From the middle of the 19th century to the beginning of the 20th century, the research on the nucleus has made considerable progress.
1. In 1875, German botanist E.A. Strassburger first described the coloring objects in plant cells and concluded that the same plants each had a certain number of coloring objects.
2. In 1880, Baranetsky described the spiral structure of colored objects, and Pfitzner discovered the colored particles the following year.
3. In 1885, German scholar C. Lable proposed the law of constant number of colored objects.
4. In 1888, W. Waldell officially named the colored objects in the nucleus as chromosomes.
5. In 1891, German scholar H. Henkin observed the X chromosome in sperm cells of insects.
6. In 1902, W.L. Stevens, E.B. Wilson and others discovered the Y chromosome.
The phenomenon of cell division has received attention during this period and has been carefully analyzed.
1. In 1867, the German botanist W. Hofmeister was in plants, and A. Schneider was in animals in 1873. They respectively described indirect divisions in more detail.
2. In 1882, German cytologist W. Fleming proposed the term mitosis to replace indirect division after discovering the longitudinal division of chromosomes. E. Heuzer described the distribution of chromosomes during indirect division; After him, EA Strassburger divided mitosis into: early, mid, late, and terminal; he and other scholars also observed meiosis in plants, and after further research, they finally distinguished haploid and diploid. Number of chromosomes.
3. In 1933, H. Bauer discovered polytene chromosomes in the Malpiki tube cells of mosquitoes.
4. In 1934, T.S. Payint discovered this structure in Drosophila, and R.L. King and H.W. Beams in Chironomid.
Polytene chromosomes are huge chromosomes that exist in some gland cells of Diptera larvae. In Drosophila, their length is about 100 times that of normal chromosomes, and each chromosome consists of many (up to 400) It is composed of dyed fibers, showing darkly stained bands and lightly stained interbands on the entire chromosome. Its formation is due to mitosis in the nucleus (only chromosomes are divided but not nuclei), so each polyline chromosome is actually formed by many chromosomes. This kind of chromosome is bulky, which facilitates the analysis of the fine structure of the chromosome. In addition, the functional activity of polytene chromosomes can be judged based on the swelling bubbles.
It was not until the 1970s that nucleosomes were observed under an electron microscope; shortly afterwards, combined with biochemical extraction, it was observed that the chromosomes in the metaphase were so-called scaffold proteins as the core, and the DNA fibers stretched out to form spirals.