Protein Targeting and the Signal Sequence · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Address tags for new proteins

Science · Organisms & Life Processes · ages 18-20
Name ______________________   Date ____________

  1. Why does the cell need sorting at all?

    • Ribosomes cannot read mRNA
    • Chains all start in the cytosol but jobs live elsewhere
    • Proteins are too big to move

  2. What is a signal sequence?

    • A short address tag on a new chain
    • A sugar added in the Golgi
    • A ribosome itself

  3. Deleting the signal from a secreted protein strands it in the cytosol. True or false?

    Circle one:   True   False

  4. A protein targeting sequence is swapped for another type. What happens?

    • Nothing changes
    • The protein follows the new tag
    • Translation stops

  5. What does the rough ER build?

    • Lipids only
    • Cytosolic enzymes
    • Proteins for export or membranes

  6. How does translation of a secreted protein differ from a cytosolic one?

    • It never pauses
    • It restarts from scratch
    • It pauses when the signal emerges, then finishes on the ER wall

  7. Where do proteins without ER signals end up?

    • Secreted outside
    • In the Golgi
    • Cytosol, nucleus or mitochondria

  8. A cytosolic enzyme gains an ER signal sequence. Where does it end up?

    • Enters the endomembrane system
    • Stays cytosolic
    • Goes to mitochondria
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Address tags for new proteins W1-mt_wVFZq8cZgn-s1

  1. Chains all start in the cytosol but jobs live elsewhere · One shared birthplace with many workplaces demands a postal system.
  2. A short address tag on a new chain · The short stretch flags the protein for the ER and its station.
  3. True · With no tag, no docking happens and the chain finishes at home.
  4. The protein follows the new tag · Presence and type of tag predict destination.
  5. Proteins for export or membranes · Docked ribosomes feed export and membrane cargo into the ER.
  6. It pauses when the signal emerges, then finishes on the ER wall · Targeting interrupts translation mid stream to dock the ribosome.
  7. Cytosol, nucleus or mitochondria · Free ribosomes supply the home market, never the export line.
  8. Enters the endomembrane system · The new tag docks its ribosome on the ER like any export cargo.
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