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Genetic Code and Translation

The genetic code is the set of triplet codons in mRNA that specify amino acids, and translation is the ribosome-mediated synthesis of a polypeptide using mRNA, tRNA, and amino acids.

In one line: Codon says the word, anticodon answers it, ribosome links the amino acids.

Concept Practice Coming Soon

Understand it

  • The genetic code is triplet, nearly universal, non-overlapping, comma-less, and degenerate, meaning more than one codon can code for the same amino acid.
  • AUG usually acts as the start codon and codes for methionine, while UAA, UAG, and UGA are stop codons. tRNA works as an adapter because it has an anticodon for mRNA recognition and an amino acid attachment site.
  • During translation, charged tRNA brings amino acids to the ribosome.
  • Codon-anticodon pairing at ribosomal sites ensures amino acids are added in the correct order to form a polypeptide.

Example

The mRNA codon AUG begins translation and brings methionine. If the next codon is UUU, a tRNA with complementary anticodon carries phenylalanine to the ribosome.

Rule to remember

Coding rule: three bases form one codon; codons are read sequentially in one reading frame from start codon to stop codon.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

An mRNA segment with 5'-AUG GGU UAA-3' starts with methionine, adds glycine, then stops at UAA, producing a short polypeptide before termination.

Method you can reuse

Attach amino acid to tRNA, bind mRNA to ribosome, start at AUG, match codon with anticodon at A site, form peptide bond with chain at P site, move ribosome, repeat elongation, stop at termination codon.

Sketch it in your answer

Label mRNA, codon, tRNA, anticodon, amino acid attachment site, ribosome small and large subunits, A site, P site, and growing polypeptide chain.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is tRNA called an adapter molecule?

Think first, then show answer
tRNA is called an adapter because one part recognizes the mRNA codon through its anticodon, while another part carries the specific amino acid for that codon.

Common wrong answer

Saying stop codons code for a special amino acid.

Don't mix this up

Students often write that one amino acid has only one codon. The code is degenerate, so several codons may specify the same amino acid.

Exam tip

For translation answers, mention mRNA codon, tRNA anticodon, aminoacylation, ribosome, initiation, elongation, termination, and peptide bond formation.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked as features of genetic code, codon interpretation, tRNA structure-function, or translation sequence.

Answer plan

Define genetic code, list key features, explain tRNA as adapter, then describe translation through initiation, elongation, and termination with codon-anticodon pairing.
  1. 1

    Enough for 1 mark

    The genetic code is the set of triplet codons in mRNA that specify amino acids, and translation is the ribosome-mediated synthesis of a polypeptide using mRNA, tRNA, and amino acids.

  2. 2

    Add for 2 marks

    Example: The mRNA codon AUG begins translation and brings methionine. If the next codon is UUU, a tRNA with complementary anticodon carries phenylalanine to the ribosome.

  3. 3

    Add for 3 marks

    The genetic code is triplet, nearly universal, non-overlapping, comma-less, and degenerate, meaning more than one codon can code for the same amino acid.

Practice

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