This document discusses eukaryotic chromosome organization. It notes that eukaryotic cells contain many chromosomes in the nucleus, with each species having a characteristic number. Chromosomes are made up of DNA and proteins like histones. DNA is wrapped around histones to form structures called nucleosomes, which are further compacted through multiple levels of coiling and folding involving other proteins. This allows the long DNA molecules to fit within cell nuclei.

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,
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,

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,

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,

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,
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,
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,
 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,
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,
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,
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,
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,
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,
Levels of Chromosome Packing in Eukaryotes
Courtesy:slideshare.net

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

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