What Is the Function of Genomic DNA?

Nucleic acids are macromolecular compounds in living organisms, including DNA and RNA. The DNA double helix model established by Watson and Crick not only clarifies the structural characteristics of DNA molecules, but also reveals the transmission of genetic information during DNA replication from parent to offspring as a molecule that performs biological genetic functions. The method and high fidelity have laid the foundation for genetics to enter the molecular level and become the most brilliant milestone in the development history of modern molecular biology.

Structure and function of DNA

DNA primary structure

Nucleic acids are made up of many

DNA secondary structure double helix structure model

In 1953, Watson and Crick proposed the famous double-helix structure model of DNA molecules, revealing how genetic information is stored in DNA molecules and how genetic traits are maintained between generations. This is a major milestone in the development of biology.
Before the establishment of the DNA double helix structure model, as early as 1868, Miescher had extracted a complex of nucleic acid and protein from pus cells, which was then called nuclide. However, the important status of nucleic acids in life activities was not recognized until the 1950s.
In the 1920s, Levene studied the chemical structure of nucleic acids and proposed the four-nucleotide hypothesis. In the late 1940s, experiments by Avery, Hershey, and Chase confirmed that DNA is genetic material. Combined with simple techniques such as paper chromatography, quantitative analysis of bases of various biological DNA, found that the base composition of DNA is regular:

(1) DNA of different tissues of the same organism

DNA supercoil

DNA

DNA

Double-helix DNA is further twisted and coiled to form its tertiary structure. Supercoil is the main form of tertiary structure of DNA. Since the supercoil of circular DNA of polyoma virus was discovered by Vinograd et al. In 1965, it is known that most prokaryotes are covalently closed circle (CCC) molecules. Into a superhelix structure (superhelix or supercoil), as shown in Figure 15-11. Some single-stranded circular chromosomes (such as × 174) or double-stranded linear chromosomes (such as phage entry) must also change their chromosomes into supercoiled forms at a certain stage of their life cycle. For eukaryotes, although their chromosomes are mostly linear molecules, their DNA is bound to proteins. The DNA between the two junctions forms a loop structure, similar to CCC molecules, and also has a supercoiled form. . Supercoil is divided into positive supercoil and negative supercoil according to its direction. In eukaryotes, when DNA and histone octamers form a nucleosome structure, there is a negative supercoil. Studies have found that all DNA supercoils are produced by DNA topoisomerases.
Nucleosome

Nucleosomes are the basic structural units that make up chromatin, making DNA, RNA, and protein tissues in chromatin a dense structural form. The nucleosome consists of a core particle and linker DNA, which can be seen as twisted beads under an electron microscope. The former includes a dense eight composed of two molecules: histones H2A, H2B, H3 and H4 Polymer (also known as core histone), and one and three-quarters of a circle of DNA strands of 146 bp in length; the latter includes about 60 bp of connecting DNA between two adjacent core particles and a group of DNA located on the junction region DNA Protein H1, the junction region, gives elasticity to chromatin fibers. Nucleosomes are the first stage of DNA compaction. On this basis, the DNA strands are further folded into six nucleosomes per circle with a fibrous structure with a diameter of 30nm. These 30nm fibers are then twisted into tadpoles, and many tadpoles surround the chromosome skeleton. (Scaffold) forms rod-shaped chromosomes that eventually compress nearly 10,000 times. In this way, only a few centimeters in length of each chromosome (for example, the average length of a human chromosome's DNA molecule is 4cm) is contained in a nucleus with a diameter of several micrometers (for example, the diameter of a human nucleus is 6-7 m).
The formation of nucleosomes and the relationship between the structure and function of DNA supercoil are not very clear, and may be related to the transcriptional regulatory control of genes.

Chromatin and nucleosomes

Chromatin

The chromasome of eukaryotes exists as chromatin for most of the life cycle of the cell. Chromatin is a fibrous structure called chromatin filaments. It is formed by stringing the most basic units, the nucleosomes. DNA is the main chemical component of chromosomes and a carrier of genetic information, accounting for about 27% of all chromosomal components. In addition, histones and non-histones account for 66%, and RNA account for 6%.

Histones are basic proteins with an isoelectric point generally above pH 10.0. They are rich in two basic amino acids (lysine and arginine). The relative proportions of histones are divided into five types. It has been proven that H1 is quite close to the core particle of the nucleosome, and it is just a custom to think that it is located on the connecting DNA.
The five histones are exactly the same in different tissues of the same organism and are similar in different eukaryotes. Histones play a very important role in the packaging of DNA in chromosomes.