basic intro about topic

2. • Histones are the major proteins involved in this packaging.
• The binding of the chromosomal DNA to histones is the first level of
packaging.
• The DNA-histone protein complex is called the chromatin.
• There are 5 types of histones namely, H1, H2A, H2B, H3 and H4.
• These are basic proteins (25% of basic amino acids lysine and
arginine) and negatively charged DNA (due to the presence of
phosphate group) can easily bind to these positively charged proteins.

3. • Imagine a positively charged histone rod surrounded by negatively
charged DNA thread.
• DNA: Histone ratio is 1:1.

4. Nucleosome model
• Nucleosome is the lowest level of chromosome organization.
• Nucleosome consists of a disc shaped structure of 11nm in diameter
• comprising of 2 parts: A core particle and a small spacer or linker
DNA.

5. • Core particle:
• consists of octamer of histones consisting of 2 copies of H2A, H2B, H3
and H4.
• A DNA strand of 146 bp is tightly wrapped around this core forming
two circles (73 nucleotide bp/turn).

6. • Spacer DNA or linker DNA:
• length varies between species 1-80 bp
• One unit of H1 is associated with it.
• This makes the lowest level of chromosome organization and
chromosomes are most condensed during metaphase.

7. • How many nucleosomes are present in a human cell?
• Each nucleosome consists of 200 nucleotide pairs (approx)
• No. of nucleotide pairs in a human cell= 6x109
• No. of nucleosomes= 6x109/200=3x107 nucleosomes.

9. • The amino acid sequences of histones are highly conserved in
evolution. The most conserved histones are H3 and H4. H1 is the least
conserved histone.
•
• There are five main types of histones called H1, H2A, H2B, H3 and H4.
The five histone are grouped into two categories: Nucleosomal
histones and H1 histones.
•

10. • Types of Histones: Nucleosomal histones and H1 histones
•
• 1. Nucleosomal histones

11. • comprises of H2A, H2B, H3 and H4.
• Each one made up of approx. 102-135 amino acids.
• These proteins are highly conserved during evolution.