Nucleosomes are the fundamental repeating subunits of eukaryotic chromatin that package DNA into a compact structure. They are composed of 146 base pairs of DNA wrapped around an octamer of histone proteins, resembling beads on a string. This represents the first order of DNA compaction. Higher orders of compaction involve the nucleosomes winding further to form solenoid fibers, scaffold loops, chromatids, and finally full chromosomes. Nucleosomes allow the long DNA molecules to fit within cell nuclei while also regulating genetic expression.

1. NUCLEOSOMESPresented by : Sheetal Narkar

2. PACKAGING OF DNA IN CHROMOSOMESAn eukaryotic chromosome consists of a linear DNA molecule.This DNA is condensed into a complex structure with histones and non histone proteins………….

3. WHY PACKAGING IS REQUIRED ????????DNA is about 3 meters long and it has to be packed in a nucleus, which is only a few micrometers in a diameter.

4. Hence highly coiled structure is required.

5. There are various orders of packaging :

6. First order of packaging : nucleosomes.

7. Second order of packaging : solenoid fiber.

8. Scaffold loop.

9. Chromatid.

10. Chromosome.First order of packaging : nucleosomes. Second order of packaging : solenoid fiber. Scaffold loop. Chromatid. Chromosome.

12. Nucleosomes are the fundamental repeating subunits of all eukaryoticchromatin (except when packaged in sperm).They are made up of DNA and four pairs of proteins called histones, and resemble "beads on a string of DNA" when observed with an electron microscope. They represent the first order of DNA compaction in the chromosome……………………..

13. FORMATION OF NUCLEOSOMESCore particle chromatosome nucleosomeCORE PARTICLE : It consists of 146 bp of DNA wrapped 1.8 times in a left handed helix around the outside of an octamer of histones.CHROMATOSOME : The core particle interacts with one molecule of histone H1 to form a particle containing ~166 bp of DNA called chromatosome.NUCLEOSOME :The chromatosome links with the linker DNA forming a nucleosome containing ~200 bp of DNA.

14. CHROMOSOMAL PROTEINSThe chromosomes of eukaryotes are made up of DNA and proteins.There are 2 major types of proteins associated with DNA in the chromatin.

15. HISTONESThey are the most abundant proteins associated with the chromosomes.They are very rich in basic proteins.At normal ph of the cell the histones have net positive charge that facilitates their binding to the negatively charged DNA. This positive charge is found mainly on the amino group of the side chains of the basic amino acids lysine and arganine.Histones lack tryptophan.

16. Histones are highly modified proteins , and the modifications include acetylation ,methylation and phosphorylation……………..5 major types of histones associated with eukaryotic DNA are :Each class of histone consists of a :N – terminal , which is hydrophobic.

17. C – terminal , which is hydrophilic.

18. Central globular structure , which forms the central molecule.H2A , H2B , H3 and H4 together form an octamer while H1 links to the linker DNA outside the octamer.

20. DEPRESS THE GENETIC ACTIVITY :As the histones increase the compaction of the DNA , it depresses the genetic activity.STRUCTURAL ROLE :They play a structural role in the packaging of DNA molecules and hence render them more compact.Functions of histones :