Are there multiple copies of circular DNA and membrane bound ribosomes IN CHLOROPLAST ? Are polysomes found in them ?

  • $\begingroup$ Wikipedia says the number of chloroplast DNA copies (nucleoids) drops from 100 to 15-20 as the chloroplast ages (en.wikipedia.org/wiki/Chloroplast_DNA#Nucleoids). No luck looking up membrane-bound ribo-/polysomes. I found lots on protein import into chloroplasts, but no export. My guess is that with many chloroplast genes migrating into the plant genome, protein flux is in rather than out (why would something the whole cell needs be made in just the chloroplast?). If you look at a list of chloroplast DNA genes, it's pretty self-contained; no proteins that would need to be exported. $\endgroup$
    – A. Kennard
    Nov 14, 2013 at 0:13
  • 1
    $\begingroup$ ribosomes are never membrane bound.. they are adsorbed on ER (in case of RER) $\endgroup$
    Nov 14, 2013 at 11:28

1 Answer 1


The chloroplast, having multiple circular copies of its chromosome and its own tRNA and ribosome genes more resembles the bacterial systems it is descended from. See @A.Kennard comment

Polysomes being found in bacteria frequently are single mRNA to which are attached multiple ribosomes. This paper shows that chloroplast polysomes were observed in the mid 1960s by centrifugal sedimentation.


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