Concept


Thomas Hunt Morgan's concept of genes as beads along the length of a chromosome changed little through the first half of the 20th century. Genes were seen as inviolate objects with fixed positions on the chromosomes. However, in the 1950s, Barbara McClintock showed that certain DNA fragments, termed transposons, can be activated to transpose ("jump") from one position on a chromosome to another. She hypothesized that transposition provides a means to rapidly reorganize genes in response to environmental stress. McClintock's work was remarkable, not only for the fact that it flew in the face of prevailing dogma, but, also, because it was based entirely on observation of chromosomes and genetic crosses. Confirmation of her ideas had to await the discovery of the modern tools of DNA analysis. This work paved the way for the modern concept of chromosomes as dynamic, changing structures.

Animation


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. 19th 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. In human mitochondria, only one noncoding region exists. Third, most mitochondrial genes lack introns -- nocoding 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.

Gallery


Ivan Wallin.

Audio/Video


Audio Glossary

Chromosome, Mitochondrial DNA, Genome

Video Interviews

Svante Paabo

Svante Paabo is the Director of the Department of Genetics at the Max Planck Institute of Evolutionary Anthropology. His group was the first to successfully isolate Neandertal mitochondrial DNA.

Clip 1 (1:27)
Mitochondrial DNA is frequently used in forensic anthropology. What is the age limit of DNA?

Clip 2 (0:45)
What contributes to DNA degradation?

Clip 3 (1:15)
How to pick the best sample from which to isolate DNA.

Clip 4 (0:29)
Comments on why is it relatively easy to reconstruct mitochondrial DNA.

Biography


 

Ivan Wallin was one of the first proponents of the idea that mitochrondria are derived from bacteria.

IVAN EMANUEL WALLIN (1883-1969)

If you were in Ivan Wallin's anatomy class, you had better be prepared or you would face the wrath of "Mitochondria Man." Instead of lecturing, Wallin simply dissected cadavers during class with his students gathered around the dissection table. As he worked, he fired off questions to the students around him. Students who gave bad answers were whacked in the chest.

Ivan Wallin wanted to be only one thing when he grew up: a teacher. The son of Swedish immigrants, Wallin was born and raised in the small farming community of Stanton, Ohio. He attended Augustana College and Princeton University before earning his B.Sc. from the University of Iowa in 1905. Wallin began teaching biology while he pursued a Master's degree at the University of Nebraska (1908) and a D.Sc. in Anatomy at New York University (1915). In 1918, Wallin moved to Boulder, Colorado to become a professor at the University of Colorado Medical School, where he instituted his eccentric teaching style.

Wallin was intimidating in the classroom but kind on the outside. He threw frequent parties for his students, and, in return, they helped him build his cabin in the North St. Vrain Canyon, 20 miles north of Boulder. At "Club Wallin," the professor and his students spent the nights playing poker and raiding the padlocked liquor cabinet. During the day, Wallin hunted duck, fly-fished for trout, and built furniture for the cabin. (The students were presumably hungover).

The biggest event on Wallin's social calendar was his annual Christmas Glogg party, attended by a hundred guests who were subjected to such Swedish delicacies as Sillsallad (pickled herring salad), lutefisk (dried codfish that has been soaked in a water and lye solution before cooking), and of course, Glogg (a high octane concoction of wine and aquavit, served hot). Wallin entertained his guests with his renditions of Swedish folk songs on the piano.

In the 1920s, Wallin tested his ideas about mitochondria in the shed behind the medical school's classrooms. At the time, most people believed mitochondria had evolved from the cell, but Wallin thought mitochondria might be bacteria, because they were indistinguishable by sight. He wrote a series of nine papers explaining his theories and experiments, and claimed in one that he had definitive proof mitochondria were bacteria: he could culture them outside of the cell.

No one believed Wallin's cultures were free from contamination, and in retrospect, they were right. It is impossible to culture mitochondria outside the cell, because the instructions for 95% of the proteins it needs reside in nuclear DNA. The heavy criticism Wallin took from the scientific community turned him away from research and he spent the rest of his career in teaching and administration. It was not until after Wallin died that evidence began accumulating that his theory was partially correct.

Factoid

Links


 

Links

The Mitochondria

Learn more about mitochondria and the cell in this short chapter from Webcytology.

Mitochondrial Disease Information

Information on mitochondrial disease from the United Mitochondrial Disease Foundation.

Death to Sperm Mitochondria

Not a rallying cry, but answers to the question, "Why do sperm mitochondria vanish from the embryo?"

Bibliography

  • Margulis, Lynn, 1971, Symbiosis and Evolution, Scientific American, 225: 48-57.

  • Margulis, Lynn, and Sagan, Dorion, 1986, Microcosmos, University of California Press, California, U.S.A.

  • Mehos, Donna C., 1984, Symbionticism as a Biological Principle, Boston University, Master's Thesis.

  • Sagan, Lynn, 1967, On the Origin of Mitosing Cells. Journal of Theoretical Biology, 14: 255-274.

Glossary


Chromosome - One of the threadlike "packages" of genes and other DNA in the nucleus of a cell. Different kinds of organisms have different numbers of chromosomes. Humans have 23 pairs of chromosomes, 46 in all: 44 autosomes and two sex chromosomes. Each parent contributes one chromosome to each pair, so children get half of their chromosomes from their mothers and half from their fathers.
Mitochondrial DNA -
Genome - All the DNA contained in an organism or a cell, which includes both the chromosomes within the nucleus and the DNA in mitochondria.

Children resemble their parents.
Genes come in pairs.
Genes don't blend.
Some genes are dominant.
Genetic inheritance follows rules.
Genes are real things.
All cells arise from pre-existing cells.
Sex cells have one set of chromosomes; body cells have two.
Specialized chromosomes determine gender.
Chromosomes carry genes.
Genes get shuffled when chromosomes exchange pieces.
Evolution begins with the inheritance of gene variation.
Mendelian laws apply to human beings.
Mendelian genetics cannot fully explain human health and behavior.
DNA and proteins are the molecules of the cell nucleus.
One gene makes one protein.
A gene is made of DNA.
Bacteria and viruses have DNA too.
The DNA molecule is shaped like a twisted ladder.
A half DNA ladder is a template for copying the whole.
RNA is an intermediary between DNA and protein.
DNA words are three letters long.
A gene is a discrete sequence of DNA nucleotides.
The RNA message is sometimes edited.
Some viruses store genetic information in RNA.
RNA was the first genetic molecule.
Mutations are changes in genetic information.
Some types of mutations are automatically repaired.
A chromosome is a package for DNA.
Some DNA does not encode protein.
Some DNA can jump.
Genes can be turned on and off.
Genes can be moved between species.
DNA responds to signals from outside the cell.
Different genes are active in different kinds of cells.
Master genes control basic body plans.
Development balances cell growth and death.
A genome is an entire set of genes.
Living things share common genes.
DNA is only the starting point for understanding human biology.
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