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Question 19N.3.hl.TZ0.13

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Date November 2019 Marks available [Maximum mark: 3] Reference code 19N.3.hl.TZ0.13
Level hl Paper 3 Time zone TZ0
Command term Explain, List Question number 13 Adapted from N/A
13.
[Maximum mark: 3]
19N.3.hl.TZ0.13

DNA, deoxyribonucleic acid, is made up of nucleotides.

(a)

List two components of nucleotides.

[1]

Markscheme

Any two correct for [1]:
pentose «sugar»
OR
deoxyribose ✔

phosphate/phosphato «group»/residue of phosphoric acid ✔

NOTE: Accept “−OPO32−/−OPO3H−/−OPO3H2” but not “PO43−”.

«organic» nitrogenous base
OR
nucleobase
OR
nucleic base
OR
purine
OR
pyrimidine ✔

NOTE: Accept the four bases together: “adenine/A, guanine/G, cytosine/C, thymine/T”.
Accept names or formulas.

(b)

Explain how the double-helical structure of DNA is stabilized once formed.

[2]

Markscheme

Any two of:
H-bonding between bases in each pair ✔
hydrophobic interactions/π-stacking between bases ✔
polar/charged/hydrophilic groups in sugar-phosphate backbone interactions with aqueous solution/water
OR
H-bonding AND ion-dipole interactions between phosphato «groups» andwater/histones ✔

Accept "phosphate groups are hydrophilic and form H-bonds with water".
Accept “H-bonding with histones”.

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