NCERT line-by-line
1. TITLE
NCERT says 'Some amino acids are coded by more than one codon' — Here's how NEET tests it.
2. THE NCERT STATEMENT
NCERT EXACT LINE:
"Some amino acids are coded by more than one codon, hence the code is degenerate."
— NCERT Class 12, Chapter 5 (Molecular Basis of Inheritance), Page 96.
3. WHY THIS LINE MATTERS
- How many times has NEET tested this? Highly tested! It appears frequently in statement-based questions, Assertion-Reason formats, and "choose the incorrect statement" MCQs regarding the features of the genetic code.
- What makes this statement tricky? Students often confuse the term "degenerate" with the code being "unambiguous" or "universal". It seems counter-intuitive that a highly specific biological code is termed "degenerate".
- What do students typically misremember? Students frequently flip the definition, mistakenly thinking that "degenerate" means one codon codes for multiple amino acids (which would cause absolute chaos in protein synthesis!), rather than the correct fact: multiple codons code for one amino acid.
4. MCQ
According to NCERT, the genetic code is considered "degenerate" because:
A) Some amino acids are coded by more than one codon.
B) A single codon codes for more than one amino acid.
C) The codon is read in mRNA in a contiguous fashion without punctuations.
D) UUU codes for Phenylalanine from bacteria to humans.
5. ANSWER + LINE BREAKDOWN
Correct Answer: A
Break down the NCERT line:
- "Some amino acids" means that out of the 20 standard amino acids, many have several different triplet combinations assigned to them.
- "are coded by more than one codon" explains the mathematical necessity of the code; since 61 codons code for amino acids, there is a surplus of codons for just 20 amino acids.
- The key word is "degenerate" because it specifically describes this genetic redundancy, ensuring that even if a point mutation changes the third base of a codon, the same amino acid might still be produced, protecting the organism.
6. HOW NEET TWISTS THIS
Show 3 ways NEET can ask the same fact:
- As direct question: "There are 64 codons and 20 amino acids. The phenomenon where multiple codons specify the same amino acid is known as...?"
- As assertion-reason:
Assertion: The genetic code is degenerate.
Reason: Some amino acids are coded by more than one codon. (Both are true and Reason is the correct explanation).
- As except question: "All of the following are salient features of the genetic code EXCEPT: A) It is degenerate B) It is contiguous C) One codon can code for multiple amino acids D) It is nearly universal."
7. MEMORY LOCK
MEMORIZE:
Many Codons $\rightarrow$ One Amino Acid = Degenerate.
Mnemonic: Degenerate means Duplicate codons for the same amino acid.
8. RELATED NCERT LINES
"Also remember these from the same page:"
- "The codon is read in mRNA in a contiguous fashion. There are no punctuations."
- "The code is nearly universal: for example, from bacteria to human UUU would code for Phenylalanine (phe)."
- "AUG has dual functions. It codes for Methionine (met), and it also act as initiator codon."
9. NCERT REFERENCE
NCERT Class 12, Chapter 5 (Molecular Basis of Inheritance), Page 96, Paragraph 5.6 (The salient features of genetic code).
10. CTA
All important NCERT lines from this chapter → Genetics and Evolution Guide