Section 4: Bioprocess Engineering and Process Biotechnology
Mass transfer in bioprocesses (kLa)
Subtopic: Fundamentals of Mass transfer in bioprocesses (kLa)
Question Statement:
A 1.2 kb DNA fragment was cloned into BamHI and EcoRI sites located on a 2.8 kb cloning vector.
The BamHI and EcoRI sites are adjacent to each other on the vector backbone. The vector contains
an XhoI site located 300 bp upstream of the BamHI site. An internal XhoI site is present in the gene
sequence as shown in the figure. The resultant recombinant plasmid is digested with EcoRI and
XhoI and analyzed through 1% agarose gel electrophoresis. Assuming complete digestion with
EcoRI and XhoI, the DNA fragments (in base pairs) visible on the agarose gel will correspond to:
(A) 2800, 700 and 500 (B) 2800, 700 and 800
(C) 2500, 700 and 800 (D) 2500, 1200 and 300
Options:
(A) 2800, 700 and 500
(B) 2800, 700 and 800
(C) 2500, 700 and 800
(D) 2500, 1200 and 300
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2016 Q.36)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Microbiology & Photosynthesis (Physiology of Cyanobacteria)
**2. Detailed Step-by-Step Analysis**:
- Let us evaluate each statement concerning cyanobacteria (blue-green algae):
- **P. They can perform oxygenic photosynthesis**: **CORRECT**. Cyanobacteria possess chlorophyll $a$ and utilize Photosystem II to split water ($2\text{H}_2\text{O} \to 4\text{H}^+ + 4e^- + \text{O}_2$), generating molecular oxygen.
- **Q. Usually filamentous forms are involved in nitrogen fixation**: **CORRECT**. Many filamentous cyanobacteria (e.g., *Anabaena*, *Nostoc*) fix atmospheric nitrogen ($N_2$).
- **R. Nitrogen fixation occurs in heterocysts**: **CORRECT**. The nitrogenase enzyme complex is irreversibly inactivated by molecular oxygen. In filamentous cyanobacteria, nitrogen fixation is physically compartmentalized into thick-walled specialized non-photosynthetic cells called **heterocysts**, which lack Photosystem II (producing no $O_2$) and maintain an anaerobic microenvironment.
- **S. They cannot grow in a mineral medium exposed to light and air**: **INCORRECT**. Cyanobacteria are photoautotrophs (and diazotrophs); they grow exceptionally well in minimal mineral salt media exposed to light and air because they fix atmospheric $\text{CO}_2$ via the Calvin cycle and atmospheric $N_2$ via nitrogenase.
- Therefore, statements **P, Q, and R** are CORRECT.
- **Option-by-Option Evaluation**:
- **Option (A) [CORRECT]**: P, Q, and R are completely correct biological facts about cyanobacterial physiology.
- **Option (B) [INCORRECT]**: S is false.
- **Option (C) [INCORRECT]**: S is false.
- **Option (D) [INCORRECT]**: S is false.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **A**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
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