Protein composition of chromatin
All proteins associated with nuclear DNA are divided into two major categories: histone and non-histone proteins.
Histones are small proteins with a low molecular weight (ranging from 11 to 22 kDa). Their amino acid composition is dominated by basic amino acids — lysine and arginine. It is these radicals that give the molecules a pronounced positive charge, which is critical for their function.
Non-histone proteins represent a highly diverse fraction. This group includes:
- Enzymes responsible for nucleic acid synthesis (DNA and RNA polymerases, helicases).
- Specific transcription and replication regulatory proteins.
- Additional proteins responsible for the higher levels of DNA compaction.
Nucleosomal organization
The fundamental structural unit of chromatin is the nucleosome. It is formed through electrostatic interactions: positively charged amino acid residues of histones are attracted to negatively charged phosphate groups of the DNA sugar-phosphate backbone.
The structure of the nucleosome includes the following elements:
- Nucleosome core: a protein octamer. It consists of eight histone molecules (two molecules of each of four different types).
- Core DNA: a segment of the double helix approximately 146 base pairs long, which wraps tightly around the protein core, making about 1.75 turns.
- Linker region: a free segment of DNA about 30 base pairs long that connects adjacent nucleosomes. A special histone — H1 — is associated with this linking segment.
Heterochromatin and repression mechanisms
Heterochromatin consists of tightly packed regions of genetic material. In this form, genes are transcriptionally inactive (stably repressed).
The stable inactivity of genes in heterochromatin is ensured by three key factors:
- Initially high level of condensation (compaction) of the DNA strand.
- Formation of nucleosomes (tight binding of the strand to histones, which physically blocks access to genes).
- Methylation of deoxycytidine in so-called CpG sequences. The addition of methyl groups alters the local conformation of chromatin, creating an insurmountable obstacle for transcription enzymes.
Euchromatin and differential gene activity
Euchromatin comprises uncoiled, active (или potentially ready for activation) chromatin regions where the active genes of the cell are located. Such areas are characterized by increased sensitivity to DNase enzymes.
Transition into the active state requires the weakening of ionic bonds within the nucleosome. This is achieved through chemical modifications of histones:
- Acetylation or methylation of amino groups in lysine and arginine radicals.
- Phosphorylation of serine residues.
These reactions reduce the net positive charge of the histone octamer, causing it to hold onto the negatively charged DNA less tightly.
> Clinical example: The set of genes is identical in all cells of the body (with the exception of lymphocytes, which undergo gene rearrangements). However, their activity is strictly tissue-specific. For example, the $\beta$-globin gene in erythrocyte precursor cells (reticulocytes) is located in the euchromatin zone and is actively transcribed, whereas in muscle tissue the exact same gene is hidden within heterochromatin and is completely switched off.
Briefly: answers to topical questions
Which specific types of histones form the nucleosome core (octamer)?
The nucleosome core (octamer) is formed by eight histone molecules of four types. This protein core includes the following histone types:
- Histone H2A — 2 molecules.
- Histone H2B — 2 molecules.
- Histone H3 — 2 molecules.
- Histone H4 — 2 molecules.
These proteins form a globule around which the DNA strand wraps, creating the structural unit of chromatin — the nucleosome.
What are the levels of chromatin compaction from nucleosome to chromosome?
The following levels of chromatin compaction from nucleosome to chromosome are distinguished:
- Nucleosomal level — DNA is wrapped around the histone octamer; nucleosomes form a beads-on-a-string fiber.
- Fibrillar level — nucleosomes are packed into a chromatin fibril, which is a 30 nm superhelix involving histone H1.
- Loop level — the fibril forms loops attached to the nuclear matrix.
- Chromonemal level — a filamentous chromatin structure; condensed regions of a single metaphase chromatid.
- Chromosomal level — the highest degree of compaction in metaphase; the packing sequence culminates in the formation of the mitotic chromosome.
To remember the amino acids that give histones a positive charge, use the phrase: "LYS and ARG carry a POSITIVE charge" (LYSine and ARGinine).
What else to study on this topic
- The mechanism of action of DNA and RNA polymerases, as well as helicases in chromatin
- The process of CpG island methylation and epigenetic regulation
- Types and functions of specific transcription regulatory proteins
- Mechanisms of genetic rearrangements in lymphocytes (an exception to the rule of genome identity)
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