As a prerequisite for genetic material, chromosomes are closely related to heredity, so people come to study the chemical composition of chromosomes to see which components in chromosomes are genetic material. The results of chemical analysis show that the main components of eukaryotic chromosomes are nucleic acid and protein, and their approximate proportions are as follows:

So, is the genetic material protein or nucleic acid?

As genetic material, at least the following four conditions must be met.

1. can accurately replicate itself in the process of cell growth and reproduction;

2. It can guide the synthesis of protein, thus controlling biological characteristics and metabolism;

3. It has the potential to store a lot of genetic information;

4. The structure is relatively stable, but it will mutate under special circumstances. After the mutation, it can continue to be copied and passed on to future generations.

There are about 20 main amino acids that make up protein. Due to different kinds and quantities of amino acids and different arrangement order, countless protein can be formed, which meets the third condition mentioned above. Protein (especially enzymes) can control the traits and metabolism of organisms, which meets the second condition. However, protein can't replicate itself, its content in chromosome is often not fixed, and its molecular structure is unstable, so it can't be passed on to future generations, so protein can't be genetic material.

Scientific research has fully proved that nucleic acid meets the above four conditions, so nucleic acid is the genetic material of organisms. Nucleic acids are divided into deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The genetic material of most organisms is DNA, and the genetic material of some viruses is RNA.

DNA is indirect evidence of genetic material. Before Avery and his colleagues completed the transformation experiment of pneumococcus, indirect evidence suggested that DNA might be genetic material. The indirect evidence that DNA is genetic material mainly includes the following points.

1.DNA is distributed in chromosomes and is the main component of chromosomes, and chromosomes are directly related to heredity.

2. The DNA content in the nucleus is very stable, and there is a parallel relationship with the number of chromosomes. In the cells of the same organism, the DNA content of somatic cells (diploids) is twice that of germ cells (haploids), and the number of chromosomes of somatic cells is exactly twice that of germ cells.

3.DNA is stable in metabolism and is not affected by factors such as nutritional conditions and biological age.

4. Some physical and chemical factors that act on DNA, such as ultraviolet rays, X-rays and nitrogen mustard. Will cause changes in the genetic characteristics of organisms.

The essence of pneumococcal transformation 1944, Avery and others proved for the first time that DNA is a transformation factor of pneumococcal, which is of great significance in molecular genetics. Avery et al. respectively extracted DNA, protein and polysaccharide from S-type pneumococcus, and put each substance into the culture medium of R-type pneumococcus. It is found that only DNA can transform some S-type pneumococci into S-type pneumococci. This kind of DNA obtained from the donor bacterium is given to another bacterium in a certain way, thus changing the genetic characteristics of the recipient bacterium, which is called transformation. The essence of transformation is the recombination of foreign DNA with the DNA of the recipient cell, so that the recipient cell can obtain new genetic information (Figure 6- 1). Experiments show that the transformation rate is related to the DNA purity of donor bacterial cells. The purer the DNA, the higher the conversion rate. If the DNA in the donor cell is degraded by dnase in advance, the transformation will no longer exist.

At present, the role of RNA in virus reproduction and inheritance has been determined. In the inheritance of most animals and plants, DNA is the main genetic material, and RNA also plays a role in inheritance. Some plant viruses and animal viruses only contain RNA, but not DNA, and their genetic traits are determined by RNA. For example, tobacco mosaic virus (TMV) is composed of protein and RNA. As early as 1957, Gilles and Shi Lamu treated the virus with carbolic acid to remove protein, leaving only RNA, and then inoculated RNA into normal tobacco, resulting in mosaic disease; If normal tobacco is partially infected with protein, mosaic will not occur. This proves that RNA plays the role of genetic material. Later, someone combined the RNA of plantain virus (HRV) with the protein of tobacco mosaic virus to form a new strain similar to "hybridization". The infection experiment was carried out with it. As a result, according to the specificity of RNA, that is, according to the RNA of Plantago virus, the virus type that the disease occurred and spread was transferred. This further confirmed the role of RNA in heredity.