The nucleus in the eukaryotic cell is the central organelle, on which vital activity and synthetic processes depend. A significant part of the contents of the nucleus is represented by filamentous DNA molecules of various degrees of compaction in combination with proteins. These are euchromatin (decondensed DNA) and heterochromatin (tightly packed parts of DNA).
In the life of the cell, euchromatin plays an important role. An “instruction” for assembling ribonucleic acid (RNA), which becomes the basis for the synthesis of polypeptide molecules, is read from it.
Does everyone have a core?
All living things, from the smallest to the gigantic, are provided with genetic information in the form of deoxyribonucleic acid. There are two fundamentally different forms of representing it in cells:
- Prokaryotic organisms (pre-nuclear) have cells that are not divided into compartments. The repository of their only circular DNA, not connected to proteins, is simply a portion of the cytoplasm called the nucleoid. Nucleic acid replication and protein synthesis occur in prokaryotes in a single cell space. We will not see them with the naked eye, because the representatives of this group of organisms are microscopic, up to 3 microns in size, bacteria.
- Eukaryotic organisms are characterized by a more complex cell structure, where hereditary information is protected by a double membrane of the nucleus. Linear DNA molecules together with histone proteins form chromatin, which actively produces RNA using multienzyme complexes. Protein synthesis occurs in the cytoplasm on ribosomes.
The decorated nucleus in eukaryotic cells can be seen during interphase. The karyoplasm contains a protein skeleton (matrix), nucleoli and nucleoprotein complexes, consisting of sections of heterochromatin and euchromatin. This state of the nucleus persists until the start of cell division, when the membrane and nucleoli disappear, and the chromosomes acquire a compact rod-shaped form.
Chief at the core
The main component of the contents of the nucleus, chromatin, is its semantic part. Its functions include the storage, sale and transfer of genetic information about a cell or organism. The directly replicated portion of chromatin is euchromatin, which carries data on the structure of proteins and various types of RNA.
The remaining parts of the nucleus perform auxiliary functions, provide appropriate conditions for the implementation of genetic information:
- nucleoli - compacted areas of nuclear content that determine the synthesis of ribonucleic acids for ribosomes;
- protein matrix orders the arrangement of chromosomes and the entire contents of the nucleus, supports its shape;
- semi-liquid internal environment of the nucleus, karyoplasm, provides transport of molecules and the course of various biochemical processes;
- the two-layer shell of the nucleus, the karyolemma, protects the genetic material, provides selective bilateral conducting of molecules and molecular complexes due to complex nuclear pores.
What does chromatin mean?
Chromatin got its name in 1880 thanks to Fleming's experiments on observing cells. The fact is that during fixation and staining, some parts of the cell manifest themselves especially well (“chromatin” means “stained”). Later it turned out that this component is represented by DNA with proteins, which, due to its acidic properties, actively perceive alkaline dyes.
In the central part of the cell, stained chromosomes forming a metaphase plate are visible in the photo.
DNA Forms
In the cells of eukaryotic organisms, the chromatin nucleoprotein complexes can be in two states.
- In the process of cell division, DNA reaches maximum torsion and is represented by mitotic chromosomes. Each thread forms a separate chromosome.
- During the interphase, when the DNA of the cells is most decondensed, chromatin uniformly fills the nucleus space or forms clots visible in the light microscope. Such chromocenters are more often detected near the nuclear membrane.
These states are alternative to each other; completely compactized chromosomes are not preserved in the interphase.
Euchromatin and heterochromatin
Interphase chromatin is a chromosome that has lost its compact form. Their loops loosen, filling the core volume. There is a direct correlation between the degree of decondensation and the functional activity of chromatin.
Its sections, completely "untangled", are called diffuse or active chromatin. It is practically not visible in the light microscope after staining. This is because the thickness of the DNA helix is only 2 nm. Its other name is euchromatin.
This condition provides access of enzyme complexes to sense DNA fragments, their free attachment and functioning. The structure of messenger RNAs (transcription) is read from diffuse regions by RNA polymerases, or DNA itself is copied (replication). The higher the synthetic activity of a cell at a given moment, the greater the proportion of euchromatin in the nucleus.
The diffuse regions of chromatin alternate with compact heterochromatin zones twisted to varying degrees. Due to its higher density, colored heterochromatin is clearly visible in interphase nuclei.
The picture shows chromatin of various degrees of compaction:
- 1 - double-stranded DNA molecule;
- 2 - histone proteins;
- 3 - DNA wrapped around the histone complex for 1.67 turns, forms a nucleosome;
- 4 - solenoid;
- 5 - interphase chromosome.
Subtleties of definition
Euchromatin at a particular point in time may not be involved in synthetic processes. In this case, it is temporarily in a more compact state and can be mistaken for heterochromatin.
Real heterochromatin, also called constitutive, does not carry a meaning and is decondensed only during replication. DNA in these places contains short repeating sequences that do not encode amino acids. In mitotic chromosomes, they appear in the region of the primary constriction and telomeric endings. They also separate sections of the tancributed DNA, forming intercalary fragments.
How euchromatin "works"
Euchromatin contains genes that ultimately determine the structure of proteins (structural genes). Decryption of the nucleotide sequence into a protein occurs with the help of a mediator, which, unlike chromosomes, can leave the nucleus - messenger RNA.
In the process of transcription, RNA is synthesized on a DNA matrix from free adenyl, uridyl, cytidyl and guanyl nucleotides. Transcription is carried out by the enzyme complex RNA polymerase.
Some genes determine the sequence of other types of RNA (transport and ribosomal) necessary to complete the synthesis of protein from amino acids in the cytoplasm.
Heterochromatin of a single chromosome is often assembled into a clearly visible chromocene. Around it are loops of despiralized euchromatin. Due to this configuration of the DNA of the nucleus, enzyme complexes and free nucleotides necessary for the realization of euchromatin functions are easily suited to the semantic parts.