Chromatin and Chromosomes: Understanding DNA in Eukaryotic Cells

Chromatin is the complex of DNA associated with special proteins called histones. This material is found in the nucleus of eukaryotic cells and appears as a web of thin threads.

Chromatin is the form in which DNA is functional, i.e., it can be read, copied in the form of messenger RNA, and taken to the cytoplasm of the cell so that the ribosome can translate it and make the corresponding protein.

When the nucleus starts the process of cellular division (mitosis or meiosis), the tangle of threads begins a progressive condensation that ends in the formation of discrete and independent entities: the chromosomes. Therefore, chromatin and chromosomes are two morphologically distinct aspects of a single cellular entity: DNA.

Chromosomes are formed so that the distribution of DNA during cell division is equitable, and each daughter cell receives the same amount of information.

Most plant and animal species have a diploid number of chromosomes, represented by 2n, i.e., pairs of homologous chromosomes. They have a complete set of chromosomes of paternal origin and a complete set of chromosomes of maternal origin.

Each pair of homologous chromosomes is composed of one chromosome from the father and one from the mother that carry information for the same characters, but this information need not be the same.

For example, in the chromosome of the pair of homologs from the parent, information for hair color may be present, and this information may indicate blond hair, while the information from the mother may also be for hair color, but this may indicate black hair.

When diploid species form their sex cells or gametes, they do so through meiosis, and the gametes have half the information because each gamete carries only one chromosome of the pair of homologs.

Gametes are haploid, therefore, represented by n, since they carry only one chromosome for each character.

The fact that a species is diploid is an evolutionary advantage over a haploid species. Having two reports for each character (each one is on one of the homologous chromosomes) means that if information for a character is defective or suffers some other problem, there is still information available, and the organism can continue to live. However, if the only information for a character in a haploid organism is damaged, it can die because it has no other information.

Humans are diploid; they have 46 chromosomes, i.e., 23 pairs of homologous chromosomes. Their diploid number is 2n = 46. All body cells have 46 chromosomes except for gametes (eggs in the case of women and sperm in the case of men), which have half, i.e., 23. n = 23.