Breeding, mutating, and modifying organisms
All earthly life is built of the same basic materials, by the same builders, using the same basic tools. What makes Bill different than Sam, and Sam different than a caterpillar, is the blueprint—that is, the DNA— used to build the organism. The diversity of life is then the result of the diversity of DNA.
Sexual reproduction is all a mother and father coming together and taking the DNA handed down to them by their parents to create new DNA. The mother and the father each have two strands of DNA interwoven about one another. One of these strands was passed down to them by their mother, and one passed down to them by their father. This is represented for the mother in Figure 1, where a single DNA “strand” is represented by three folders of instructions. Each folder codes information for a certain trait. For instance the top folder (folder 1) may code for the time of the year the crop flowers, the middle folder (folder 2) may code for the plant's height, and the bottom folder (folder 3) may code for the size of the plant's seed. Each folder of the same number codes for the same thing, but is represented by different colors to emphasize the fact that the content of the folder (that is, the content of the DNA) may contain different instructions. For instance, in Figure 1, the Grandma’s DNA may say the plant should flower early whereas the Grandpa’s DNA may say the plant should flower late.
Figure 1—Metaphor for DNA
The purpose of mating is for one strand of the mother's DNA to combine with one strand of the father's DNA, the result of which is a blueprint for a new offspring with a unique set of genetics. However, when the mother donates half of her DNA she doesn't just give the whole strand of the Grandma's DNA, nor does she donate the whole strand of the Grandpa's DNA. Instead, at each level of the DNA, either the Grandma’s or Grandpa’s DNA is randomly selected for use in the new offspring’s DNA. The two strands of the DNA for a single parent are shuffled to create a new unique offspring. In Figure 2, the DNA for time-of-flowering (folder 1) is taken from the Grandma, folder 2 is taken from the Grandpa, and folder 3 is taken from the Grandma. These choices are random. There was an equal chance that folder 3 could have been taken from the Grandpa instead. Because the DNA are shuffled to create a new offspring’s DNA, and because in reality there are an enormous number of ’folders“ (scientists refer to the folders as alleles) two parents can give birth to an almost endless variety of unique offspring.
Figure 2—Shuffling the DNA segments of Grandma and Grandpa
Everything that happens on the mother’s side happens on the father’s side, and in reproduction one randomly generated DNA strand for the mother is matched with one randomly generated DNA strand from the father to create a new offspring like that in Figure 3.
Figure 3—An offspring’s DNA
When another offspring is create a different, randomly generated DNA strand from the mother and father is once again combined. Perhaps this second offspring will have the genetic code shown in Figure 4. Note that if the DNA from the Grandma and Grandpa were not randomly shuffled, but instead either the Grandma’s or Grandpa’s DNA were taken as a whole segment, two parents could have only four genetically distinct children. The diversity of individual members of a species is then due to this shuffling of DNA.
Figure 4—Another offspring’s DNA
It should be recognized that this random shuffling of genes does not alter the placement of genes in the DNA strand. The folder labeled “1” is always located at the top—never in the middle, never on bottom.
Improving genetics through selective breeding
Ireland experienced a famine in the mid-18th century as a plant disease destroyed virtually all of it’s potato crop. One disease caused so much damage because there was little genetic diversity in the variety of potato crops planted, and so if one individual potato plant was unable to protect itself from the disease it was doubtful that other potato plants could either.
The country desperately needed to inject more diversity in its potato crop genetics, so it went to South America, the place of the potato’s origin, where many different varieties of potatoes still existed. Bring some of these varieties back to Ireland, they bred it with their traditional varieties to create potatoes with new DNA. For instance, maybe now potatoes had a red gene for folder 2, and perhaps this particular gene helps the plant defend itself against disease?
Figure 5—Another offspring’s DNA
So long there exists considerable diversity in the genetic portfolio of a species, selective breeding (where humans make careful decisions about which individuals to breed) can alter crops and livestock in ways to improve productivity and adapt to changing environments.
The patience of nature
Nature does indeed endow species with genetic diversity as a defense against an uncertain future. Changes in climate are foreseeable, but with enough variety of genes in a species natural selection makes the species’ survival likely.
Just in case the existing genetic variability within a species is not enough to help it survive, nature has another trick: genetic mutation. When a parent copies and passes a strand of DNA onto its offspring the copy is usually an exact copy, not not always. Random errors in copying, and random failures in the machinery used to check DNA copies, can result in an altered gene. The gene mutates. Most humans contain one or two mutated genes (more accurately, one or two altered alleles).
These mutations are errors, not something that was intended in reproduction, and usually the mutation makes the organism less fit to survive, but every now and then the mutation offers the organism an advantage. Several thousand years ago a human was born with a mutated gene that allowed it to digest milk, allowing the individual to not only acquire nutrients from the meat and the cheese the cow can produce, but the milk as well, giving it a more efficient source of nourishment. It proved so advantageous that most of its descendants in Eurasia possess that mutated gene, which has now become the “normal” gene.
