Eukaryotic Chromosome Organisation

Levels of packing in chromatin of eukaryotes are described. Role of histones and non-histone protein in chromosome packing is explained.

1. Eukaryotic Chromosome Organization Dr C R Meera Assistant Professor & HOD Department of Microbiology St. Mary’s College, Thrissur

2. Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India Eukaryotic Chromosomes Eukaryotic cells contain large amount of DNA in the nucleus. Unlike prokaryotes, eukaryotes have many chromosomes Each eukaryotic species has a characteristic number of chromosomes Courtesy: microbenotes.com Courtesy: alevelbiology.co.uk

3. Eg : In humans 46 chromosomes seen. 23 pairs of chromosomes Total amount of DNA in a haploid genome of a species is called the “Species C value” . Courtesy: ghr.nlm.nih.gov Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India

4. C value Total amount of DNA in a haploid genome of a species is called the “Species C value” . Amount of DNA found in different organisms varies considerably. Amount of DNA can not be correlated with the relatedness of organisms. Also, no direct relationship between C value and structural and organizational complexity of organisms – “C value Paradox”. Its due to the occurrence of repetitive sequence DNA in the genome. Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India

5. Chromosome organization in Eukaryotic cells Eukaryotic cell cycle has 4 phases called G1, S, G2 & M Courtesy: irrecenvhort.ifas.ufl.edu G 1 phase- Single chromosome S phase- Chromosome duplicate to 2 “sister chromatids” joined by centromere G 2 phase- same as S phase M phase- centromeres separate- “ daughter chromosomes ” formed from sister chromatids Courtesy: legacy.Hopkinsville.kctcs.edu Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India

6. G1 Phase- One linear DS DNA molecule is complexed with protein. Protein is twice the amount of DNA. S Phase- Sister chromatids with one linear DS DNA running the length of each chromatid Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India DNA + Proteins = Chromatins Structure of chromatin is identical in all eukaryotes Courtesy: academia.edu

7. Chromatin structure Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India Chromatins are made up of DNA and two types of proteins called histones & nonhistones . Both proteins necessary for physical structure of chromosome Histones Most abundant proteins Small basic proteins with large amount of Arginine & Lysine Bind to – ively charged DNA 5 types- H1, H2A, H2B, H3 & H4 Amount of histone is equal to amount of DNA in chromatin Courtesy: hayounlee.weebly.com

8. Histones Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India H2A, H2B, H3 & H4 amino acid sequences are very similar & highly conserved between even distantly related species Histones play some vital role in eukaryotes in organizing DNA in chromosome Human DNA is 700 times greater than E coli DNA in chromosome of a single human cell is >2m long Several levels of packing is necessary to pack it into the nucleus Histones play very important role in this chromatin packing

9. Non-histone proteins Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India All proteins associated with DNA except histones Eg : Proteins that bind to DNA during DNA replication, repair, transcription, gene regulation, recombination etc. Mainly with – ive charge, hence bind to + ive histones Each eukaryotic cell will have different non-histones Differ in number & type from cell to cell in same organism Also differ at times in the same cell Also differ from organism to organism

10. Levels of packing in Chromatin Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India Chromatin structure differs in different stages of the life cycle G1 phase-Least compact & most organised before cell division (Metaphase) Least compact structure is called “10 nm chromatin fibre” Appear as “Beads on a string” Beads are 10 nm in diameter and form part of nucleosomes 10 nm chromatin fibre Courtesy: albertkharris.com

11. Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India Nucleosome- 1st level of chromatin packing Courtesy: en.Wikipedia.org 11 nm in diameter A core of 8 histone proteins – 2 each of H2A, H2B, H3 & H4 Surrounded by DNA of 147bp length wound around by 1.65 times This makes DNA compact by a factor of about 6 Strands of DNA between nucleosomes are called “linker DNA” Amount of linker DNA not constant within & among organisms In humans, linker DNA range between 38-53 bp , thus total amount of DNA per nucleosome is between 185-200 bp

12. Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India Solenoid Fibre- 2nd level of packing Courtesy: basicmedicalkey.com H1 Histones responsible for second level of condensation H1 binds to linker DNA at one end of nucleosome and to middle of DNA segment around histone core- A nucleosome core plus H1 is called Chromatosome . DNA spiral helically & becomes regular pattern with 6 nucleosome per turn with diameter of 30 nm - “30 nm chromatin fibre” or “Solenoid fibre”.

13. Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India Chromosome Scaffold Non histone proteins cause next level of packing in chromatin Non histone proteins bind to 30 nm chromatin fibre to form 30-90 kb Chromatin loops of DNA- 180-300 nucleosome per loop Chromatin loops attach to Chromosome Scaffold made up of non histone proteins and form “Rosettes of chromatin loops” Scaffold associated Regions (SARs)- Stretches of DNA that bind to non-histone proteins to determine the loop 15 loops per turn- chromosomes are now 10,000 times shorter & 400 times thicker than naked DNA Courtesy: quora.com

14. Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India Levels of Chromosome Packing in Eukaryotes Courtesy: slideshare.net

15. Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India Levels of Chromosome Packing in Eukaryotes Courtesy: bmcbiophys.biomedcentral.com

16. Eukaryotic Chromosome Organisation , Dr C R Meera, St.Mary’s College, TCR, Kerala, India