CHROMOSOME MODELS SMG

A brief account of Chromosome models - Dupraw model (Folded fibre model ) & Nucleosome model .

1. Dr. Saji Mariam George Associate Professor (Retired) Assumption College Autonomous Changanacherry CHROMOSOME MODELS

2. CHROMOSOME MODELS Dupraw model (Folded fibre model) Proposed by Dupraw (1965) The body of the chromatid is represented by a single DNA – Protein Fibre . The 20 A° DNA double helix is spirally packed in protein to form a fibril . This fibril is coiled to form a 80 – 100 A ° fibre called Type A fibre – DNA is packed in a packing ratio of at least 6: 1 (Extended length / Packed length) .

3. Type A fibre is coiled to form a 200 – 350 A ° Type B fibre – Packing ratio 56 : 1 – extensively folded both longitudinally and transversely to form the chromatid . The folded fibre model applies to both the interphase and metaphase chromosome.

4. Dupraw model (Folded fibre model) Images: https :// www.easybiologyclass.com

5. Nucleosome Model Proposed by Roger Kornberg (1974) Electron microscopic studies have shown that chromatin is formed of a series of repeating units – Nucleosomes - bead like structures which are about 10 nm in diameter. Each Nucleosome contains a histone octamer consisting of two of each of the four histones H2A, H2B, H3 and H4 with about 200 base pairs of DNA coiled on the outside of the nucleosome (Repeating units of chromatin consisting of 200 bp of DNA coiled around a histone octamer ).

6. Image : https://en.wikipedia.org

7. After extensive nuclease digestion, a 146 nucleotide pair long segment of DNA remains present in each nucleosome . This nuclease – resistant structure is called the nucleosome core – It consists of a 146 – nucleotide pair length of DNA and two molecules each of histones H2A, H2 B, H3 and H4. The whole structure has the shape of a flattened disc. The Histones protect the segment of DNA in the nucleosome core from cleavage by endonucleases .

8. Nucleosome model https://biology.stackexchange.com

9. Each nucleosome is separated from the next by a region of linker DNA (spacer DNA) which is generally 54 bp ( bp =base pair) long and contains single H1 histone protein molecule. The formation of nucleosome converts a DNA molecule into a chromatin thread about one – third of its initial length.

10. The chains of nucleotides can be folded into a 30nm fiber forming a solenoid structure having six nucleosomes per turn. This structure is stabilized by interactions between different H1 molecules. The folding of the nucleosome chain to a solenoid structure results in condensation of the chromatin. The solenoid is further coiled into a super solenoid structure which results in extreme condensation.

11. © 2013  Nature Education  Adapted from Pierce, Benjamin.  Genetics: A Conceptual Approach , 2nd ed. Image: https://www.nature.com

12. A single chromosome Image:http :// www.biologymad.com

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