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Define the following: (a) Polyribosome (b) Plasmids (c) Cristae

GeneralClass 12AllAnswered 27 Mar 2026
Answer

(a) Polyribosome (Polysome)

  • Definition: A cluster of multiple ribosomes (often 5–10) attached to a single mRNA strand
  • Occurrence: In both prokaryotes and eukaryotes
  • Function: Simultaneous translation of mRNA by multiple ribosomes → rapid protein synthesis
  • Significance: Increases translational efficiency; visible as "beads on a string" in electron micrographs

(b) Plasmids

  • Definition: Extra-chromosomal, circular DNA molecules in prokaryotic cytoplasm
  • Characteristics:
    • Replicate independently of chromosomal DNA
    • Size: Few thousand base pairs
    • Carry non-essential genes (antibiotic resistance, toxins, metabolic enzymes)
  • Applications:
    • Genetic engineering (cloning vectors)
    • Antibiotic resistance transfer (horizontal gene transfer)

(c) Cristae

  • Definition: Infoldings of the inner mitochondrial membrane projecting into the matrix
  • Structure: Shelf-like or tubular extensions
  • Function:
    • Increase surface area for oxidative phosphorylation
    • House electron transport chain complexes and ATP synthase
  • Variation: Number and shape vary with cell's energy demand (more cristae in cardiac muscle than liver)

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