h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Was in a biochemistry class the other day learning about the organelles in the sub-cellular features of eukaryotic cell.

We progressed from learning that within the nucleus, the nucleolus is where ribosomes are produced.

We then proceeded to look at the interior structure of the mitochondria, which has it's own DNA and ribosome, but no nucleus.


So my question is where does Mitochondria get it ribosome's from?

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I'm already familiar with the postulation that mitochondria are in a symbiotic relationship with us, having been originally absorbed into a cell at an early period in life's history. Hence their own DNA is not our own, but they are passed down via the egg, which is why mitochondrial DNA can be used to trace back ancestry to early females.

What I want to know is how comes they have ribosomes with no nucleus.

I asked the teacher, but he didn't know.

Hi Clive.
The quote below is from W*k*pedia and states that the ribosome is produced in the nucleolus and cell cytoplasm in eukaryotes and I can verify this from my copy of Principles of Anatomy and Physiology by Gerard J. Tortora & Nicholas P. Anagnostakos (a rather ancient copy from 1990!).

"In bacterial cells, ribosomes are synthesized in the cytoplasm through the transcription of multiple ribosome gene operons. In eukaryotes, the process takes place both in the cell cytoplasm and in the nucleolus, which is a region within the cell nucleus. The assembly process involves the coordinated function of over 200 proteins in the synthesis and processing of the four rRNAs, as well as assembly of those rRNAs with the ribosomal proteins."

http://en.wikipedia.org/wiki/Ribosome

t.

Which distills down to "In bacterial cells, ribosomes are synthesized in the cytoplasm through the transcription of multiple ribosome gene operons."

Bacteria have no nucleus (and hence no nucleolus), yet they do have ribosomes. As you have noted there are some very strong resemblances between mitochondria and bacterial cells so it would be a good postulate that they are produced in the same way as in bacteria. I.e. by uncompartmentalised (as far as we can tell) biosynthesis within the cytoplasm.

This link gives some interesting reading on the subject although it doesn't reach any hard conclusion regarding your question.


http://books.google.co.uk/books?id=iXeQ1Bi9P7cC&pg=PA383&lpg=PA383&dq=ribosome+synthesis+in+mitochondria&source=bl&ots=xJS2GiMRKh&sig=iEHWBUkkU9o-7es5KjPva16git4&hl=en&ei=SyO6SsWIJ9GgjAeJ1eXyBQ&sa=X&oi=book_result&ct=result&resnum=3#v=onepage&q=ribosome%20synthesis%20in%20mitochondria&f=false

Thanks guys!

Off to go read your link now Orcus

clive

hi

Goddidit

I'd have thought that they'd probably be synthesised within the mitochondrial cytoplasm itself... they're pretty self-contained organells really, though I think now as they've become more an integrel part of eukaryotic cells, some of their DNA is actually now located within the 'host' cells' nuclear DNA/nucleous....
I'm sure I recall that anyhow...

Interesting. Having done a bit of reading, it sees that the mitochondria do contain 'special ribosomes'.

However, the mitochondrial DNA encodes only 13 peptides, all of which are apparently assoiciated with its inner membrane. Although 2 ribosomal RNAs (rRNA) are found within human mitochondria.

So the peptidyl part of the mitochondrial ribosome cannot be encoded by the mitochondrion's DNA but must come from the main cell nuclear DNA.

So* I conclude that the mitochondrial ribosome is part mitochondrial in origin (some of the rRNA) and part main cell origin.

Interesting.




*assuming the protein part of the mitochondrial RNA is genetically encoded - highly likely I think.

Well now that *is* interesting.

So that ---> * would mean there is ...for want of a better word.... transference between the nucleolus and the mitochondria.

Shall have to make mention of this in the lecture next week.

(..So I asked some biochemist buddies of mine and...)