Post-Unit Worksheet: Unit 5 (based on Problem Set 5.1.6) Let’s
look at an experiment that studies how cells inherit chloroplasts.
Recall that chloroplasts, like mitochondria, originate from an
endosymbiotic event, and therefore descended from free-living,
photosynthetic prokaryotes. Experimental Set-up: Euglena gracilis
is an excellent organism for studies of chloroplast inheritance
because it is a facultative heterotroph, meaning that in the
absence of light and/or a functional chloroplast it can still grow,
as long as appropriate carbohydrates are provided in the growth
medium. As chloroplasts have a bacterial ancestral origin, they can
be inhibited by certain antibiotics, without damaging the rest of
the eukaryotic cell. Streptomycin is an antibiotic that binds
specifically to ribosomes in the chloroplast (but not the ones in
the cytosol), which has the effect of inhibiting chloroplast
translation. If Euglena gracilis cells are exposed to streptomycin
for several generations of growth, and then the streptomycin is
removed to allow translation to restart, only a small fraction of
the cells will recover full photosynthetic function. These cells
can be easily differentiated as they turn green in the presence of
light, indicating the presence of chlorophyll. Cells that are
unable to photosynthesize remain white.
To help you get started:
• Think about the process of translation in chloroplasts. Why
are chloroplasts capable of translating proteins independently?
• A cell undergoes mitosis to form two daughter cells. What
happens to the chloroplasts in the original cell when it forms two
daughter cells? Would streptomycin affect this process?
Questions: a) Why do some cells recover photosynthesis after
streptomycin treatment and others don't?
b) Why do those that don't form green colonies immediately after
streptomycin treatment never develop photosynthetic activity at
later times?
c) If you were to examine the green and white cells using
microscopy, which endosymbiotic organelles would you expect to find
in the cytoplasm? Mitochondria? Chloroplasts? Both? Neither?
Why?