Basic Biochemistry & Biomolecules

• Golden Rice Engineering (Ingo Potrykus & Peter Beyer): Genetically engineered to synthesize provitamin A ($\beta$-carotene) in the rice endosperm by introducing two key biosynthetic genes:

• psy (Phytoene Synthase) from daffodil (Narcissus pseudonarcissus) or maize (Zea mays), converting geranylgeranyl diphosphate (GGPP) to phytoene.
• crtI (Phytoene Desaturase / Carotene Desaturase) from the bacterium Pantoea ananatis (Erwinia uredovora), performing multiple desaturation steps to yield lycopene.
  • Conclusion: The engineered genes are Phytoene synthase and Phytoene Desaturase.

• Ribozyme / RNase P $

ightarrow$ (IV) Cleavage of RNA genome: Catalytic RNA molecules that specifically bind target viral RNA through complementary base pairing and cleave phosphodiester bonds.

• Coat Protein $

ightarrow$ (III) Interferes with uncoating: Transgenic expression of viral coat protein (CP-mediated resistance) prevents the disassembly and uncoating of invading virions upon initial cell entry.

• cDNA of RNA satellite $

ightarrow$ (I) Interferes with replication: Satellite RNAs attenuate symptom severity by competing with helper virus genomic RNA for viral replicase machinery.

• Antisense RNA $

ightarrow$ (II) Binds to RNA preventing function: Hybridizes with complementary sense viral mRNA or genomic RNA, blocking translation and triggering enzymatic degradation.

Correct option: (4) (A) - (IV), (B) - (MN), (C)~ (), (B) - (HN)

• Tomato, Bt $\rightarrow$ (IV) Heliothis armigera: Transgenic tomato expressing cry1Ac confers resistance against tomato fruit borer (Helicoverpa armigera).
• Maize, Bt cryII $\rightarrow$ (I) European corn borer: Transgenic Bt maize controls European corn borer (Ostrinia nubilalis).
• Sugarcane, Bt cryI $\rightarrow$ (II) Diatraea saccharalis: Engineered sugarcane controls the devastating sugarcane stalk borer (Diatraea saccharalis).
• Rice, Bt cry1 gene $\rightarrow$ (III) Striped stem borer: Transgenic Bt rice expressing Cry1Ab or Cry1Ac protects against striped stem borer (Chilo suppressalis).

Correct option: (3) (A) - (IV), (B) - (D), (G)- (), (D) ~ (atl)

• Hammerhead Ribozyme Mechanism: The hammerhead ribozyme is a small catalytic RNA motif comprising three base-paired helical stems surrounding a conserved core.
• Cleavage Chemistry: It catalyzes a site-specific phosphodiester bond transesterification wherein the 2'-hydroxyl oxygen attacks the adjacent 3'-phosphorus in an in-line $S_N2$ nucleophilic substitution, producing a 2',3'-cyclic phosphate and a 5'-hydroxyl terminus without requiring a protein cofactor.
• Conclusion: Option 2 correctly identifies its cleavage mechanism.

Correct option: (2) It cleaves RNA sequences using a water molecule in a mechanism similar to protein enzymes.