Silent jumping gene in human body
Biologists often say that the most dangerous thing a cell does is to divide. This is because divided cells need DNA replication, and this process may be wrong, that is, gene mutation. Gene mutation may kill living things, but it is also the key to evolution. Through gene mutation and natural selection, organisms can evolve into various kinds and forms.
When cells divide, some DNA fragments can be randomly transferred in the genome and run from one place to a new place. This DNA fragment is called a jumping gene (also called a transposon). Jumping gene plays the role of unsung hero in the process of evolution, because it can change the composition of genome.
However, jumping genes account for half of all mammalian genetic material. If these jumping genes keep jumping during cell division, isn't the cell always in danger? Scientists have found that mammalian cells have a special ability, that is, they are very good at preventing jumping gene transfer, or silencing jumping genes. Therefore, there is no need to worry about jumping genes in our bodies-most of them are dormant.
But new research finds that the brain is different. In 2000, Rusty Gage, a neurobiologist at Salk Institute of Biology in the United States, and his colleagues studied the development process of human nervous system, that is, the process of immature stem cells developing into mature nerve cells. When investigating the gene expression of these stem cells, they were puzzled to find that most of the jumping genes in these cells were active. They are not silent, but happy.
Jumping genes in the human brain are particularly active.
Gage was surprised by this discovery, but also felt a little uneasy. If jumping genes randomly tamper with the genomes in these stem cells, it means that the genomes of each mature brain cell are slightly different, and each one has different behaviors.
The above-mentioned phenomenon of genome tampering is called gene chimerism, which rarely occurs in other tissues of the body. For example, the genomes of ciliated cells and blood cells that protect our lungs are exactly the same, although ciliated cells look like anemones and blood cells look like lollipops. They look different because they have different ways of gene expression (the process by which cells produce protein according to gene instructions). On the contrary, although nerve cells look very similar, they are very different at the genetic level.
Gage et al. sequenced the DNA of single nerve cells of some human corpses in the next few years and found that it was true. In the same brain, every nerve cell is genetically different. Gage soon realized that this new discovery might help to answer a long-standing question in the field of neurology: how on earth does nature assemble such a complex system as the brain?
There are more than 20,000 genes in Caenorhabditis elegans that biologists often study, and the total number of nerve cells is 302. In contrast, we humans also have more than 20,000 genes, but we always have more than 80 billion nerve cells. It's like giving two chefs seemingly the same ingredients. The first chef only wants to come up with a few recipes, but the second chef can come up with hundreds of thousands of recipes! This is because the raw materials of the second chef occasionally change. For example, celery can sometimes be turned into potatoes or eggplant. According to Gage's discovery, jumping gene makes this transformation possible, which can make nerve cells complex and diverse.
Jumping genes shape each unique brain.
Not only that, but Gage's discovery also shows that each brain is unique. Because the specific mode of operation of jumping genes is not inherited. The ecosystem of the daughter's brain is obviously different from that of her mother, even different from her twin sister.
This also means that it is sometimes difficult for scientists to rule out individual genetic differences in experiments.
For example, C57BL/6 is a mouse commonly used in the laboratory. They are inbreeding, so they can have almost the same genome. So, can this avoid genetic differences? The fact is that genetic differences still exist. Gage put forward a hypothetical experiment: put some mice in a big box. They are identical twins of the same sex. If we test their behavior, we will find that their temperament will be very different. One may be stunned, and the other will go crazy in circles. This is because jumping genes make the genes in each mouse's brain different.
This explains to some extent why it is difficult for scientists to find the genetic basis of nervous system diseases, because there is no unified genome in the brain. In fact, many brain diseases are related to the activation of jumping genes in the brain. But it is not clear whether there is a causal relationship between the two.
Fundamentally speaking, our diverse brains may have deepened the differences in everyone's personality. From this perspective, this strange jumping gene finally helps each of us achieve our unique self.
What's more interesting about jumping genes?
If we compare apes and humans, we will also find more interesting places about jumping genes.
20 13, Gage et al. compared the difference of jumping gene activity between apes and human stem cells. They found that jumping genes in apes are much more active than those in humans. Therefore, the genetic difference between two different groups of chimpanzees separated by thousands of meters is greater than that between any two people on earth. In fact, there is another similar gene in both apes and humans, which can inactivate the jumping gene. However, the expression ability of these genes in humans is stronger than that in apes, which is 20 times as high as their 10.
In this way, compared with apes, our genetic diversity is relatively poor, which is not a good thing from an evolutionary point of view. However, Gage believes that this may force us to move away from the evolution at the genetic level and towards the evolution at the cultural level. In other words, we will use our medical knowledge to fight diseases, and we can also pass on our knowledge to the next generation through channels such as language, instead of relying on genetic variation to randomly find ways to fight diseases.
(This article comes from the article number. 1, the mystery of science * 20 16)