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From: pi@hpcuhb.HP.COM (Paul Ilgenfritz)
Newsgroups: sci.bio
Subject: Re: Organelle Reproduction
Message-ID: <5180003@hpcuhb.HP.COM>
Date: 18 Jul 90 00:50:47 GMT
References: <5180001@hpcuhb.HP.COM>
Organization: Hewlett Packard, Cupertino
Lines: 61


Here are some emailed responses with names removed to protect the innocent:

----------------------------------------------------

In article <5180001@hpcuhb.HP.COM> you write:
>
>I read a book on evolution which left me curious about how organelles in a
>cell reproduce after a cell divides.  In particular, mitochondria,
>ribosomes, and chloroplasts.  Do they divide themselves or are new ones
>produced under direction from the nucleus?  The book implys that they
>divide since they have their own DNA and RNA but it is never directly
>stated.
>
   A brief answer is that the more-or-less autonomous organelles, including
mitochondria, plastids, and centrioles, reproduce themselves but coordinate
their timing of reproduction with the rest of the cell.  Ribosomes are not
autonomous, but are produced from DNA templets in either the nucleus or
an organelle.  A long answer would include discussion of the symbiotic
origin of the eukaryotic cell, gradual transfer of genes from the organelles
to the nucleus (most dramatic in the contrast between animal mitochondria
which have a "stripped down" genome and higher plant mitochondria which
retain more of their original genes), and lots of details about the 
regulation of gene expression, packaging and transport of proteins, export 
of mRNA's from the nucleus, and regulation of the timing of cell division.

   The larger organelles consist of a mix of proteins coded by their own   
genomes and by the cell nucleus.  To take chloroplasts as an example, the
mix ranges from some algae where almost nothing comes from the nucleus to
higher plants where a number of enzymes critical for chloroplast function
are coded in the nucleus and produced using nuclear-type ribosomes.  
   
   A very accessable and visually attractive book, that deals with the
varieties of cell structure in different groups, is called  _Five Kingdoms_
and is by Lynn Margulis and Karen Schwartz.  Most larger libraries should
have it, and there is a second edition in print in paperback.

--------------------------------------------------------------------
Subject: Re: Organelle Reproduction
Newsgroups: sci.bio
In-Reply-To: <5180001@hpcuhb.HP.COM>

In article <5180001@hpcuhb.HP.COM> you write:
> 
> I read a book on evolution which left me curious about how organelles in a
> cell reproduce after a cell divides.  In particular, mitochondria,
> ribosomes, and chloroplasts.  Do they divide themselves or are new ones
> produced under direction from the nucleus?  The book implys that they
> divide since they have their own DNA and RNA but it is never directly
> stated.

As I recall, chloroplasts divide semi-independently... their own DNA is
relativly complete.  I've read that the genome of chloroplasts shows a
strong relationship to the genome of cyanophytes ("blue-green algae"),
and that it's generally believed that chloroplasts evolved from
symbiotic cyanophytes.

Mitochondria also contain their own DNA... but I believe that it's much
less complete than chloroplast DNA, and that mitochondria are dependent
on products of nuclear DNA for much of their ability to function.  I
don't recall how mitochondria reproduce.