Chromosome term was given by Waldeyer.
- Chromosomes are seen during the Metaphase stage of cell division.
- Chromatin reticulum seen during the interphase coils to form chromosomes during cell division.
Chromatin shows 2 types of coiling behavior:
- Euchromatin: loosely coiled chromatin. It is transcriptionally active.
- Heterochromatin: densely coiled chromatin. It is transcriptionally inactive and consists of repetitive DNA. It is present in centromere, telomere & satellite.
Packaging of DNA in chromatin:
Chromatin is made of DNA and histone proteins, in the ratio 1:1.
Histone proteins are basic proteins, rich in Lysine and Arginine.
- They are of 5 types H1, H2A, H2B, H3 & H4.
Chromatin consists of repeating units called Nucleosomes.
- Nucleosomes appear like ‘beads on a string’.
- Kornberg proposed the Nucleosome model.
- A nucleosome consists of the core particle and a linker DNA.
- Core particle is an octamer of 4 histone proteins; H2A, H2B, H3 & H4 each present twice. DNA strand of 146 bp surrounds the octamer. It makes 1¾ coil around the octamer.
- Linker DNA; is a DNA strand of about 8 to 114 bp that joins 2 core particles & completes the 2nd coil around the octamer. H1 histone protein stabilizes the 2 coils around the octamer.