Concept of chromatin and its biological role
In the nuclei of somatic cells in most eukaryotes, including humans, genetic material exists as dense complexes. The basis of this structure is DNP (deoxyribonucleoprotein), a compound of nucleic acid and proteins.
The term chromatin refers to a specific complex of DNA and histone proteins. It got its name due to its pronounced ability to be intensely stained by specific cellular dyes.
Proteins within chromatin perform several critically important functions simultaneously:
- They carry out the physical packaging and conformational changes of the thin DNA strand into a bulky, compact structure.
- They regulate gene expression processes by opening or closing access to them.
- They take direct part in DNA replication (doubling) and repair (restoration).
Dynamics of the structure during the cell cycle
The state of genetic material is not constant and directly depends on the cell's life phase:
- During interphase, chromosomes are long, intertwined, and decondensed DNP threads. It is in this "relaxed" chromatin state that the synthesis of new molecules is possible—replication takes place.
- During cell division, chromatin changes its configuration, transitioning into a highly condensed (maximally dense) state, and forms clearly distinguishable chromosomes. The main goal of such tight condensation is to ensure strictly uniform segregation of duplicated chromosomes between the newly forming daughter cells.
Chromosome set and types of chromosomes
Human somatic cells feature a diploid set (designated as 2n), which totals 46 chromosomes. Half of this set (50%) is inherited from the maternal organism, and the other half from the paternal.
Chromosomes in the karyotype are divided into pairs. Homologous chromosomes are paired structures that possess the following features:
- They contain an identical set of genes.
- They have morphological similarity (identical structure).
- They are capable of synapsis (pairing up and joining) during meiosis.
- At the same time, they may carry different allelic forms of the same gene.
Sex chromosomes are distinguished separately. In humans, these are the X and Y chromosomes, which are non-homologous to each other.
Stages of DNA compaction
The packaging process of genetic material proceeds in a strictly sequential manner. Each stage forms a structure of a specific diameter and increases the overall degree of molecule compaction.
- Double-stranded DNA
The basic level, representing a free double helix with a diameter of 2 nm. A short turn of the helix contains about 5 turns.
- Nucleosomal level ("beads on a string")
The first compaction step increases packing density by 6–7 times, with a thread diameter of 11 nm.
- The main unit is the nucleosome. This is a protein core consisting of 8 histones (an octamer comprising two molecules each of H2A, H2B, H3, and H4 proteins).
- DNA wraps around this octamer, making 1¾ turns, which covers a region of approximately 146 base pairs (bp).
- Between nucleosomes is the linker, a free DNA segment 50–70 bp long that is not bound to the protein core and serves as a connecting bridge.
- Nucleomeric (solenoid) level
The nucleosome thread coils into a helix (solenoid or nucleomere), forming a chromatin fibril with a diameter of 30 nm. The packaging degree reaches 40 times. One turn of the helix contains 6–8 nucleosomes. The structure is rigidly stabilized by an additional histone protein, H1, whose task is to shorten the linker DNA.
- Chromomeric (loop) level
The diameter reaches 300 nm, and the degree of compaction reaches 700 times. The chromatin fibril folds into large loops. These loops attach to a special scaffold formed by non-histone proteins. One such loop represents a domain containing several genes (its length ranges from 50 to 200 thousand bp).
- Chromonemal level
Loop domains undergo further coiling. A filamentous chromatin structure with a diameter of 700 nm is formed, providing 1500-fold compaction. Externally, this looks like condensed regions of one of the metaphase chromatids.
- Chromosomal level
The final stage of chromonemal thread packaging. A fully-fledged metaphase chromosome with a diameter of 1400 nm is formed. At this stage, the ultimate degree of compaction is achieved—DNA is packed 10,000 times denser than its initial state.
Briefly: answers to topical questions
Which non-histone proteins participate in the formation of the chromomeric level of compaction?
Acidic non-histone proteins participate in the formation of the chromomeric level of compaction.
At this level:
- the chromatin thread forms loops;
- loops gather into groups—rosettes;
- the bases of the loops in a rosette attach to a common protein center;
- protein centers of rosettes anchor to the nuclear matrix;
- fibrils are connected to non-histone proteins that form the scaffold.
To memorize the dimensions of structures at different packaging levels, learn the numerical series of diameters: 2 → 11 → 30 → 300 → 700 → 1400 nanometers.
Class and exam questions
What is the difference between chromatin and DNP?
DNP (deoxyribonucleoprotein) is a general term for any complex of DNA and proteins. Chromatin is a narrower concept denoting a specific complex of DNA specifically with histone proteins.
What is the purpose of linker DNA?
Linker DNA is a segment of the chain (50–70 base pairs long) that does not wrap around the protein octamer. It is necessary for connecting adjacent nucleosomes to each other.
What function does histone H1 perform?
Histone H1 operates at the nucleomeric (solenoid) level. It binds to DNA and shortens linker regions, stabilizing the structure of the chromatin fibril.
What is a nucleosome?
This is a bead-like structure consisting of a protein core (8 histones: two molecules each of H2A, H2B, H3, H4) and a DNA thread wrapped around it (1¾ turns or about 146 base pairs).
What else to study on this topic
- Structure of histone proteins and their amino acid composition
- Non-histone proteins: functions and participation in chromosome scaffold formation
- Mechanisms of homologous chromosome synapsis in meiosis
- Regulation of gene expression via changes in chromatin compaction degree
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