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5.2.3. Genetic material: chromatin and chromosomes

Chromatin

The chromatin is the way that presents the DNA in the cell nucleus. It is the basic substance of eukaryotic chromosomes, which corresponds to the association of DNA, RNA and proteins found in the interphase nucleus of eukaryotic cells and that constitutes the genome of these cells. Proteins are of two types: histones and non-histone proteins.

The basic units of chromatin are nucleosomes.

The interphase chromatin provides the genetic information for the transcription and translation processes. It distinguishes:

  • The heterochromatin or dense chromatin (with packaging similar to the chromosome fragments of DNA that are not transcribed). It has a more compact appearance.
  • The euchromatin or diffuse chromatin (less packed, less stained, contains most of the active genes). This chromatin contains the DNA fragments that are transcribed.

Chromosomes

When the cell is about to divide (mitosis or meiosis), the chromatin condenses into bodies called chromosomes. Chromosomes, like chromatin, are made up of DNA associated with histones.

The number of chromosomes varies between species, but is constant in all cells of an individual.

The chromosome is a DNA molecule that contains genes, DNA segments that contain the information necessary to synthesize a protein or character of the individual.

A somatic chromosome is made up of:

  • Two identical chromatids from DNA duplication, which is why they are called sister chromatids.
  • The centromere or primary constriction is the region by which the two sister chromatids of the replicated chromosome are held together, the chromosome presenting four arms.
  • The kinetochore, where the microtubules of the mitotic spindle are inserted.
  • The satellite, segment of the chromosome separated by the secondary constriction).
  • The telomere is the end of the chromosome, with special properties that protect the chromosome.

Chromosome shape

All somatic cells of individuals of a species have chromosomes with the same shape. To distinguish it, we will look at the centromere, which will divide the chromosome into two arms (short or p arm and long or q arm ) and on the constrictions of the chromatids.

According to the position of the centromere, chromosomes are classified into:

  • Metacentric. The centromere is located in the middle of the chromosome and the two arms are the same length.
  • Submetacentric. The length of one arm of the chromosome is slightly greater than that of the other.
  • Acrocentric. One arm is very short (p) and the other long (q).
  • Telocentric or Subtelocentric. Only one arm of the chromosome can be seen as the centromere is at the end.

Tipos de cromosomas

Chromatin and chromosomes: similarities and differences

  • Similarities between chromatin and chromosomes:
    • Chromatin and chromosomes are made up of DNA and proteins.
  • Differences between chromatin and chromosomes:
    • Degree of condensation:
      • The chromatin is less condensed, type "pearl necklace", solenoid or loop. It has a fibrillar appearance, little compacted and very metabolically active (genes are transcribed and translated).
      • The chromosome is highly condensed and can be seen with the light microscope. The metaphase chromosome is made up of two sister chromatids joined at the centromere. It is not metabolically active.
    • When does it appear:

Fundamental ideas about chromatin and chromosomes

DNA, depending on the  cell phase , can be found in the form of chromatin or chromosomes:

  • Chromatin
    • In the cell nucleus.
    • Formed by DNA, RNA and proteins (histones) found in the  interphase nucleus of eukaryotic cells.
    • It is made up of a succession of nucleosomes. It contains the genetic information.
    • Two forms of chromatin:
      • The heterochromatin  or  dense chromatin  (with packaging similar to the chromosome fragments of DNA that are not transcribed).
      • The euchromatin or diffuse chromatin. This chromatin contains the DNA fragments that are transcribed.
  • Chromosomes
    • They appear by condensation of chromatin when the cell is about to divide (mitosis or meiosis).
    • Chromosomes, like  chromatin, are made up of DNA associated with histones.
    • The chromosome contains the genes .
    • Components of a somatic chromosome  :
      • Two identical chromatids from DNA duplication, which is why they are called sister chromatids. The chromatid is each of the two equal parts of a chromosome.
      • The centromere or primary constriction is the region by which the two sister chromatids of the replicated chromosome are held together, the chromosome presenting four arms.
      • The kinetochore , where the microtubules of the mitotic spindle are inserted.
      • The satellite, segment of the chromosome separated by the secondary constriction).
      • The telomere is the end of the chromosome, with special properties that protect the chromosome.