IB Biology HLDNA, RNA & the Genetic CodePaper 1 & 2~10 min read
Nucleic Acid Structure & Function
DNA and RNA are built from the same kind of parts, but three small differences change everything about what each one does. Then there is the rule that makes accurate copying possible: A always pairs with T, and C always pairs with G.
📘 What you need to know
Three differences between DNA and RNA: sugar (deoxyribose vs ribose), base (thymine vs uracil), and number of strands (double vs single).
RNA polynucleotide chains are also much shorter than DNA.
Complementary base pairing: adenine pairs with thymine, cytosine pairs with guanine.
Two hydrogen bonds form between A and T. Three hydrogen bonds form between C and G.
Because of this, the base sequence on one strand determines the sequence on the other — one strand acts as a template.
This allows DNA to be copied very precisely during replication.
Only 4 bases, but they can be arranged in any order and any length, giving DNA an almost limitless capacity to store information.
A human nucleus holds about 3.2 gigabases, roughly 2 metres of DNA.
DNA and RNA compared
Property
DNA
RNA
Pentose sugar
Deoxyribose
Ribose
Bases
Adenine, cytosine, guanine, thymine
Adenine, cytosine, guanine, uracil
Number of strands
Double-stranded (double helix)
Single-stranded
Length
Very long
Relatively short
Everything else about the two nucleotides is built the same way — phosphate to sugar, sugar to base, all covalent bonds.
🧠
Memory trick: “U are RNA”
Uracil belongs to RNA. Thymine belongs to DNA — and both “thymine” and “double” have the sound of the longer, more permanent molecule. Deoxyribose vs ribose: DNA’s sugar is the one with the extra word, and the one missing the extra oxygen.
Complementary base pairing
The bases on opposite DNA strands always pair up in the same specific way:
This is not a coincidence. The hydrogen bonds that hold the two strands together can only form between these particular pairs — the shapes and charges do not fit otherwise.
C–G pairs are held slightly more strongly than A–T pairs because of that third hydrogen bond.
Why this matters so much
Because the pairing is fixed, the base sequence on one strand determines the sequence of the other. We say one strand acts as a template for the other.
If you know one strand reads A–T–G–C, the other must read T–A–C–G. Nothing else is possible. This is what allows DNA to be copied very precisely during replication, which in turn ensures the genetic code is accurately copied into newly formed cells.
A useful exam consequence. Because A always pairs with T and C always pairs with G, the amount of A always equals the amount of T, and the amount of C always equals the amount of G. If a question tells you a DNA molecule is 20% adenine, then it is also 20% thymine, leaving 60% shared between C and G — so 30% each.
DNA as an information storage molecule
Despite the genetic code containing only four bases, they can combine to form a very diverse range of sequences in molecules of different lengths. This gives DNA an almost limitless capacity for storing genetic information.
Measuring the storage
One way to measure it is by counting the number of genes in an organism’s DNA. Even very simple organisms carry several thousand.
Organism
Approximate number of genes
Human
20 000
Dog
19 000
Water flea
31 000
Bacterium (E. coli)
4 300
Rice plant
41 500
Look carefully at that table before you assume “more complex organism = more genes”. A rice plant has roughly twice as many genes as a human. Gene number and complexity are not the same thing, and examiners love data questions built on exactly this surprise.
The other way to measure storage is the number of base pairs in the genome:
The DNA in the nucleus of a human cell contains about 3.2 gigabases — roughly 109 base pairs.
Stretched out, that DNA would be about 2 metres long.
All of it fits inside a microscopic nucleus, which shows how tightly it is packaged.
So DNA stores an enormous amount of data with great economy.
Worked examples
WE 1
Calculate base percentages in a DNA molecule
A sample of DNA is found to contain 32% guanine. Calculate the percentage of each of the other three bases. (3 marks)
Step 1: use complementary base pairing
Guanine always pairs with cytosine, so cytosine = 32%.
Step 2: find what is left over
G + C = 32 + 32 = 64%, so A + T together make 100 − 64 = 36%.
Step 3: split the remainder
A pairs with T, so the remainder is shared equally: 36 ÷ 2 = 18% each.
G = 32%, C = 32%, A = 18%, T = 18%always start by pairing up the base you are given — the rest is just arithmetic
WE 2
Explain the importance of complementary base pairing
Explain how complementary base pairing allows genetic information to be copied accurately. (3 marks)
Point 1: state the rule
Adenine always pairs with thymine and cytosine always pairs with guanine, because hydrogen bonds can only form between these pairs.
Point 2: the consequence for the strandsThe base sequence on one strand therefore determines the sequence on the other, so one strand acts as a template.
Point 3: link to replication
During replication each strand can be used to build an exact new partner strand, so the genetic code is copied precisely into new cells.
Fixed pairing → each strand is a template → accurate replicationthe word “template” is usually worth a mark by itself
💡 Exam tips
Learn the three DNA/RNA differences as a set: sugar, base, strands. Comparison questions almost always want all three.
Remember the bond counts: A–T = 2, C–G = 3. A good way to keep it straight is that C and G are both curved letters and both get the bigger number.
For percentage questions, start with the pair rule: A = T and C = G.
Use the word template when explaining replication accuracy.
Know the key figures: about 3.2 gigabases and about 2 metres of DNA per human nucleus.
Do not assume more genes means a more complex organism — the rice plant proves otherwise.
⚠ Common mistakes
Pairing A with C or G with T. The bases pair as A–T and C–G, never any other way.
Swapping the hydrogen bond numbers. A–T has two, C–G has three.
Saying RNA is always single-stranded with no exceptions. It is usually single-stranded.
Forgetting that RNA is much shorter than DNA — this is a valid comparison point.
Writing that hydrogen bonds join nucleotides within a strand. Within a strand the bonds are covalent.
Dividing the leftover percentage wrongly. If A + T = 36%, each one is 18%, not 36%.
Up next: DNA Structure — the 3′ and 5′ ends, why the strands run in opposite directions, and how Watson and Crick worked out the double helix.
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