The term "genome" first appeared in the genetic literature in 1922, which refers to the complete set of chromosomes contained in haploid cells. With the development of molecular biology, the core of chromosome structure is confirmed to be a DNA molecule, and then the genome is defined as all the DNA molecules contained in the whole set of chromosomes. There are 23 chromosomes in the human genome. Because the two sex chromosomes X and y are not completely homologous, the human genome includes all the DNA molecules in 24 chromosomes, that is, 22 autosomes and the DNA molecules in X and Y sex chromosomes.
Scientific research shows that each DNA molecule is a linear molecule composed of four base pairs, A-T, T-A, C-G and g-c. there are about 3 billion base pairs in 24 DNA molecules. The main task of the human genome project is to find out the sequence of 3 billion base pairs in 24 DNA molecules, which is a genetic code in our body. If we compare four base pairs to four bricks of different colors, for example, A-T is a red brick, G-C is a black brick, and DNA molecule is a path made of bricks, then 24 human DNA molecules are equivalent to 24 paths with different numbers of bricks of four colors.
If the size of each brick is equivalent to a 5-point Chinese character, then the base pairs (bricks) are arranged one after another, and the length of 60 base pairs is about 10 cm. According to this calculation, the total length of 3 billion base pairs is equivalent to the distance from Montreal, Canada to London, England. It can be seen that it is very difficult to understand the sequence of 3 billion base pairs on 24 DNA molecules. However, as long as we know the sequence of these 3 billion base pairs on 24 DNA molecules, it will be of great significance to understand the origin of human beings, the causes of individual differences, the causes of diseases, longevity and aging and other life phenomena.