The DNA or deoxyribonucleic acid is the biomolecule that contains the genetic information to make cell proteins. Proteins, especially enzymes, are responsible for the regulation of all vital processes, including growth, tissue repair and reproduction.
The DNA is found in the nucleus of eukaryotic cells and is part of the chromosomes.
STRUCTURE OF CHROMOSOMES
The chromosomes are formed by a material called chromatin. Chromatin is the association of DNA with proteins (histones). When the cell is at rest, the chromatin is dispersed throughout the nucleus, and the chromosomes are not visible. Before cell division, the DNA is doubled, and the chromatin condenses, forming shorter and thicker fibres, which are the chromosomes.
The chromosomes are shaped like a cane, with a narrower zone called a centromere, which divides the chromosome into two arms of the same or different length.
During cell division, as a result of DNA duplication, each chromosome appears duplicated, that is, formed by two identical chromatids joined by the centromere.
NUMBER AND TYPE OF CHROMOSOMES
The number, shape and size of chromosomes are characteristic of each species. The number is not related to the complexity of the organism.
The human karyotype consists of 23 pairs of chromosomes. All diploid cells in the body contain these 23 pairs, that is, 46 chromosomes. The gaments are haploid cells and contain a single set, that is, 23 chromosomes.
The chromosomal formula in the human species is 2n = 46, where n is the number of homologous pairs of chromosomes.
In mammals, it is called X and Y. In the human karyotype, it is represented in par 23. The other chromosomes are called autosomes.
GENES ARE LOCATED IN CHROMOSOMES
Each chromosome contains numerous genes arranged in a linear fashion. The genes of two chromatics of each chromosome are identical since they are the result of DNA duplication.
The genome is the set of genes in an organism. The human genome contains about 35,000 genes.
The fact that you are a human being and not a bird, the colour of the hair, the skin or the eyes, the strokes of the face, the stature or how long your life will be, the inheritance of certain hereditary diseases, depends totally on the genes.
However, not all genes are active in all cells: the genes responsible for the colour of your eyes are activated in the cells of the iris, but they are not active in those of the liver or in those of a muscle, for example. The genes responsible for producing insulin are activated in the cells of the pancreas, but not in the others.
GENES AND HOMOLOGICAL CHROMOSOMES
The chromosomes of a cell are grouped into pairs of homologous chromosomes. The two chromosomes of each pair are the same shape and size; one comes from the mother and one from the father.
Each of the chromosomes of a pair of homologues contains genes that determine the same character, for example, eye colour or hair type, are located in the same position of the chromosome.