ARCH-B-06: DNA Base Composition (%A = %T, %G = %C), Triplet Coding Ratios & Point Mutations

Full 15-Q Timed Drill

Step 1 of 6 • The Core Idea

ARCH-B-06: Why the 125s Classroom Method Fails Under the 90s Clock

Standard Classroom Method (Too Slow)~125s (Blows 90s Clock)

In double-stranded DNA, base pairing ensures

\%\text{A} = \%\text{T} = 18\%

, so

\text{A} + \text{T} = 36\%

. The remaining

100\% - 36\% = 64\%

is split equally between

\text{G}

and

\text{C}

, giving

\%\text{G} = \frac{64\%}{2} = 32\%

(~90s).

The Fast ShortcutTarget: 30s (Saves 95s)

Line 1 (50% Purine-Pyrimidine Invariant): In dsDNA,

\%\text{A} + \%\text{G} = 50\%

ALWAYS.
Line 2 (Subtract):

\%\text{G} = 50\% - 18\% = 32\%

(10s).

Where Students Lose Time: In 19 past UAT-UK questions (8.15% of Biology), candidates lose 95s by forgetting to subtract both %a and %t (2 * 20% = 40%) from 100% before halving for %g. Instead, apply B5 with Maths 1 skill M3.5.

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