The document discusses eukaryotic chromosomes, focusing on chromatin condensation during mitosis and the structural organization of chromatin. It describes the role of histones and non-histone proteins in chromatin packaging and the modifications that regulate chromosome structure and gene activity. Current models of chromosome organization are also examined, highlighting the processes of nucleosome formation and higher-order chromatin folding.

1. EUKARYOTIC CHROMOSOME
and
CHROMOSOMAL CONDENSATION
Dr. Nagabhushan C M
Assistant Professor, Dept. of Studies in Zoology,
Vijayanagara Sri Krishndevaraya University, Ballari
9880121090
nagabhushancm@vskub.ac.in

2. The changes occurring during cell division,
especially the fate of chromatin, have been
fascinating biologists since the 19th century when
chromatin could be visualized with novel cellular
staining techniques.
The iconic structure of X-shaped mitotic chromosomes
not only decorates numerous covers of scientific
journals but also anchored in human knowledge.
The history

3. In animal cells, the nuclear envelope breaks down with
the onset of mitosis so that a variety of cytoplasmic proteins
can access chromatin.
Although chromatin changes are already detectable before
nuclear envelope breakdown, this leads to further chromatin
condensation and allows assembly of the mitotic spindle,
which will capture, move and align the individualized
chromosomes at the metaphase plate and segregate the
disengaged chromatids. ……
How ? This way..!!

4. At the end of mitosis a nuclear envelope reforms around
the segregated and decondensing chromatin in each of the
emerging daughter cells.
Although in some yeast species the nuclear envelope is
not dis-assembled and reassembled during mitosis, cytosolic
factors similarly get access to the nucleoplasm with entry into
mitosis allowing chromatin condensation and
intra-nuclear spindle formation to occur.
After chromatin segregation, the compacted chromatin is
decondensed to re-establish its interphase state.
How ? This way..!!

5. The difference
Escherichia coli genome (4.6 Mb)
would be 1,000 times longer than the cell.
The human genome (3.4 Gb)
would be 2.3 m long if stretched linearly.
Solutions
1. Super coiling DNA double helix is twisted in space about its
own axis, a process is controlled by
topoisomerases.
2. Looped domains

6. The structure of eukaryotic chromosome
Chromatin is a complex of DNA and chromosomal proteins
~ twice as much protein as DNA
Two major types of proteins:
1. Histones are abundant, basic proteins with a positive charge that bind
to DNA.
5 main types: H1, H2A, H2B, H3, H4
~equal in mass to DNA
evolutionarily conserved
1. Non-histones remaining other proteins associated with DNA.
differ markedly in type and structure.
amounts vary widely
>> 100% DNA mass
<< 50% DNA mass

7. The HISTONES of nucleosomes
1. HISTONES are the major PROTEIN component of chromatin.
2. Are SMALL, and BASIC PROTEINS (lys & arg) to neutralise the CHARGED DNA.
3. The assembly of histones with DNA required a molecular chaperone,
nucleoplasmin.
1. H1 = Clamp For DNA Wrapped Around Nucleosome
2. H2A = Found In Two Copies Of Each In The Nucleosome.
3. H2B = Found In Two Copies Of Each In The Nucleosome.
4. H3 = Found In Two Copies Of Each In The Nucleosome.
5. H4 = Found In Two Copies Of Each In The Nucleosome.
Forming OCTAMER.
9. H3 AND H4 are very conserved species of proteins upto 98%.
10. ~only 1 % change in the last 600,000,000 years.

8. NUCLEOSOMAL PACKAGING OF DNA
In the presence of H1 protein,
175-200 bp DNA is associated
with the NUCLEOSOME.
Around 146 bp is wrapped
around the OCTAMER.
H1 helps CLAMP the DNA onto
nucleosome and participates in
higher-order chromatin folding.

