Alevel Bio question help

The two DNA strands of a particular gene contain 168 guanine bases
between them. The relationship between the numbers of guanine bases
(G), adenine bases (A), thymine bases (T) and cytosine bases (C) in these
two strands of DNA is shown in the following equation.
G = 4(A + T) – C
Use this information and your understanding of DNA structure to calculate
the maximum number of amino acids coded by this gene.
Show your working.

Mark scheme: Correct answer for 2 marks = 70;;
Accept for 1 mark,
A = 42 and T = 42
OR
420 (total bases in gene)
OR
210 (bases in template strand)
I'm not sure how they calculated the answer to this question as the mark scheme doesnt specify how they have answered the question.

Reply 1

kaorimiyazono

19

So you know that G = 4(A + T) – C and that there are 168 Gs.
You also know that no. G = no. C and no. A = no. T due to complementary base pairing.

Therefore you can substitute 168 into the G and the C of the equation like so:
168 = 4(A + T) – 168

A and T will be the same number (x) so you can replace them with 2x so you're left with:
168 = 4(2x) – 168

Then solve for x:
168 = 4(2x) – 168
168 = 8x – 168
(+168 to left)
336 = 8x
(/left by 8)
42 = x

Now we know that the total number of bases (in both strands) is:
168 + 168 + 42 + 42 = 420
The template strand will have half of that so 210 bases.
One amino acid is coded by 3 bases so the total number of AAs coded for by the template strand is 210/3 = 70.

Feel free to ask me to clarify anything that doesn't make sense :smile:

Reply 2

kit2247

OP

9

Original post by kaorimiyazono

So you know that G = 4(A + T) – C and that there are 168 Gs.
You also know that no. G = no. C and no. A = no. T due to complementary base pairing.

Therefore you can substitute 168 into the G and the C of the equation like so:
168 = 4(A + T) – 168

A and T will be the same number (x) so you can replace them with 2x so you're left with:
168 = 4(2x) – 168

Then solve for x:
168 = 4(2x) – 168
168 = 8x – 168
(+168 to left)
336 = 8x
(/left by 8)
42 = x

Now we know that the total number of bases (in both strands) is:
168 + 168 + 42 + 42 = 420
The template strand will have half of that so 210 bases.
One amino acid is coded by 3 bases so the total number of AAs coded for by the template strand is 210/3 = 70.

Feel free to ask me to clarify anything that doesn't make sense :smile:

This was well explained. Thank you! :smile:

Reply 3

kaorimiyazono

19

Original post by kit2247

This was well explained. Thank you! :smile:

Np