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6.11.3. The mitotic nucleus or nucleus in division

The mitotic nucleus or nucleus in division

During cell division, chromatin filaments condense to form chromosomes. We will see the changes that occur in the nucleus when we treat mitosis, now we will focus on the chromosomes.

Chromatin fibers are so dispersed and tangled that they cannot be distinguished. In mitosis, in the S phase the DNA is duplicated, and an identical copy of each chromosome is created, so that thereafter, each chromosome is made up of two identical subunits, called chromatids.

When the M phase begins, the two chromatids on each chromosome spiral and shorten, the chromatin fibers coil and bundle around non-histone proteins to form the mitotic chromosome.

The active chromosome corresponds to the 300 Å fiber. The mitotic chromosome is the way to distribute the genetic material between the two daughter cells without difficulty, since the chromatin would be very difficult to separate equally.

It would also be useful to review the  morphology of the chromosomes that we saw in the topic of nucleic acids.

Number of chromosomes

All cells of an individual, except sex cells, and all individuals of the same species, have the same number of chromosomes.

Most organisms are diploid (2n), having a maternal and paternal set of chromosomes.

Gametes (ovules and sperm), fern and moss spores , certain algae, and the hyphae of some fungi are haploid (n), so they contain a single set of chromosomes.

There are also other exceptions, such as wheat, which contain more than two chromosome sets and are called triploids (3n), tetraploids (4n) and, generally, polyploids, when they contain up to several hundred times the normal amount of DNA.

Humans have 46 chromosomes, of which 44 are autosomes, and the other two are sex chromosomes: XX in women and XY in men. These chromosomes can be distinguished into metaphase cells .

Chromosome endowment in different animal and plant species:

Ascaris: 2 or 4 Hyacinth: 8 Potato: 48 Chimpanzee: 48
Onion: 16 Cat: 38 Corn: 20   Drosophyla: 8
Jewish: 22 Dog: 78 Pea: 14 Mouse: 40
Tobacco: 48 Male: 46 Tomato: 24 Horse: 64

Chromosomes can be stained with dye and photographed. If the 46 chromosomes are cut out, and they are grouped by their shape, we will obtain the graphic representation of the chromosomes or karyogram . When doing the karyogram, it will have been observed that the chromosomes are the same two by two.

These chromosomes that are identically shaped, and are genetically similar, since they contain information for the same characters, are called homologous chromosomes. One of them comes from the father and the other from the mother, so the DNA of the homologous chromosomes is not the same. On the other hand, the DNA of the sister chromatids is the same, since one chromatid is created by duplication (DNA replication) of the other.

Chromosome laws

Chromosomes follow the following laws:

  1. Chromosome individuality. They continue to exist as an individualized structure during the interface, although since they are decondensed they cannot be distinguished.
  2. Numerical constancy. All somatic cells have the same number of chromosomes (2n), except for reproductive cells (ovules and sperm), which contain half (n) of the chromosomes. When fertilization occurs, the new individual will again be diploid (2n).
  3. Formation of pairs of homologous chromosomes. There is always an even number of chromosomes, since each chromosome has another homologue. This is how pairs of homologous chromosomes are formed.