Question

Explain the likely evolutionary origin of mitochondrial and chloroplast genomes. How have the sizes of the mitochondrial and chloroplast genomes changed since their origin? How has this occurred?

Short answer

Mitochondria and chloroplasts likely evolved from prokaryotic cells through a process called endosymbiosis. Over time, their genomes have greatly reduced in size due to gene loss and transfer of genes to the nuclear genome. This occurred through the mechanisms of recombination or transposons, and selective environmental pressures.

Step-by-step solution

Explain the Evolutionary Origin

Both mitochondria and chloroplast likely evolved from prokaryotic cells. According to the endosymbiotic theory, a host eukaryotic cell would have engulfed a prokaryotic cell which eventually became an organelle within the eukaryotic cell. The prokaryotic cell that was engulfed is believed to be similar to modern-day bacteria (for mitochondria) and cyanobacteria (for chloroplasts).

Discuss the Changes in Genome Size

Over time, the genomes of mitochondria and chloroplasts have reduced in size. This is mainly due to gene loss and the transfer of genes to the nuclear genome of the host cell. The current mitochondrial genome is much smaller than that of a typical bacterium. Similarly, chloroplast genomes have also shrunk considerably.

Describe the Mechanism of Changes

The reduction in size of the mitochondrial and chloroplast genomes is the result of gene transfer mechanisms. These mechanisms include the incorporation of the endosymbiont's DNA into the host cell's DNA by recombination or transposons. Some genes might have been lost if they were no longer needed in the new intracellular environment. The transferred genes are returned back to organelles as proteins, via special transport mechanisms.