IB Biology HLDNA, RNA & the Genetic CodePaper 1 & 2~9 min read
The Basis of the Genetic Code
Four bases have to specify twenty amino acids. Reading them one at a time gives only four options, and two at a time gives sixteen — still not enough. Reading them in threes gives sixty-four. That is why the genetic code is read in triplets.
📘 What you need to know
DNA carries the genetic code as a sequence of nitrogenous bases in its nucleotides.
The strand carrying the sequence that is read by enzymes is the coding strand.
The code is read as a triplet of bases, called a codon. Each codon codes for one amino acid.
There are 20 different amino acids that can be coded for.
The sequence of amino acids determines the shape and function of the protein produced.
The genetic code is universal — almost every organism uses the same code, with a few rare exceptions.
This is evidence for a universal common ancestor, and it is why genetic engineering is possible.
Sequences that have stayed the same across organisms are called conserved sequences.
How the code is written
DNA molecules carry the genetic code as a sequence of nitrogenous bases — adenine, guanine, cytosine and thymine. Nothing else about the molecule varies, so the order of these four bases is the whole message.
One of the two DNA strands carries the base sequence that will be read by enzymes. That strand is known as the coding strand. Sections of it that carry instructions for a protein are called genes.
The central idea
base sequence → order of amino acids → shape of the protein → what it does
The code is read in groups of three. Each sequence of three bases is a codon, and each codon codes for one amino acid. With 20 amino acids to specify, three bases at a time is the smallest group that works.
Change one base and you may change one codon, which may change one amino acid — and that can change the shape of the whole protein.
Why shape matters so much. The sequence of amino acids determines how the polypeptide folds. The folded shape is what lets an enzyme fit its substrate, or a receptor fit its signal. So a change in the base sequence can change what the protein does.
From gene to protein
A gene is simply a length of DNA whose base sequence codes for one polypeptide.
The genetic code is universal
Here is one of the most important ideas in the whole course. The genetic code is universal: almost every organism on Earth uses the same code. There are a few rare and minor exceptions, but the rule holds.
That means the same triplet codes for the same amino acid in all living things — the codon that means “glycine” in a bacterium means glycine in a whale.
Two big consequences
Consequence
Explanation
Genetic information is transferable between species
A gene taken from one organism will be read the same way by another. This is exactly why genetic engineering works — a human insulin gene can be put into a bacterium and the bacterium will make human insulin
Evidence for a universal common ancestor
If the code had arisen separately many times, different groups would use different codes. The fact that they all share one code points to all living organisms having evolved from a single ancestor
Conserved sequences
Over time, mutations have changed some of the base sequences of organisms. All of an organism’s base sequences together form its genome.
Some sequences code for proteins — these are coding sequences.
Others lie in regions that do not code for proteins — non-coding sequences.
Many coding and non-coding sequences have remained unchanged across all organisms. These are called conserved sequences.
Highly conserved sequences are usually found in the genes coding for:
Proteins involved in transcription and translation
Histone proteins, which package DNA tightly into the nucleus
Think about why those genes in particular stayed the same. They handle jobs so fundamental that almost any mutation would be fatal, so natural selection removed the changes. Their similarity across species is strong evidence for universal ancestry.
Worked examples
WE 1
Explain how DNA determines the structure of a protein
Explain how the base sequence of a gene determines the structure and function of a protein. (4 marks)
Point 1: what carries the code
The genetic code is carried as the sequence of nitrogenous bases on the coding strand of DNA.
Point 2: how it is readThe code is read in triplets called codons, and each codon codes for one amino acid.Point 3: what that produces
The order of the codons therefore determines the order of the amino acids in the polypeptide.
Point 4: link to function
The amino acid sequence determines how the protein folds, and its shape determines its function.
Base sequence → codon order → amino acid order → protein shape → functionwork along the chain in order and each link is a mark
WE 2
Why is the universality of the code significant?
Explain the significance of the genetic code being universal. (3 marks)
Point 1: state what universal means
Almost all organisms use the same code, so the same triplet codes for the same amino acid in every species.
Point 2: the practical consequenceGenetic information can be transferred between species, which makes genetic engineering possible.
Point 3: the evolutionary consequence
It provides evidence that all living organisms evolved from a universal common ancestor.
One shared code = transferable genes and a shared ancestrygive both consequences — practical and evolutionary. Most students only give one.
💡 Exam tips
Use the word codon, not “triplet code word”. A codon is three bases coding for one amino acid.
Remember the number 20 — that is how many amino acids the code has to specify.
Always finish the chain with shape → function. Stopping at “the amino acid order changes” loses the last mark.
Say “almost every organism” — there are a few rare exceptions, and the IB likes precision here.
Link universality to both genetic engineering and common ancestry.
Know the difference between coding and non-coding sequences, and that both can be conserved.
⚠ Common mistakes
Saying one base codes for one amino acid. It takes three — a codon.
Confusing the coding strand with the template strand. The coding strand carries the sequence that matches the mRNA.
Saying the genetic code is the same in all organisms with no exceptions. There are rare, minor exceptions.
Treating “universal” as meaning every organism has the same genes. It means they use the same code, not the same sequences.
Forgetting that conserved sequences can be non-coding too.
Writing that mutations always change the protein. Not every base change alters the amino acid produced.
Up next: Nucleic Acid Structure & Function — a direct comparison of DNA and RNA, and why complementary base pairing lets DNA be copied so accurately.
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