'Semi-autonomous Organelles';

There are two organelles which contain their own DNA (coding for about 50% of the organelle) and reproduce independently of the nucleus. They are said to be 'semi-autonomous organelles'. mitochondrial DNA mutates at a known, constant, rate and is ONLY inherited from the mother, so it can be used to track purely female genetic lines. In the same way, the Y chromosomes is (obviously) only passed on from father to son and so can be used to track the purely male genetic line.

• Mitochondria (sing. Mitochondrion). these are sausage-shaped organelles (2-5 micro.m long) where aerobic respiration takes place in eukrayotic cells.Mitochondria are surrounded by a double membrane: the outer membrane is quite permeable, but the inner membrane is highly folded into cristae, which give it a large surface area. It is studded with ATPase, the enzyme which is the main site of ATP synthesis. This is where the last stage of respiration-the ETC takes place. The space enclosed by the inner membrane is called the mitochondrial matrix and contains small circular strands of DNA and 70S ribosomes. This is the site of the TCA or Kreb's cycle stage of respiration . 
• Chloroplasts. Bigger and fatter than mitochondria ( so settle first when cells are homogenised and centrifuged), chloroplast are the site of photosynthesis, so are only found in photosynthetic cells (plants and algae). Like mitochondria a double membrane enclose them, but chloroplast are contain membranes arranged in disks called thylakoids. Thylakoids contain chlorophyll and other photosynthetic pigments and carry out the light reaction of photosnythesis. The thylakoids are then stacked into piles called grana. The space between the inner membrane and the thylakoid is called the stroma- the site of the light-independent (or 'carbon-fixing') stage of photosynthesis. Chloroplasts also contain starch grains, 70S ribosomes and circular DNA.

Endosymbiosis (= probable evolution of mitochondria and chloroplasts)

Prokaryotic cells are far older and more diverse than eukaryotic cells. Prokaryotic cells have probably been around for 3.5 billion years, while eukaryotic cells arose only about 1 billion years ago. It is thought that eukaryotic cell organelles like mitochondria and chloroplasts are derived from prokaryotic cells that became incorporated inside larger prokaryotic cells. This idea is called endosymbiosis, and is supported by these observations:

• organelles contain circular DNA, like bacteria cells. 
• contain 70S ribosomes, like bacteria cells
• Organelles have double membranes, as though a single-membrane cell had been engulfed and surrounded by a larger cell.
• Organelles reproduce by binary fission, like bacteria. 
• Organelles are very like some bacteria that are alive today
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