Mary and Joe are identical twins. They live in an ordinary and happy family. As like as two peas, Marie and Joe both loved sports and art before they were 20 years old. They did equally well in school. In the eyes of teachers and classmates, the twins looked almost the same regardless of their appearance or preferences. But after the age of 20, the situation changed and Mary began to have hallucinations and delusions. With the passage of time, Mary's hallucinations and delusions became more and more serious, and finally became a schizophrenic, while Joe was all right
Can the genome decide everything
According to the available data, about one in every 260 pairs of identical twins has the same obvious difference as Mary and Joe. Scientists are very confused. From the perspective of genetics, identical twins have exactly the same genome and live in the same environment. There should be no obvious differences. However, many facts show that identical twins often have great differences in personality, health and disease susceptibility when they grow up.
In fact, this is similar to another phenomenon in the human body: there are more than 200 different types of cells in the same person, such as liver cells, nerve cells, skin cells, etc. different types of cells have their own characteristics, but the genomes in different types of cells are the same. Why are there huge differences between cells with the same genome? Scientists are also troubled by this. Now it has been found that there are about 22000 genes in the human genome, but in each special type of cell, only thousands of genes really play a role, that is, only a specific part of genes play a role (expression) in different types of cells.
Gene expression is like a wonderful puppet show performed in the human body. Each puppet comes and goes on stage just right. After playing, they make their own exquisite actions to make the puppet show lively and colorful, sometimes make the audience smile and sometimes make people cry. Such a wonderful puppet show, in addition to the puppets with different looks, can't be done without the matchmaker who hides behind the scenes and controls the puppets. In the cells of any organism, those diverse genes are just like the puppets in the Puppet Troupe. Without the matchmaker who regulates and controls the puppets, the puppet show cannot be staged. So where are the "matchmakers" who regulate and control genes?
Who's manipulating genes
In order to find the "matchmaker" who regulates and controls genes, scientists have conducted in-depth research on the existence of genomic DNA in the nucleus. The results show that the DNA carrying genetic information does not float and exist alone in the cell, but combines with a protein called histone to form a complex called chromatin. The DNA about 3 meters long is wound on the "beads" composed of four histones to form a long "bead chain", and each "bead" wound with DNA is called "nucleosome", A piece of DNA between two nucleosomes is called connecting DNA. The distance between nucleosomes in chromatin and the number of turns of DNA around nucleosomes are strictly fixed. Once it is changed due to the regulation of epigenetic mechanism, the chromatin structure will change, which is chromatin remodeling.
Chromatin remodeling will affect which genes play a role (expression) and which genes do not play a role (non expression), which is like a matchmaker in a puppet show, which puppets perform on the stage and which puppets step down to rest. What causes chromosome remodeling is the "dressing up" of nucleotides in DNA and amino acids in histones. If a segment of DNA molecule is intensively methylated, the chromatin will be condensed and the gene will not be expressed. On the contrary, if the methyl added to the nucleotide falls off (demethylation), the distance between nucleosomes will increase, and the originally unexpressed genes can be expressed.
In addition to methylation affecting gene expression, histones in nucleosomes can also regulate gene expression by "wearing a hat". If histones "wear" acetyl groups, this "hat" is called "acetylation". Acetylated histones will also cause chromosomal remodeling, usually causing gene expression. It can be seen that DNA methylation and histone modification are also "matchmakers" for regulating control genes.
The difference already exists in the fetal period
There are many kinds of specific cells in the same organism because of the difference of a specific group of genes. The difference between identical twins is also due to the difference of some expressed genes. Some scientists have made a comparative analysis of DNA methylation and histone acetylation of 40 pairs of identical twins. The results show that 65% of the 40 pairs of identical twins have the same degree of DNA methylation and histone acetylation, and 35% have obvious differences. It is this 35% difference that leads to the difference of expressed genes between identical twins with the same genome. The study also found that the difference in DNA methylation existed in the fetus, and the difference between fraternal twins was greater than that between identical twins. This result shows that the fetus has differences in the womb. After birth, it is constantly affected by the internal and external environment, and the differences will continue to accumulate. By the age of 50, the differences between identical twins have been very significant.