DNA and RNA are built from the same kind of parts, yet they do very different jobs. This page pins down exactly how they differ, why DNA is such a good place to store information, and how to handle the base-percentage calculations examiners love.
📚 What you need to know
DNA has deoxyribose, RNA has ribose.
DNA uses thymine, RNA uses uracil instead. The other three bases are shared.
DNA is double-stranded and long; RNA is usually single-stranded and short.
Three types of RNA: mRNA, tRNA and rRNA, each with its own job in making proteins.
Because A pairs with T and C pairs with G, A = T and C = G in any double-stranded DNA sample.
Four bases in any order and any length gives an almost limitless number of possible sequences.
Human DNA holds around 3.2 billion base pairs in each set of chromosomes.
DNA and RNA side by side
There are only three real differences, and every exam question on this is built from them: the sugar, one base, and the number of strands.
Spot the U on the right where the DNA strand would have a T. Everything else about the building blocks is the same shape.
Property
DNA
RNA
Pentose sugar
Deoxyribose
Ribose
Bases used
A, C, G and T
A, C, G and U
Number of strands
Two (a double helix)
Usually one
Typical length
Very long — millions of bases
Short — hundreds to a few thousand bases
Main role
Long-term store of genetic information
Carrying and using that information to build proteins
If you can only hold one line in your head walking into the exam, make it this: DNA is Double-stranded with Deoxyribose; RNA is single-stranded with Ribose and Uracil. The letter D does a lot of work there.
The three types of RNA
RNA is not one molecule doing one job. Three different types work together during protein synthesis.
Type
What it does
Where you find it
mRNA (messenger)
Carries the copied instructions for one gene
Made in the nucleus, then travels to the cytoplasm
tRNA (transfer)
Brings the correct amino acid to the ribosome
Cytoplasm
rRNA (ribosomal)
Forms part of the ribosome itself
Ribosomes
An easy way to keep them straight: mRNA is the message, tRNA is the taxi that brings amino acids in, and rRNA builds the workbench the job happens on. The first letter of each name gives you the clue.
Why DNA is such a good store of information
This comes up as an “explain” question, so it is worth having three separate reasons ready.
🧩 Three reasons, ready to use
Huge variety. Only four bases, but they can be arranged in any order and in molecules of any length. The number of possible sequences is effectively limitless.
It can be copied accurately. Complementary base pairing means each strand acts as a template for the other, so replication produces an exact copy.
It is stable and well protected. The bases sit on the inside of the double helix, shielded by the sugar–phosphate backbone, and the two strands are held by many hydrogen bonds.
How much can it actually hold?
Gene numbers give one measure of storage. Look at these figures and notice that they do not line up neatly with how complicated the organism looks.
Organism
Approximate number of genes
Human
20 000
Dog
19 000
Water flea
31 000
Rice plant
41 500
E. coli (a bacterium)
4 300
A water flea has more genes than you do. That surprises most students, and it is a useful reminder that complexity comes from how genes are controlled and combined, not from simply having more of them.
The other measure is the number of base pairs. Human DNA holds roughly 3.2 billion base pairs per set of chromosomes, and if you stretched the DNA from a single cell end to end it would be about 2 metres long — all folded into a nucleus you need a microscope to see.
Where does “2 metres” come from? Each base pair adds about 0.34 nm to the length. So 3.2 × 109 pairs × 0.34 nm gives about 1.1 m for one set of chromosomes. A body cell carries two sets, one from each parent, giving roughly 2.2 m in total.
Base pairing calculations
Because A always pairs with T and C always pairs with G in double-stranded DNA, the amounts must match:
The rule you need
A = T and C = G so A + T + C + G = 100%
The bar always adds to 100%. Find the pair you are given, double it, subtract from 100, then halve what is left.
🧩 The method for any base percentage question
Write down the base you are given and its partner — they are equal.
Add those two together to get the share taken by that pair.
Subtract from 100% (or from the total number of bases) to find what is left for the other pair.
Halve it, because those two bases are equal as well.
Check that your four values add back up to 100% or to the original total.
Worked examples
WORKED EXAMPLE
A sample of double-stranded DNA contains 22% adenine. Calculate the percentage of guanine.
Step 1: adenine pairs with thymine, so they are equalA = 22%, so T = 22%Step 2: find what the A–T pair takes up22 + 22 = 44%Step 3: what is left belongs to C and G100 − 44 = 56%Step 4: C and G are equal, so halve it56 ÷ 2 = 28Guanine = 28%check: 22 + 22 + 28 + 28 = 100
WORKED EXAMPLE
A DNA molecule contains 8000 bases in total, of which 1800 are thymine. Calculate the number of cytosine bases.
Step 1: thymine pairs with adenineT = 1800, so A = 1800Step 2: total taken by A and T1800 + 1800 = 3600Step 3: bases left for C and G8000 − 3600 = 4400Step 4: split equally between C and G4400 ÷ 2 = 22002200 cytosine basessame method as percentages – only the total changes
WORKED EXAMPLE
A student measures the bases in a sample and finds 31% adenine and 19% thymine. Suggest what this tells you about the sample.
Step 1: check the rule
In double-stranded DNA, A must equal T. Here 31% does not equal 19%.
Step 2: work out what could explain itthe equal-amounts rule only applies when every base has a partnerStep 3: give the conclusion
So the sample cannot be normal double-stranded DNA. It is likely to be single-stranded.
The sample is not double-stranded, so A does not have to equal Tquestions like this are testing whether you know why the rule works, not just the rule
💡 Exam tip
Learn the DNA vs RNA differences as three items: sugar, base, number of strands. That structure makes a 3-mark question easy.
Always state both halves of a difference — “DNA has thymine whereas RNA has uracil”. A one-sided answer often loses the mark.
In base calculations, always check your four values add up to the total. It takes five seconds and catches most slips.
The A = T, C = G rule only holds for double-stranded DNA. Watch for questions that quietly say “single-stranded”.
If asked why DNA is good for storing information, give variety, accurate copying and stability as three separate points.
Know what each RNA type does — mRNA, tRNA and rRNA come up as one-mark matching questions.
⚠ Common mix-up
Saying RNA has no thymine and stopping there. There are three differences, and the sugar is the one most often forgotten.
Applying A = T to RNA. RNA is single-stranded, so its bases do not have to be in matching amounts.
Mixing up bases and base pairs. 3.2 billion base pairs means 6.4 billion bases.
Halving before subtracting in percentage questions. Subtract first, then halve.
Thinking more genes means a more complex organism. The gene-count table shows that is not true.
Calling mRNA, tRNA and rRNA “types of DNA”. They are all RNA.
Up next: DNA Replication — now that you know why the two strands are complementary, you are ready to see how a cell uses that to make a perfect copy of its entire genome.
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