The chromosomes of eukaryotic cells are a highly coiled, coiled DNA molecule in regular spaces around proteins called histones.
Along the DNA molecule are located the genes; therefore, it is responsible for commanding and coordinating all cellular functioning and for transmitting hereditary characteristics.
At the beginning of cell division, the chromosomes are duplicated in two identical filaments, the chromatids , united by a region called the centromere . The centromere is present in all types of chromosomes and divides each chromatid into two arms.
In the course of cell division, the two chromatids of the chromosome are independently condensed. Thus, observed under the microscope, the chromosome is presented as two rods joined by the centromere.
During chromatin spiralization, the heterochromatin regions are less condensed than those of Euchromatin, resulting in regions of narrowing in the chromosomes called constrictions. All chromosomes have at least one constriction. The chromatids joined by the centromere are called sister chromatids .
The shape of the chromosome varies throughout the life cycle of the cell. If the genetic material remains condensed throughout the life of the cell, the activity of the genes is impaired by the lack of space for the enzymes responsible for DNA duplication to act.
On the other hand, the condensation of chromosomes during the period of cell division facilitates movement and distribution of the genetic material to the daughter cells formed, avoiding damage. It is in the period of cell division that the morphology of the chromosomes is studied, because it is the moment when they are more visible due to their high degree of condensation.
Thus, it is essential that the cell synthesize all the molecules necessary for cell division before it begins, during the interphase period, since no transcription occurs during the process. At the end of the cell division, the chromosomes decondensate and assume the chromatin form again.
The chromosomes are classified according to the position of the centromere and the size of the chromosome arms. This classification can be made by interrupting the cell division in the metaphase phase of mitosis, in which the chromosomes show a maximum degree of condensation.
- Metacentric : chromosome with centromere in the central region, with two arms of the same size.
- Submetacentric : The most common type in human species, the centromere is displaced to one of the extremities and arms of different sizes.
- Acrocentric : the centromere is almost at the extremity and one of the arms is larger than the other, as in the Y chromosome.
- Telocentric : the centromere is located at the end, which makes each chromatid show only one arm. No such chromosome is observed in the human species.
Observe in the chart below the graphical representation of each type of chromosome described.
Karyotype and human chromosomes
The set of characteristics concerning the number, shape and size of a cell’s chromosomes corresponds to its karyotype (from the Greek karyon , “nucleus”).
In the human species, for example, the karyotypes of males and females are equal up to par 22 of chromosomes. Pair 23 is the sex chromosome – XX in women and XY in men. The chromosomes that do not vary between men and women are the autosomes (from Greek autos , “own”).
22 AA (autosomes) + XX or 46, XX
22 AA (autosomes) + XY or 46, XY
In the assembly of the human karyotype, we adopt a technique based on cell culture in vitro, performed in an area called cytogenetics. Cells of different types, such as white blood cells (leukocytes) from the blood, may be used.
Thus, the human species has 46 chromosomes in total, pairs 1 to 22 are the same for men and women, presenting only difference in pair 23.
Number of chromosomes
According to the number of chromosomes, the cells may be haploid or diploid. Cells exhibiting homologous chromosome pairs are diploid cells (from the Greek diplo , “two, double”), represented by 2n . Cells that have only one representative of each pair of homologs are haploid cells (from the Greek haplo , “simple”), represented by n . Diploid cells have the total number of chromosomes of their species; the haploids, half that number.
All human somatic cells are diploid, that is, they have two sets of 23 chromosomes or two genomes; a set from the father, and another from the mother, a total of 46 chromosomes. However the human gametes (oocytes and spermatozoa) are haploid cells, consisting of only 23 chromosomes, one of each pair of homologues. When fertilization occurs, the haploid sets of each gamete unite to form the zygote, or egg-cell, the primordial cell of a new being.