John Walshaw (ubcg08r@iona.cryst.bbk.ac.uk)
Mon, 25 Mar 1996 19:01:48 +0000 (GMT)

Here are some points about ribosomes which cropped up in the BioMOO
today- can any cell biologists help?

In textbooks it says something to the effect that mitochondria and
chloroplasts 'contain complete genetic systems', but that the proteins
which mediate these systems are mostly encoded by the nuclear genome.
By the way, I seemed to remember from my undergraduate cell biology (some
time ago admittedly) that some of the rRNA molecules of cytoplasmic
ribosomes are encoded by mitochondrial DNA, and mentioned this- but looks
like my memory failed me here, as this is wrong. E.g., the map of the
complete human mitochondrial genome includes a 16S rRNA gene and a 12S
rRNA gene (none of whose products are present in the cytoplasmic/ER
ribosomes, which consist of 18S, 5.8S, 28S and 5S rRNA components).

But is it correct to assume that the products of these mitochondrial genes
arent enough to make a mitochondrial ribosome? I.e., the organelles must
import additional rRNA molecules coded by the nuclear genome. If so, are these
imported rRNAs the same as those included in cytoplasmic ribosomes, or
are they specific 'mitochondrial ribosome rRNAs' encoded by the nuclear

Another thing- the implications of the different genetic codes found in
1)chloroplasts 2)mitochondria and 3)both prokaryotes, and eukaryote
nuclear genomes.

What are the implications for the endosymbiont theory for the origin of
these organelles (i.e. one prokaryote ingesting another prokaryote). I
can think of a couple of points, but can anyone enlighten us?

John Walshaw