Mutations happen only rarely, so rarely that scientists can track our ancestors according to whether they happen to possess that mutation or not. I once sent my DNA to National Geographic where my Y chromosone (a particular gene passed down from father to son) was sequenced and compared with information on the DNA of others to determine the path of my ancestors. The results told me that one of my ancestors around 30,000 years ago was part of a group of humans to reach Europe. Many Europeans share this genetic mutation, while few in Africe and the Middle East, thus providing an essential clue that some of my ancestors were long-time inhabitants of Europe. By the genetic mutations occurring on my Y chromosone with those of other human populations, scientists believe that [some of] my ancestors left Africa, traveled up through the Middle East, and then westward into Brittania and the Iberian Penninsula
Figure 6—The path of Dr. Norwood’ as determined by analysis of DNA mutations
While nature is patient humans are not, and in the race to keep food supplies ahead of world population growth scientists cannot just wait for natural mutations of genes to provide a better crop, nor are they satisfied with what they can achieve with selective breeding. Instead, they have learned to quicken the pace of genetic mutation by inducing mutations themselves, either through radiation or exposure to certain chemicals. The wheat crop shown below was created by using chemicals to encourage genetic mutations in the plant’s DNA, most of which made for a poor crop but some of which allowed the wheat to resistant certain herbicides. This resistance allows the farmer to spray herbicides right on top of the wheat plant, killing all weeds but leaving the wheat plant undisturbed.
Figure 7—Wheat variety made from chemically-induced genetic mutation
The Cavendish banana plant supplying virtually all of America’s bananas under severe threat from multiple plant diseases. These plants are particularly vulnerable because the plant is farmed not by breeding certain plants but by making clones, thus leading to little genetic diversity. In scientists’ desperate attempts to locate a Cavendish plant that can withstand the diseases, The Economist magazine reports that scientists have “bombarded plants with gamma rays; three of the resulting mutants have shown resistance in the laboratory to [the Black Sigatoka disease](E1)
Figure 8—Radiation or chemical-induced genetic mutation
Here we go ... time for genetically modified organism
All of the plant breeding techniques thus far are not controversial, but the next one—genetic modification—is the most controversial agricultural issue today. The controversy itself we will postpone to another lecture. For now, our goal is to make sure we understand what a genetically modified organism (GMO) is.
Plant breeding has become much more precise in the last few decades. Now, when scientists want to create an improved variety of soybeans, they don't just want different genes, they often know exactly what gene they want altered in a plant’ DNA. Figure 5 provides an illustration, where the plant breeder would like a red gene in the #2 slot. Moreover, if they can just acquire a copy of red gene they could use GM technology to insert it right into the #2 slot.
Sometimes such a red gene is already present in the soybean’s DNA, just not in the #2 slot. Perhaps it is found lower on a DNA strand, like the #5 slot. If this is the case they can simply remove the red gene from the #5 slot of one soybean plant and insert it into the #2 slot of a different soybean plant. If they are able to convince the organisms’ reproductive machinery that this new red gene belongs in the #2 slot, that newly altered DNA will be copied to create new offspring, and a genetially modified organisms (GMO) is created.
Figure 9—Creating GMOs: cisgenic organisms
Sometimes the gene desired to insert into, say, a soybean plant, isn’t found elsewhere on the soybean’s DNA but is present on the DNA of a completely different organism. When Monsanto sought a gene for resistance to the pesticide Round Up, it searched decontamination ponds where residues of the Round Up pesticide were held, hoping to find colonies of bacteria that were not harmed by the residues. Bacteria can evolve quickly, and it was suspected that some bacteria had developed a resistance to the pesticide, and they were right. After finding such bacteria and identifying the gene(s) bestowing it with resistance, they then removed these gene(s) and inserted it into the DNA of a soybean. It wasn’t easy, nor was it cheap, but it was possible, and the transgenic soybean it created is one of the most successful (and for some, the most hated) of all the GMOs.
To help corn farmers deal with the many pests that prey upon their crops, Monsanto took a gene from the bacterium Bacillus thuringiensis (Bt) that allows the bacterium to produce chemicals toxic to those pests, and inserted it into corn’s DNA, thereby allowing corn to produce it’s own insecticide.
Figure 10—Creating GMOs: transgenic organisms
It is important to recognize that all DNA are made of the same materials, just like both Ford and Hondas are made of the same iron, aluminum, and and plastic steel. The creation of a GMO does not alter the substance of the DNA, only the instructions it contains.
Recall that the Cavendish banana plant which supplies virtually all of the U.S. is under attack from diseases like the Black Sigatoka and the Panama disease. Although scientists are making progress against the Black Sigatoka using chemical-induced mutations, they are looking to genetic modification to thwart the Panama disease. Resistance to the Panama disease has been found in an Asian fruit, and if they can just transfer those genes bestowing the fruit with resistance can be transfered to the Cavendish banana plant then the world’s precious but vulnerable banana tree might be saved.(E1)
So you see, genetic modification allows us to inject genetic diversity in cloned banana plants. Although humans can reduce the genetic diversity within a plant species, they can also increase it.
Pick your poison remedy
Which makes you more uncomfortable: creating new plant varieties by zapping them with radiation, or creating new plant varieties by precisely inserting a gene into a plant’s DNA? Perhaps you are like me, and neither scare you. But if they do, I would have thought that zapping plants with radiation would be labeled as “frankenfoods“, not the transgenic / cisgenic plants. In reality, the opposite is true. I have never heard a single food activist complain about the use of radiation to alter crops, but activists often give the impression that GMOs are the single biggest threat to humankind.
Perhaps there is no logical explanation, but activists seemed to really get energized when Monsanto developed a GM soybean that was resistant to the same pesticide sold by Monsanto, and a corn created its own insecticide. These developments made evident how fast a corporation could alter a crop, and if you believe that pesticide corporations are irresponsible, then GMOs seemed just a way for corporations to endanger the public faster. This is not a belief that I hold, by the way, but only because I place more trust in the Food and Drug Administration, the Environmental Protection Agency, and the United States Department of Agriculture than food activists. Indeed, the GMO controversy has as much to do with trust in corporations and regulators than the GM technology itself.
