Mitochondria  are cellular organelles that contain their own genome in
   the  form  of  a  circular  DNA molecule. In humans, the mitochondrial
   genome  encodes 13 proteins required for oxidative phosphorylation and
   ATP  production, 22 tRNA molecules, and 2 rDNA genes. The organization
   and  function  of  the  mitochondrial genome bear many similarities to
   those  of  bacterial  genomes.  Therefore, mitochondria are thought to
   derive  from  bacteria;  that  is, they are descended from free-living
   prokaryotic organisms that became intracellular symbionts in nucleated
   host cells. Once incorporated into a nucleated cell, the genome of the
   mitochondrion  became smaller over time, and the reduced genome we now
   see in mitochondria is thought to be the product of gene loss and gene
   transfer  to  the  nucleus  of  the host cell. However, many questions
   about  the  origin and evolution of mitochondria remain. Among them is
   one  relating to the bacterial origins of mitochondria: Which group of
   bacteria are they descended from?

   [Page   658]   Researchers   investigating  this  question  have  used
   information  derived  from  nucleotide sequencing of the mitochondrial