What is the definition of "bacterium" that applies to mitochondrial DNA? Being an integral part of (most?) human cells, how can it be referred to as a "bacterium"?
Also, how much information has been developed about the "exchange of genetic information"? What is known about the mechanism?
Watson and Crick realized that the C and T were complementary to the G and A nucleotides and that this suggested a means for replication. It would seem that some of the coding constitute markers for delineating the extent of exchangeable sections of the chromosome. Is any of this understood yet?
Thanks for the clarifications. It's handy having a molecular biologist available. But then you've probably heard that before.
The mitochondria of all eukaryotic cells, and the chloroplasts of plant cells, are both remnants of early bacteria that formed a symbiotic relationship with a larger unicellular organism. I do not know what kind of bacteria the chloroplast is, but there is evidence that the mitochondria are a type of rickettsia. (Note: other species of rickettsia (spread by ticks) are obligate intracellular parasites that can cause severe disease.) The mitochondrial chromosome has all the characteristics of a bacterial chromosome--it is circular, it contains no introns, and most of the DNA is coding (that is, it transcribes directly into message RNA for protein production). In addition, the mitochondrial DNA uses a slightly different triplet code than the nuclear DNA. Many of the mitochondrial genes have been integrated into the nuclear DNA (DNA tends to move around) over the course of evolution. At this point, the mitochondrial are absolutely incapable of independent living. We also cannot live without them.
Another interesting tidbit is that we also have thousands of viruses living in our genome. In some cases, we are dependent on those viruses--a protein necessary for placental development actually is encoded in a virus! A few years ago, I read about some researchers who were reconstituting one of those viruses from the information in our genome. Since they did not know what the virus might do, they did all this work in high biocontainment. Right now, koalas are being infected by a retrovirus that appears to be making itself a permanent part of their genome. The last such event in humans probably took place over 100,000 years ago.
Also, how much information has been developed about the "exchange of genetic information"? What is known about the mechanism?
Watson and Crick realized that the C and T were complementary to the G and A nucleotides and that this suggested a means for replication. It would seem that some of the coding constitute markers for delineating the extent of exchangeable sections of the chromosome. Is any of this understood yet?
There are several mechanisms for genetic information exchange. One is the crossing-over that happens during gametogenesis. I won't talk about all of the mechanisms, but I will discuss one discovered by my hero, Dr. Barbara McClintock. She is my hero because people didn't believe her research was valid, and they were mean to her (in the scientific way, meaning they used big words to insult her), yet she persevered with her work and eventually won the Nobel Prize. What happens is that little pieces of DNA excise themselves from the chromosome and insert themselves somewhere else, either in the same chromosome or a different one. There are enzymes involved that facilitate this process. This system uses the fact that some DNA sequences are repeated throughout the genome; it inserts and removes itself wherever it finds a matching sequence. These transposons, as they are called, can disrupt gene function if they insert themselves into the gene's control region. In corn--the system that McClintock studied--the action of transposons causes the colors of kernels on the same ear to be different. This is especially noticeable in "Indian corn." When a transposon inserts itself into the pigment gene in the germ cell, the resulting kernel is white. All eukaryotes, including humans, have transposons, and those transposons are (IIRC) always moving around.
And, in answer to your question, I will say that yes, quite a bit is understood about how DNA is exchanged between chromosomes. Homologous chromosomes line themselves up on the basis of matching DNA sequences. Hmm, I will have to cut the explanation short, partly because I must go to work, and partly because this explanation really needs pictures.
Thanks for the clarifications. It's handy having a molecular biologist available. But then you've probably heard that before.
Thank you!
:)