9. HISTONE MODIFICATIONS
HISTONEs are modified in order to regulate;
1. Compact Chromosome structure
2. Gene activity.
The main types of modification include;
1. Phosphorylation of serines.
2. Methylation of lysines.
3. Acetylation of lysines, nutralization of positive charges......contd

10. Modification takes place at the N-terminal tails
1. ACETYLATION:
a. It is carried out by HAT
(histone acetyl transferase)
b. Acetyl groups are removed by HDACs (Histone
deacetylases)
c. Acetylation neutralizes lysine + charges and
LOOSENS histone / DNA interactions.
d. Thus provides chromatin decondensation.

11. Modification takes place at the N-terminal tails
2. METHYLATION:
a. Methylation of lysines is carried out by
histone methyl transferases. (HMT)
b. It promotes formation of highly compact
chromatin (eg. HETEROCHROMATIN)

12. Modification takes place at the N-terminal tails
3. PHOSPHORYLATION OF SERINES:
A. It is carried out by KINASES,
B PHOSPHATASEs remove phosphates.
C Phosphorylation helps to pack nucleosomes
together and thus TENDS TO promote higher
levels of COMPACT CHROMATIN.
(eg.metaphase chromosome)

13. Metaphase
chromosome

15. Metaphase chromosome
It is formed by LOOPING and
COILING of condensed
chromatin. (association with
H1 phosphorylation)

16. Hierarchical
Folding
Scaff
old
Current Opinion in
Cell Biology
Two classes of models
describe the structure of mitotic
chromosomes.
Hierarchical folding models (left)
suggest that chromatin fibers are
folded into consecutive higher-order-
structures starting from initial 11-nm-
fibers (‘beads-on-a-string’).
The scaffold model (right) predicts
the existence of a continuous,
proteinaceous core at the center of
chromosome arms to which loops are
attached.

17. H
3 T
3
Ac
K16
A
c
Ac
Ac
A
c
H
4 H3 T3
phosphorylation
T
3
Aurora
B
P
S1
0
P S10 P
Loop
formation
Ac
K16
A
c
Ac
Ac
A
c
H3 S10
phosphorylation
P S10
P
P P
P P P
Ac
K16
A
c
Ac
Ac
A
c
H4 K16
deacetylation
Axial
Compression P P
P P
P
Lateral
Compaction
Current Opinion in Cell
Biology
Condensin-driven condensation
(left) in prophase leads to loop
formation, which are subsequently
compacted in axial and lateral
direction.
Histone-driven condensation (right)
promotes local chromatin
compaction mediated by
interactions between neighbouring
nucleosomes and controlled by
post-translational modifications.

18. Packaging of DNA into chromosomes
Level 1 = Winding of DNA around histones to create a nucleosome
Level 2 = Nucleosomes connected by strands of linker DNA like beads on string
Level 3 = Packaging of nucleosomes into 30-nm chromatin fiber.
Level 4 = Formation of looped domains.
1

19. Packaging of DNA into chromosomes
Level 1 = Winding of DNA around histones to create a nucleosome
Level 2 = Nucleosomes connected by strands of linker DNA like beads on string
Level 3 = Packaging of nucleosomes into 30-nm chromatin fiber.
Level 4 = Formation of looped domains.
2

20. Packaging of DNA into chromosomes
Level 1 = Winding of DNA around histones to create a nucleosome
Level 2 = Nucleosomes connected by strands of linker DNA like beads on string
Level 3 = Packaging of nucleosomes into 30-nm chromatin fiber.
Level 4 = Formation of looped domains.
3

21. Packaging of DNA into chromosomes
Level 1 = Winding of DNA around histones to create a nucleosome
Level 2 = Nucleosomes connected by strands of linker DNA like beads on string
Level 3 = Packaging of nucleosomes into 30-nm chromatin fiber.
Level 4 = Formation
of looped
domains.
4

22. FINAL packaging of DNA into chromosomes

23. 1. Current opinion in cell biology
2. Chromosome condensation and decondensation muring mitosis by
Wolfram Antonyn and Heinz Neumann, Elsevier, science direct Journal.
3. Chromosome organisation and dynamics during interphase mitosis by
Choon-Lin-Tiang, Yan He, Wojciench P.Pawlowski, Dept of genetics,
Cornwell university, Ithaca, New York.
Acknowlegement and references

24. thank you