DNA has only four letters, yet it has to spell out every protein in every living thing. The trick is that the letters are read in groups of three. Once you see why it has to be three, the rest of this topic falls into place.
📚 What you need to know
The genetic information is stored as the order of the bases along a DNA strand.
The bases are read in groups of three. Each group of three is a codon.
One codon codes for one amino acid. There are 20 amino acids used to build proteins.
Only one strand carries the message that gets read — the coding strand.
Three bases give 43 = 64 possible codons, which is more than enough for 20 amino acids.
The code is universal: the same codon means the same amino acid in almost every organism.
Some parts of the genome are coding sequences (they make proteins) and some are non-coding.
Sequences that have barely changed across species are conserved sequences, and they point to a shared ancestor.
The information is in the order
Look back at the structure of DNA and one thing stands out: the sugar is always the same and the phosphate is always the same. The only part that varies from one nucleotide to the next is the base.
So all of the information a cell holds is stored in one thing — the order of A, T, C and G along the strand. A gene is simply a section of that order which codes for one polypeptide.
Think of the backbone as the paper and the bases as the letters printed on it. The paper is identical everywhere; what makes one page a recipe and another a poem is only the order of the letters.
Why the code is read in threes
Here is the problem the cell has to solve. There are 20 different amino acids that proteins are built from, but DNA only has 4 different bases. So how many bases does it take to name 20 different things?
Each extra base multiplies the options by four, because any of the four bases can sit in the new position. That is why the jump from 16 to 64 is so big.
Two bases would only give 4 × 4 = 16 combinations, which is short of 20. Three bases give 4 × 4 × 4 = 64. That is comfortably more than enough, and it explains something students often find odd: several different codons can code for the same amino acid.
Number of possible codons
4 × 4 × 4 = 43 = 64 codons for 20 amino acids
Codons and amino acids
A codon is a group of three bases. Reading a gene means starting at one end and taking the bases three at a time, without skipping or overlapping. Each codon is then matched to one amino acid, and the amino acids are joined in that exact order to build a polypeptide.
Twelve bases here give four codons and therefore four amino acids. Divide the number of bases by three and you have the number of amino acids.
Which strand gets read?
DNA has two strands, but they do not both carry the message. Only one of them, the coding strand, holds the base sequence that gets read to build the protein. The other strand acts as the template that the coding sequence is copied from.
A neat way to remember why we still need both strands: the second strand is the backup copy. Because of complementary base pairing, if you have one strand you can always rebuild the other exactly. That is the whole basis of DNA replication.
Sequence changes and their effect
Because each codon is read as a fixed block of three, changing even one base can change the codon, which can change the amino acid, which can change the shape of the finished protein. And since a protein’s shape decides its job, a change in shape can stop it working.
That said, a base change does not always cause a problem. With 64 codons for only 20 amino acids, some changes land on a different codon that still codes for the same amino acid, so nothing changes at all.
The chain of cause and effect
base sequence → codon → amino acid order → protein shape → protein function
The code is universal
Here is the striking part. The same codon means the same amino acid in a bacterium, a mushroom, an oak tree and a human being. There are a handful of tiny exceptions, but the code is essentially universal.
Two big consequences come out of that, and both are common exam questions:
Genetic engineering works. A human gene can be put into a bacterium and the bacterium will read it correctly and make the human protein — this is how insulin is produced. It only works because both organisms use the same code.
It is evidence for a common ancestor. If life had started several separate times, we would expect several different codes. One shared code across everything suggests all life on Earth traces back to a single origin.
Conserved sequences. Over long stretches of time, mutations change base sequences. But some sequences have stayed almost identical across wildly different species — these are conserved sequences. They tend to be genes for jobs that every cell depends on, such as the proteins involved in reading DNA and building proteins, and the histone proteins that package DNA. If a sequence is that important, almost any change to it is harmful, so those changes do not get passed on.
Coding and non-coding sequences
Not every part of the genome codes for a protein. Coding sequences are the parts that do. Non-coding sequences do not code for proteins, but many of them are far from useless — they include regions that control when genes are switched on and off.
Worked examples
WORKED EXAMPLE
A coding sequence contains 900 bases. How many amino acids will the polypeptide it codes for contain?
Step 1: recall the rule
3 bases = 1 codon = 1 amino acid.
Step 2: divide900 ÷ 3 = 300300 amino acidsbases to amino acids, divide by 3 – amino acids to bases, multiply by 3
WORKED EXAMPLE
A polypeptide is 146 amino acids long. What is the minimum number of bases needed in the sequence that codes for it?
Step 1: each amino acid needs its own codon146 × 3 = 438 basesStep 2: think about where the reading stopsa stop codon is also needed to mark the end, and it is 3 bases long too438 + 3 = 441 bases438 bases code for the amino acids; 441 including a stop codonread the question – if it just says “codes for the amino acids”, 438 is the answer
WORKED EXAMPLE
A human gene for a hormone is inserted into a bacterium, and the bacterium makes the human hormone correctly. Explain why this is possible.
Step 1: name the property
The genetic code is universal.
Step 2: say what that means
The same triplet of bases codes for the same amino acid in bacteria as in humans.
Step 3: link it to the result
So the bacterium reads the inserted gene in exactly the same way a human cell would.
Same code = same amino acid order = the same proteinthree short linked sentences score far better than one long vague one
💡 Exam tip
Learn 43 = 64 and 20 amino acids as a pair of numbers. Questions about why the code is a triplet almost always want both.
Use the word codon, not “triplet of bases”, if the question uses it — but knowing both terms is safest.
For “explain why genetic engineering is possible”, the mark is for the word universal plus what it means.
When asked what a change in base sequence does, build the chain: base → codon → amino acid → shape → function. Each link can be a mark.
Remember that a base change is not automatically harmful, because more than one codon can code for the same amino acid.
“Conserved sequence” questions want the idea that the sequence is essential, so changes to it do not survive.
⚠ Common mix-up
Saying one base codes for one amino acid. It is three bases, always.
Writing that there are 64 amino acids. There are 64 codons and 20 amino acids.
Thinking “universal” means every organism has the same genes. It means they use the same code, not the same DNA.
Assuming non-coding DNA does nothing. Much of it has a controlling role — it just does not code for a protein.
Forgetting the stop codon when a question asks for the total length of a coding sequence.
Saying a mutation always changes the protein. Sometimes the new codon codes for the same amino acid, so nothing changes.
Up next: Nucleic Acid Structure & Function — we will put DNA and RNA side by side, meet the three types of RNA, and see just how much information one cell’s DNA can hold.
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