How to Use Chargaff’s Rule on the MCAT

Chargaff’s rule is one of those MCAT topics where the biology is usually straightforward, but the math can still feel annoying under time pressure. A question might give you the percentage of adenine in an organism’s genome and ask for the percentage of guanine. If you try to do that without a structure, it is easy to rush or pick the answer that looks close.

The good news: this is a very pattern-based question. Once you know which bases pair with each other and how to set up the percentages, the math becomes predictable.

What Is Chargaff’s Rule?

Chargaff’s rule says that in double-stranded DNA, adenine and thymine are present in equal amounts, and cytosine and guanine are present in equal amounts.

• A = T
• C = G

This happens because DNA is double-stranded. Adenine pairs with thymine, and cytosine pairs with guanine. Those paired bases are held together by hydrogen bonds, so the amounts have to match.

On the MCAT, this often shows up as a percentage problem. You may be told the percentage of one nucleotide and asked to calculate another nucleotide or the total AT or GC content.

The Simple Setup: Always Write A, T, C, G

Even if you know the rule cold, write out the bases on your scratch paper:

• Even if you know the rule cold, write out the bases on your scratch paper:

That small setup gives you a place to put the numbers and keeps you from holding every step in your head. On test day, structure matters.

If a genome has 40% adenine, it must also have 40% thymine. That means A + T = 80%. Since the total nucleotide composition has to add up to 100%, the remaining 20% belongs to C + G. Divide 20% by 2, and you get 10% cytosine and 10% guanine.

So the full composition would be:

• A = 40%
• T = 40%
• C = 10%
• G = 10%

The ratio is 40:40:10:10, or 4:4:1:1. More importantly, the process is repeatable.

MCAT Example: If Adenine Is 23%, What Is Guanine?

Here is a classic MCAT-style version of the question:

• An organism’s genome is composed of 23% adenine. What percentage of the genome is guanine?

Then place what you know. Adenine is 23%. Because A = T, thymine must also be 23%.

• A = 23%
• T = 23%

That means 46% of the genome is adenine or thymine. Subtract 46% from 100% to find the total CG content.

• 100% - 46% = 54%

That 54% is shared between cytosine and guanine. Because C = G, split it evenly:

• 54% ÷ 2 = 27%

So guanine is 27%. Cytosine is also 27%. The full breakdown is:

• A = 23%
• T = 23%
• C = 27%
• G = 27%

How to Check Your Answer Quickly

One of the most useful parts of this problem type is that you can sanity-check your answer quickly. Your four percentages should add up to 100%. In this example:

• 23 + 23 + 27 + 27 = 100

You can also eliminate answer choices before finishing every bit of math. If adenine is 23%, guanine does not have to equal 23%. Adenine equals thymine, not guanine. If all four bases were 25%, every base would be equal, but that is not what the question gave you.

If the remaining CG content is 54%, guanine must be half of that. So 46% is too high, and 11% is probably too low. Even when you are moving quickly, the numbers can tell you which choices make sense.

The Trick Question: Single-Stranded DNA

Chargaff’s rule depends on base pairing in double-stranded DNA. That means it does not automatically apply to single-stranded DNA, mRNA, or RNA in general.

If the MCAT asks whether you can determine the nucleotide composition of a single-stranded DNA molecule, be careful. Unless the question gives you the actual sequence or enough extra information, you cannot assume A = T and C = G.

This is the trap: Chargaff’s rule is powerful, but only when the context supports it. Look for wording like an organism’s genome, chromatin, or normal double-stranded DNA. If the passage emphasizes a single strand, pause before applying the rule.

MCAT Takeaway

For Chargaff’s rule questions, do not freestyle the math. Set up the bases, pair what must be equal, subtract from 100%, and divide the remaining pair by two.

1. Write A, T, C, G.
2. Use A = T and C = G for double-stranded DNA.
3. Add the known pair together.
4. Subtract from 100%.
5. Divide the remaining percentage equally between the other pair.

That is the goal on test day: not just knowing the rule, but knowing when it applies.