Read the universal genetic code
The genetic code is read as nucleotide triplets; codons specify amino acids or start and stop signals for translation.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain DNA, RNA and protein synthesis. Written practice is self-checked; it does not replace teacher review or practical assessment.
Before you begin
- Recognise that DNA stores genetic information and that proteins are made from amino acids.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many bases are in one codon?
What you will learn
- Explain that triplets of DNA or mRNA bases code for amino acids or start/stop signals
- Use a supplied genetic-code table with mRNA codons
- Describe the universal nature of the genetic code
Triplets form codons
The genetic code is read in groups of three bases. In translation, mRNA codons are read sequentially at the ribosome. Each codon specifies an amino acid or a start or stop signal.
A start codon initiates translation and a stop codon ends it; stop codons do not specify an amino acid. Use the code table supplied with a question and read its codons in the stated direction, usually 5′ to 3′.
“Universal” describes shared meanings
The genetic code is described as universal because the same codon generally specifies the same amino acid in different organisms. The code is also degenerate: more than one codon can specify the same amino acid.
Universal does not mean organisms have identical DNA sequences or make identical proteins. Different genes contain different codon sequences, so they can produce different polypeptides while using the same code.
Pause and explain
What does a stop codon do?
Worked example
A supplied table shows that UUU and UUC both specify phenylalanine. What property of the genetic code does this illustrate?
Show the worked solution
- Two distinct codons specify one amino acid.
- This is degeneracy.
Answer This illustrates degeneracy: more than one codon can specify the same amino acid.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain the terms codon and degenerate genetic code.
Give me a hint
Define a triplet and relate multiple codons to one amino acid.
Compare my reasoning
- A codon is a triplet of bases on mRNA.
- Degeneracy means more than one codon can specify the same amino acid.
A codon is a triplet of bases on mRNA. The genetic code is degenerate because more than one codon can specify the same amino acid.
Look for these in your work
- Codon defined
- Degeneracy defined
A sequence contains 30 coding bases and is translated without interruptions. How many codons does it contain?
Give me a hint
Divide the number of bases by three.
Compare my reasoning
- Each codon contains three bases.
- 30 ÷ 3 = 10 codons.
The sequence contains 30 ÷ 3 = 10 codons.
Look for these in your work
- Triplet grouping applied
- Count correct
Two organisms have different DNA sequences but the same codon meanings. Explain why this is consistent with a universal genetic code.
Give me a hint
Distinguish the code from the sequences that use it.
Compare my reasoning
- A universal code assigns the same general meaning to a given codon.
- Different genes can contain different codon sequences.
A universal genetic code means that a given codon generally specifies the same amino acid in different organisms. The DNA sequences can differ, so organisms can make different polypeptides using the same code.
Look for these in your work
- Universal meaning explained
- Different sequences distinguished
Common mistakes
- Reading a codon as one or two bases rather than a triplet
- Assigning an amino acid to a stop codon
- Interpreting universal to mean every organism has identical genes
What can you explain now?
How many bases are in one codon?
Compare with the explanation
Three
A codon is a triplet.
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Make it stick
How are codons grouped and read, what do start/stop codons do, and what does “universal” mean?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.