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5.2.1.3.2. Tertiary structure of DNA in eukaryotes

Nucleosomal fiber

The nucleosomal fiber, or string of beads, or 100 Å chromatin fiber or beaded structure, appears in eukaryotic somatic cells in interface (not dividing) forming chromatin.

The nucleosomal fiber is made up of a succession of nucleosomes, particles about 100 Å in diameter.

The nucleosome is a structure that constitutes the fundamental unit of chromatin, which is the form of organization of DNA in eukaryotic cells. It is double - stranded DNA of approximately 200 pairs of nucleobases. Nucleosomes are made up of:

  • The nucleus or core is made up of:
    • An octamer of histones (proteins rich in basic amino acids), formed by two molecules of each of the histones H2a, H2b, H3 and H4.
    • About 146 nitrogenous base pairs of DNA that are arranged around the octamer by 1.75 turns.
  • Spacer or linker DNA is the DNA between two consecutive nuclei. It consists of 54 base pairs, about 27 nitrogenous base pairs on each side.

Between two consecutive nucleosomes there is an internucleosomal loop of between 15 and 100 DNA bases. Between two nucleosomes, histone H1 associates with 10 pairs of nucleotides in each nucleosome, making two complete turns.

When the nucleosomal fiber condenses during mitosis and meiosis, the chromosomes become visible .

The crystal structure

In sperm, the nucleus has only half the DNA of somatic (non-reproductive) cells, and it is made up of DNA and protamines.

The protamines are basic and small proteins that histones. The greater attraction between DNA and protamines causes them to be much more packed than the nucleosomal fiber, constituting the crystalline structure .