3.3 DNA structure – summary of past mark schemes
3.3.1
Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and phosphate.
Mark Scheme
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
K.
L.
M.
N.
3.3.5
subunits are nucleotides;
one base, one deoxyribose and one phosphate in each nucleotide;
description / diagram showing base linked to deoxyribose C1 and phosphate to C5;
four different bases – adenine, cytosine, guanine and thymine;
nucleotides linked up with sugar-phosphate bonds;
covalent / phosphodiester bonds;
two strands (of nucleotides) linked together;
base to base;
A to T and G to C;
hydrogen bonds between bases;
antiparallel strands;
double helix drawn or described;
3’  5’ links between nucleotides;
two bonds between A-T and 3 between G-C;
Draw and label a simple diagram of the molecular structure of DNA.
Mark Scheme
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
K.
L.
labelled phosphate linked to labelled sugar by a single bond;
labelled base linked to a labelled sugar by a single bond;
correctly linked base, sugar and phosphate labelled as nucleotide;
bases toward center;
A—T, G—C base pair;
twisted ladder;
two polynucleotides (two strands shown);
covalent bond(s) labelled between sugar and phosphate / between sugar and base;
at least two nucleotides linked by a single sugar-phosphate bond;
at least two nucleotides linked by base-base bonds;
hydrogen bonds labelled between bases;
phosphate and base shown linked to correct carbon atoms on sugar;