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Why do identical twins look different?

Can genome decide everything

According to available data, about one in 260 pairs of identical twins is as different as Mary and Joe. Scientists are puzzled. From the perspective of genetics, identical twins have exactly the same genome and live in the same environment, so there should be no obvious difference. However, many facts show that when identical twins grow up, there will be great differences in personality, health and disease susceptibility.

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's body, such as liver cells, nerve cells, skin cells, etc. different types of cells have their own characteristics, but the genomes of different types of cells are the same. Why do cells with the same genome have huge differences? This also puzzles scientists. Now it has been identified 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 to say, in different types of cells, only a certain part of genes are working (expressing).

Gene expression is like a wonderful puppet show in the human body. Each puppet comes on and goes out just right. After going on the stage, each puppet makes a wonderful action, which makes the puppet show vivid and colorful. Sometimes it makes the audience happy, sometimes it makes people sad and cry. Such a wonderful puppet show, in addition to the appearance of different puppets, if there is no hidden behind the scenes to control the puppet's matchmaker is certainly not enough. In the cells of any organism, those diverse genes are just like the puppets in the Puppet Troupe. If there is no one who regulates and controls the puppet, the puppet show can not be staged. So where are the 'matchmakers' who regulate and control genes?

Who is 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 gene information does not float and exist alone in cells, but combines with a kind of protein called histone to form a complex called chromatin. The 3-meter-long DNA is wound on a 'bead' composed of four histones, thus forming a long 'bead chain'. Each 'bead' winding DNA is called a 'nucleosome', and two nucleosomes A piece of DNA between them is called connecting DNA. The distance between nucleosomes in chromatin and the number of turns of DNA around nucleosomes are strictly determined. Once the structure of chromatin changes due to epigenetic mechanism, chromatin remodeling is called chromatin remodeling.

Chromatin remodeling will affect which genes play a role (expression) and which genes don't play a role (non expression), which is just like the puppet show in which puppets are put on stage and which puppets are put off stage to rest. The amino acids in DNA and chromosomes are caused by the 'grooming' of proteins. If a segment of the DNA molecule is intensively methylated, the chromatin will be condensed and the gene will not be expressed. On the contrary, if the methyl group added to the nucleotide falls off (demethylation), the distance between nucleosomes will increase, and the gene that was not expressed can be expressed.

In addition to the effect of methylation on gene expression, histones in nucleosomes can also regulate gene expression by "wearing a cap". If histones are "wearing an acetyl cap", it is called "acetylation". Acetylated histones can also cause chromosome remodeling, usually resulting in gene expression. It can be seen that DNA methylation and histone modification are also 'matchmakers' that regulate and control genes.

The difference already exists in the fetus

There are many kinds of specific cells in the same organism because of the different expression of a 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 causes the difference of gene expression between identical twins with the same genome. The study also found that the difference of DNA methylation existed in the fetus, and the difference of heterozygous twins was greater than that of identical twins. The results show that there are differences between the fetus in the womb. After birth, the differences will accumulate under the influence of internal and external environment. By the age of 50, the differences between identical twins are very significant.