The nucleus in a 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 varying degrees of compaction in combination with proteins. These are euchromatin (decondensed DNA) and heterochromatin (densely packed pieces of DNA).
Euchromatin plays an important role in the life of the cell. It reads the "instruction" for the assembly of ribonucleic acid (RNA), which becomes the basis for the synthesis of polypeptide molecules.
Does everyone have a core?
All living beings, from the smallest to the giant, 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 non-compartmented cells. The repository of their only non-protein-bound circular DNA is just a patchcytoplasm called nucleoid. Nucleic acid replication and protein synthesis take place 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 with the help of polyenzyme complexes. Protein synthesis occurs in the cytoplasm on ribosomes.
The formed nucleus in eukaryotic cells can be seen during interphase. The karyoplasm contains a protein backbone (matrix), nucleoli and nucleoprotein complexes consisting of sections of heterochromatin and euchromatin. This state of the nucleus persists until the beginning of cell division, when the membrane and nucleoli disappear, and the chromosomes acquire a compact rod-shaped form.
Main in the core
The main component of the contents of the nucleus, chromatin, is its semantic part. Its functions include the storage, implementation and transmission of genetic information about a cell or organism. The directly replicated part 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 the proper conditions for the implementation of genetic information:
- nucleoli -compacted areas of nuclear content that determine the sites for the synthesis of ribonucleic acids for ribosomes;
- protein matrix organizes the arrangement of chromosomes and the entire contents of the nucleus, maintains its shape;
- The semi-liquid internal environment of the nucleus, karyoplasm, ensures the transport of molecules and the flow of various biochemical processes;
- The two-layer shell of the nucleus, the karyolemma, protects the genetic material, provides selective bilateral conduction of molecules and molecular complexes due to complex nuclear pores.
What does chromatin mean
Chromatin got its name in 1880 thanks to Flemming's experiments on observing cells. The fact is that during fixation and staining, some parts of the cell are especially well manifested ("chromatin" means "stained"). Later it turned out that this component is represented by DNA with proteins, which, due to its acidic properties, actively perceives alkaline dyes.
Stained chromosomes forming a metaphase plate are visible in the central part of the cell in the photo.
Forms of DNA existence
In the cells of eukaryotic organisms, the nucleoprotein complexes of chromatin can be in two states.
- In the process of cell division, DNA reaches its maximum twist and is represented by mitotic chromosomes. Each strand forms a separate chromosome.
- During interphase, when cell DNA is most decondensed, chromatin evenly fillsspace of the nucleus or forms clumps visible in a light microscope. Such chromocenters are more often detected near the nuclear membrane.
These states are alternative to each other, fully compacted chromosomes are not preserved in the interphase.
Euchromatin and heterochromatin
Interphase chromatin is a chromosome that has lost its compact shape. Their loops are loosened, filling the volume of the nucleus. There is a direct relationship between the degree of decondensation and the functional activity of chromatin.
Its parts, completely "unraveled", are called diffuse or active chromatin. It is practically invisible under a light microscope after staining. This is because the DNA helix is only 2 nm thick. Its other name is euchromatin.
This state provides access for enzyme complexes to semantic DNA fragments, their free attachment and functioning. The structure of messenger RNA (transcription) is read from diffuse regions by RNA polymerases, or the DNA itself is copied (replication). The higher the synthetic activity of the cell at the moment, the greater the proportion of euchromatin in the nucleus.
Diffuse sections of chromatin alternate with compact, twisted to varying degrees zones of heterochromatin. Due to the greater density, stained heterochromatin is clearly visible in interphase nuclei.
The figure shows chromatin of varying degrees of compaction:
- 1 - double-stranded DNA molecule;
- 2 - histoneproteins;
- 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, it is also called constitutive, does not carry a semantic load and decondenses only in the process of replication. The DNA at these locations contains short, repetitive sequences that do not code for amino acids. In mitotic chromosomes, they are in the region of the primary constriction and telomeric endings. They also separate sections of transcribed DNA, forming intercalary (intercalary) fragments.
How euchromatin "works"
Euchromatin contains genes that ultimately determine the structure of proteins (structural genes). The decoding of the nucleotide sequence into a protein occurs with the help of an intermediary capable, unlike chromosomes, of leaving the nucleus - messenger RNA.
During transcription, RNA is synthesized on a DNA template 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 processes of protein synthesis in the cytoplasm fromamino acids.
Heterochromatin of a single chromosome is often assembled into a well-marked chromocenter. Around it are loops of despiralized euchromatin. Thanks to this configuration of the core DNA, enzyme complexes and free nucleotides, necessary for the implementation of the functions of euchromatin, easily fit the semantic parts.
