Q1. Source: CSIR NET November 2020 (Session I)
Following statements were made about catalytic introns:
A. Group I introns may undergo self-splicing by transesterification.
B. Group II introns do not require any factor/protein for autosplicing either in vitro or in vivo.
C. Certain introns of both Group I and Group II classes may contain open reading frames which are translated into protein.
D. Generally, Group I introns migrate by DNA-mediated mechanisms, whereas Group II introns migrate by RNA-mediated mechanisms.
E. Ribonuclease P (RNase P) is essential for bacteria and archaea but not eukaryotes.
Which one of the following combinations represents statements which are all correct?
a. A, B, C
b. B, D, E
c. A, C, D
d. C, D, E
Statement Analysis
A. Group I introns are self-splicing ribozymes. They remove themselves from precursor RNA through a series of transesterification reactions initiated by an external guanosine cofactor— correct statement
B. While Group II introns can self-splice in vitro, many require proteins called maturases or other host factors for efficient splicing in vivo. Therefore, saying they require no factor either in vitro or in vivo is incorrect.
C. Some Group I and Group II introns contain open reading frames (ORFs) that encode proteins such as homing endonucleases or maturases, which aid intron mobility and splicing— correct statement.
D. Group I introns typically move via DNA-mediated homing mechanisms, often involving homing endonucleases.
Group II introns generally move through RNA-mediated retrohoming, involving reverse transcription of the intron RNA– correct statement.
E. RNase P is involved in tRNA maturation and is found across all domains of life, including eukaryotes. Therefore, it is not exclusive to bacteria and archaea– incorrect statement.
Correct Answer: (c) A, C, D
Exam insights
- Group I → External guanosine + DNA-mediated mobility
- Group II → Lariat formation + RNA-mediated mobility (retrohoming)
Q2. Source: CSIR NET November 2020 (Session II)
Which one of the following conditions will switch on the lac operon in E. coli?
a. + Glucose, + Lactose
b. + Glucose, – Lactose
c. – Glucose, – Lactose
d. – Glucose, + Lactose
Concept
The lac operon is maximally expressed only when:
- Lactose is present = removes repression by inactivating the Lac repressor.
- Glucose is absent = increases cAMP levels, allowing the CAP-cAMP complex to activate transcription.
Correct Answer: (d) – Glucose, + Lactose
Memory Trick
Lac operon loves Lactose and hates Glucose.
✔ Lactose present = Repressor removed
✔ Glucose absent = Activator recruited
Exam Insights
For lac operon questions, always check two signals:
| Lactose | Glucose | Operon Status |
|---|---|---|
| + | + | Low expression |
| + | – | Maximum expression |
| – | + | OFF |
| – | – | OFF |