Features of the DNA double helix
The spatial organization of Acidum deoxyribonucleicum has a number of strict biochemical regularities. The outer backbone of the helix is formed by deoxyribose-phosphate residues.
The inner part of the molecule requires reliable helix stabilization. Nitrogenous bases within the molecule are stacked one upon another, forming a structure resembling a "stack". Powerful hydrophobic interactions arise between these bases, which keep the structure from falling apart.
The geometric parameters of the molecule are very stable: exactly 10 nucleotide pairs are contained per each turn of the helix. In addition, the spatial folding of the chains forms two characteristic grooves on the molecular surface — the major groove and the minor groove.
Principle of complementarity
The two strands of the molecule are joined strictly according to the rule of complementarity, which is implemented via hydrogen bonds between the nitrogenous bases:
- Adenine (A) — Thymine (T): 2 hydrogen bonds are always formed between these bases.
- Guanine (G) — Cytosine (C): this pair is bound more strongly since 3 hydrogen bonds are formed between them.
This selectivity serves as the foundation for all processes of copying and reading genetic information.
Tertiary structure and chromatin
To fit the massive molecule inside the microscopic cell nucleus, tertiary DNA structure is formed. It arises due to the tight interaction of the DNA strand with specialized proteins. The main goal of this process is supercoiling and subsequent compaction (packaging of the molecule into a chromosome).
The complex of DNA with proteins is called chromatin. During interphase (when the cell is in a resting state), chromatin is uniformly distributed throughout the volume of the nucleus.
Chromatin consists of two main groups of proteins:
- Histones — the main structural proteins responsible for packaging.
- Non-histone proteins — perform other regulatory and structural functions.
Nucleosome structure
The nucleosome acts as the basic structural unit of chromatin. The process of DNA wrapping around proteins is strictly organized and includes several components:
- Nucleosome core: represents a protein octamer. It consists of 8 histone molecules of four different types. This core includes two molecules each of H2A, H2B, H3, and H4 histones.
- DNA wrapping: the double helix wraps around this histone core, making approximately two turns around it.
- Connecting elements: individual nucleosomes are joined into a single chain by means of linker DNA (which connects adjacent cores). A special histone H1 binds to this linker DNA. Its function is to bring nucleosomes closer together, thereby actively participating in chromatin compaction.
Briefly: answers to topical questions
Which nitrogenous bases of DNA are purines and which are pyrimidines?
Nitrogenous bases in the DNA molecule are divided into two groups: purines and pyrimidines. These compounds form pairs based on the principle of complementarity, connecting to each other via hydrogen bonds.
- Purine bases — adenine (A) and guanine (G), which are common to both DNA and RNA molecules.
- Pyrimidine bases — thymine (T) and cytosine (C).
What specific functions do non-histone chromatin proteins perform?
Non-histone chromatin proteins are specifically noted for their role at the chromomeric level of DNA packaging: the chromatin fibril forms loops that attach to a scaffold made of non-histone proteins. Chromatin proteins as a whole are noted for ensuring DNA "packaging" and the execution of intranuclear processes.
Remembering the bonds is simple: "A–T" (Adenine-Thymine) — short pair, 2 bonds. "G–C" (Guanine-Cytosine) — rounded letters, 3 bonds.
Class and exam questions
What forces stabilize the DNA double helix?
The helix is stabilized by hydrophobic interactions between nitrogenous bases stacked on top of each other.
How does histone H1 differ from other histones?
Histones H2A, H2B, H3, and H4 form the nucleosome core (octamer) around which DNA is wrapped. Histone H1 is not part of the core, but binds to the linker DNA, bringing nucleosomes together.
What is chromatin and when is it uniformly distributed in the nucleus?
Chromatin is a complex of DNA with histone and non-histone proteins. It is uniformly distributed throughout the nucleus during cell rest (interphase).
What else to study on this topic
- Mechanism of hydrophobic interactions between nitrogenous bases
- Functional significance of the major and minor grooves of DNA
- The role of non-histone proteins in chromatin structure
- The process of DNA supercoiling and the enzymes involved in it
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