Concept 30 Higher cells incorporate an ancient chromosome.

Get Adobe Flash player

Ivan Wallin presents his idea that mitochondria and chloroplasts were once free-living organisms.

I'm Ivan Wallin. In the 1920s, while using my microscope, I noticed that certain structures inside animal cells – called organelles – look a lot like bacteria cells. I proposed that these organelles were once free-living bacteria. But first let me tell you about the microscopic world of cells. Cells are classified into two groups. Lower cells, including bacteria, evolved about 3.5 billion years ago and do not have an organized nucleus (karyon). Hence, they are termed prokaryotes ("before the nucleus"). Higher cells – all plant, animal, and fungi cells – evolved from prokaryotes about 2.7 billion years ago and have a nucleus surrounded by a specialized membrane. They are called eukaryotes ("true nucleus"). Notice, they are much larger. In addition to the nucleus, eukaryotic cells have various organelles that carry out specific functions. For example, the endoplasmic reticulum and Golgi apparatus are involved in the production and export of proteins. The origin of two other organelles – mitochondria and chloroplasts – has sparked special interest because they resemble bacteria. Let's focus on the mitochondria. Mitochondria, found in all eukaryotic cells, release energy from food in a process called respiration, and store the energy in ATP molecules. ATP is then shipped from mitochondria to provide power for all of the cell's activities. Similarly, chloroplasts, found only in the cells of green plants, take energy from sunlight to make food. This process is called photosynthesis. Nineteenth century microscopists first noted the striking similarities between plant chloroplasts and single-celled algae, such as Chlorella. They proposed that a prokaryotic ancestor of plant cells had absorbed photosynthetic algae but didn't digest them. This established a symbiotic relationship; the algae produced food through photosynthesis, while the larger cell provided protection. A similar present-day symbiotic relationship can be seen in the green paramecium, which gets its green color from symbiotic Chlorella that live inside the cell. I proposed that mitochondria arose through the same sort of endosymbiosis between an ancestor of eukaryotes and a bacterium. But my claims of isolating and growing the mitochondria in the lab were later refuted, and my hypothesis was ignored. In 1967 Lynn Margulis resurrected the idea, and discoveries in the DNA world provided new evidence that both mitochondria and chloroplasts were once free-living organisms. Let's focus on mitochondria, which are found in both plant and animal cells. Like free-living organisms, mitochondria still have their own chromosomes and replicate independently within the cell. Furthermore, cells cannot produce mitochondria by themselves. If mitochondria are experimentally removed from cells, daughter cells will not contain mitochondria. The mitochondrial chromosome preserves many bacteria-like features. First, they are usually circular molecules. The human mitochondrial chromosome, only 16,549 base pairs long, is not much larger than small circular DNA in bacteria called plasmids. Second, the mitochondrial chromosome is tightly packed with genes, unlike nuclear chromosomes, which have large intergenic regions of noncoding DNA between genes. Second, the mitochondrial chromosome is tightly packed with genes, unlike nuclear chromosomes, which have large intergenic regions of noncoding DNA between genes. In human mitochondria, only one noncoding region exists. Third, most mitochondrial genes lack introns – noncoding information within nuclear genes. The human mitochondrial genome encodes only 37 genes, which are involved in the process of oxidative phosphorylation — the storage of energy in ATP. The human mitochondrial chromosome, like those of other eukaryotes, has been vastly reduced through evolutionary time. The free-living ancestor of mitochondria, perhaps similar to a Rickettsia, must have had a complement of at least 850 genes. Over time, genes for functions that could be provided by the host were lost. Also, some genes needed for respiration were transferred to the nucleus. Over millions of years of evolutionary time, this reduction resulted in the small mitochondrial chromosomes found in humans and other eukaryotes.

factoid Did you know ?

Symbiotic relationships with bacteria are essential to some organisms. For example, cows need the cellulose-digesting bacteria in their guts to survive.


What are the advantages and disadvantages of symbiosis?