The genetic code is the set of rules by which the sequence of bases in mRNA is read in groups of three, called codons, to build a protein. Each codon (triplet of three bases) codes for one amino acid. Its main salient features are: triplet, degenerate, unambiguous and specific, non-overlapping, commaless (no punctuation), and nearly universal. Memory hook: Triplet, Degenerate, Unambiguous, Non-overlapping, Commaless, Universal = "Try Doing Uber Nice Coding Universally".
The mRNA is read in triplets (codons). AUG starts and codes methionine; AAA and AAG both code lysine (degeneracy); UAA is a stop codon. Each base is read once, with no punctuation between codons.
Your doubts, answered
What exactly is a codon?
A codon is a group of THREE adjacent bases in mRNA that codes for one amino acid. Because there are 4 bases (A, U, G, C) read in triplets, there are 4x4x4 = 64 possible codons. Out of these 64 codons, 61 code for amino acids and 3 (UAA, UAG, UGA) are stop codons that do not code for any amino acid.
Why is the genetic code called degenerate? Does degenerate mean bad?
No, degenerate does NOT mean faulty. The code is degenerate because some amino acids are coded by MORE THAN ONE codon. For example, lysine is coded by both AAA and AAG. Since 61 codons code for only about 20 amino acids, many amino acids must have more than one codon. This is a very common NEET point.
What is the difference between 'degenerate' and 'unambiguous'?
These two look opposite but both are true at the same time. Degenerate: one amino acid can have many codons (many-to-one). Unambiguous and specific: one codon codes for one and only one amino acid (never two). So AAA always means lysine (unambiguous), but lysine can also come from AAG (degenerate). Read the NEET question direction carefully.
Is the genetic code universal or nearly universal?
It is NEARLY universal, not fully universal. The same codon codes for the same amino acid from bacteria to humans (for example UUU codes for phenylalanine in both). But there are a few exceptions found in mitochondrial codons and in some protozoans. In NEET, choose 'nearly universal' over 'universal'.
What does 'commaless' or 'non-overlapping' mean?
Commaless means the code has NO punctuation. After one amino acid is coded, the next codon starts immediately with no gap or comma. Non-overlapping means each base is read only once and belongs to only one codon; codons do not share bases. Together they mean the code is read continuously, three bases at a time.
Which codon is both a start codon and codes for an amino acid?
AUG has a dual function. It codes for the amino acid methionine (Met) AND it acts as the initiator (start) codon. Do not confuse it with phenylalanine (coded by UUU/UUC). This dual role of AUG is a repeated NEET trap.
⚠️ The NEET trap ✗ The genetic code is universal, ambiguous and overlapping. ✓ The genetic code is nearly universal, unambiguous (specific), and non-overlapping. It is also degenerate, triplet and commaless. 🧠 NEET loves swapping ONE word: 'universal' (should be NEARLY universal) and 'ambiguous' (should be UN-ambiguous). Also UGA is a STOP codon, never an initiator. Only AUG is the start codon.
Real NEET questions
2019 (Odisha)
From the following, identify the correct combination of salient features of Genetic Code.
A · Universal, Non-ambiguous, Overlapping
B · Degenerate, Overlapping, Commaless
C · Universal, Ambiguous, Degenerate
D · Degenerate, Non-overlapping, Non-ambiguous ✓
Solution: The genetic code is degenerate (some amino acids have more than one codon), non-overlapping (each base read once), and non-ambiguous (one codon codes only one amino acid). It is NOT overlapping and NOT ambiguous, which rules out the other options. Note it is 'nearly' universal, so any bare 'Universal' option is weaker.
2023 (Phase 2)
The salient features of genetic code are: (A) The code is palindromic (B) UGA act as initiator codon (C) The code is unambiguous and specific (D) The code is nearly universal. Choose the most appropriate answer.
A · (A) and (B) only
B · (C) and (D) only ✓
C · (B) and (D) only
D · (A) and (C) only
Solution: (C) unambiguous and specific and (D) nearly universal are correct salient features. (A) is wrong because the code is NOT palindromic. (B) is wrong because UGA is a STOP codon, not an initiator; the initiator codon is AUG. So the answer is (C) and (D) only.
2023 (Phase 1)
Statement I: The codon 'AUG' codes for methionine and phenylalanine. Statement II: 'AAA' and 'AAG' both codons code for the amino acid lysine. Choose the correct answer.
A · Statement I is correct but statement II is false
B · Statement I is incorrect but statement II is true ✓
C · Both Statement I and Statement II are true
D · Both Statement I and Statement II are false
Solution: Statement I is FALSE: AUG codes for methionine and acts as the initiator codon, NOT phenylalanine (phenylalanine is coded by UUU/UUC). Statement II is TRUE: AAA and AAG both code for lysine, which shows the degeneracy of the code. So Statement I is incorrect but Statement II is true.
Solved Molecular Basis of Inheritance NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
How many codons are there in total and how many code for amino acids?
There are 64 codons in total. Out of these, 61 codons code for amino acids and 3 codons (UAA, UAG, UGA) are stop or terminator codons that do not code for any amino acid.
Who proposed that the genetic code should be a triplet of three nucleotides?
The physicist George Gamow proposed it. He argued that since there are only 4 bases and they must code for 20 amino acids, the code should be a combination of three bases (a triplet).
Who deciphered the genetic code experimentally?
Marshall Nirenberg's cell-free system for protein synthesis helped decipher the code. Har Gobind Khorana's chemical synthesis of RNA with defined base sequences and Severo Ochoa's enzyme (polynucleotide phosphorylase) were also key.
Is the genetic code overlapping?
No. The genetic code is non-overlapping. Each base belongs to only one codon and is read only once; codons do not share bases.
Which feature of the genetic code lets bacteria make human insulin?
The 'nearly universal' feature. Because the same codon means the same amino acid from bacteria to humans, a human gene put into bacteria is read the same way, so bacteria can make human insulin (recombinant DNA technology).
