failed to discuss nucleotides explicitly.

Well answered.

In general, scores for this question were quite low. Candidates were too vague in their answers. Many failed to discuss chemical modification of DNA and the role of non-coding sequences in the regulation of gene expression.

Syllabus sections

Core » Topic 3: Genetics » 3.2 Chromosomes

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