SME
Grade 9 Science · Problem type
Base pairing: Chargaff’s rule
Use this for “A DNA sample is 30% adenine. What percent is guanine?”
Diagram
A pairs with T and G with C, so A = T = 30% and G = C = 20%: 6 of each of A and T and 4 of each of G and C in 20 bases.
This is an example. Type your own values to start a new problem.
A
%
+
G
%
=
50%
T
Percent thymine
calculated · %
C
Percent cytosine
calculated · %
nAT
A–T pairs per 10 pairs (average)
worked out (not typed)
nGC
G–C pairs per 10 pairs (average)
worked out (not typed)
H
Hydrogen bonds per 10 pairs (average)
worked out (not typed)
Formulas
T
=
A
30 = 30
G
=
50
−
A
20 = 50 − 30
C
=
G
20 = 20
nAT
=
A
+
T
10
6
=
30 + 30
10
nGC = 10 − nAT
4 = 10 − 6
H
=
2
×
nAT
+
3
×
nGC
24 = 2 × 6 + 3 × 4
Assumptions
•
A always pairs with T, and G with C, so DNA has as much A as T and as much G as C.
•
The four percents add to 100%, so A + G = 50%.
•
An A–T pair is held by 2 hydrogen bonds and a G–C pair by 3, so DNA rich in G and C holds together more tightly.
Step-by-step
WE KNOW
A = 30%
FIND
percent thymine (T), percent guanine (G), percent cytosine (C), A–T pairs per 10 pairs (average) (nAT), G–C pairs per 10 pairs (average) (nGC), hydrogen bonds per 10 pairs (average) (H)
Step 1 · Find percent thymine (T)
Use T = A
Every
A
pairs
with
a
T
,
so
there
are
as
many.
T
=
30%
Step 2 · Find percent guanine (G)
Use G = 50 − A
A
and
T
take
2
A
of
the
100%;
G
and
C
share
the
rest
equally:
100 − 2
A
2
=
50
−
A
.
G
=
50
−
30
G
=
20%
Step 3 · Find percent cytosine (C)
Use C = G
Every
G
pairs
with
a
C
,
so
there
are
as
many.
C
=
20%
Step 4 · Find A–T pairs per 10 pairs (average): nAT
Use nAT = (A + T) ÷ 10
A
–
T
pairs
are
(
A
+
T
)%
of
all
pairs,
and
(
A
+
T
)%
of
10
pairs
is
A
+
T
10
.
nAT
=
30 + 30
10
nAT
=
60
10
nAT = 6
Step 5 · Find G–C pairs per 10 pairs (average): nGC
Use nGC = 10 − nAT
Every
pair
that
is
not
A
–
T
is
G
–
C
.
nGC = 10 − 6
nGC = 4
Step 6 · Find hydrogen bonds per 10 pairs (average): H
Use H = 2 × nAT + 3 × nGC
Two
hydrogen
bonds
hold
each
A
–
T
pair
and
three
hold
each
G
–
C
pair.
H
=
2
×
6
+
3
×
4
H
=
12
+
12
H
=
24
Check
Put the numbers back in. Both sides should match.
30 = 30 ✓
20 = 50 − 30 ✓
20 = 20 ✓
6
=
30 + 30
10
✓
4 = 10 − 6 ✓
24 = 2 × 6 + 3 × 4 ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.