Steps involved in DNA coiling

2. CHROMOSOMALCHROMOSOMAL
COILINGCOILING
PRESENTERPRESENTER
MARYAM HAMEEDMARYAM HAMEED
ROLL # 13ROLL # 13
Bs. ZOOLOGY (REG.)Bs. ZOOLOGY (REG.)
DEPT. OF BIO SCIENCESDEPT. OF BIO SCIENCES
U.O.SU.O.S

3. OUTLINEOUTLINE
In which form does DNA occur inIn which form does DNA occur in
cell?cell?
Prokaryotic chromosomesProkaryotic chromosomes

DNA Super coilingDNA Super coiling
Eukaryotic chromosomesEukaryotic chromosomes
Eukaryotic chromatin compactionEukaryotic chromatin compaction
Levels of coilingLevels of coiling
Heterochromatin Vs EuchromatinHeterochromatin Vs Euchromatin

4. In which form does DNA andIn which form does DNA and
occur in a cell?occur in a cell?
Never naked!Never naked!
 Always associated with proteinsAlways associated with proteins
 From small virus genome to big genomeFrom small virus genome to big genome
of a complex organisme.of a complex organisme.
 Proteins associated with DNA play aProteins associated with DNA play a
significant role in regulation of genesignificant role in regulation of gene
expression/repression.expression/repression.

5.  In bacteria the chromosome is a circular, double-strandedIn bacteria the chromosome is a circular, double-stranded
DNA molecule.DNA molecule.
 The Prokaryotic e.g. bacterial chromosome is found in aThe Prokaryotic e.g. bacterial chromosome is found in a
region called theregion called the nucleoidnucleoid
The nucleoid is not membrane-boundedThe nucleoid is not membrane-bounded
So the DNA is in direct contact with the cytoplasmSo the DNA is in direct contact with the cytoplasm
PROKARYOTICPROKARYOTIC
CHROMOSOMESCHROMOSOMES

6.  DNA supercoilingDNA supercoiling is a important way to compactis a important way to compact
the bacterial chromosomethe bacterial chromosome
Supercoiling within loops creates
a more compact DNA

7.  Eukaryotic species contain one or more sets ofEukaryotic species contain one or more sets of
chromosomeschromosomes

Each set is composed of several different linear chromosomesEach set is composed of several different linear chromosomes
 The total amount of DNA in eukaryotic species isThe total amount of DNA in eukaryotic species is
typically greater than that in bacterial cellstypically greater than that in bacterial cells
 Chromosomes in eukaryotes are located in theChromosomes in eukaryotes are located in the
nucleusnucleus

To fit in there, they must be highly compactedTo fit in there, they must be highly compacted
• This is accomplished by the binding of many proteinsThis is accomplished by the binding of many proteins
• The DNA-protein complex is termedThe DNA-protein complex is termed chromatinchromatin
EUKARYOTIC CHROMOSOMESEUKARYOTIC CHROMOSOMES

8.  There are two major types of protein associatedThere are two major types of protein associated
with DNA in chromatin: histones and nonhistones.with DNA in chromatin: histones and nonhistones.
 The DNA is wrapped around a core of histoneThe DNA is wrapped around a core of histone
molecules, and the nonhistones are somehowmolecules, and the nonhistones are somehow
associated with that complex.associated with that complex.
 Histone proteinsHistone proteins are basicare basic

They contain many positively-charged amino acidsThey contain many positively-charged amino acids
• Lysine and arginineLysine and arginine
 The histones are relatively small basic proteins, atThe histones are relatively small basic proteins, at
normal pH of a cell they have a net positivenormal pH of a cell they have a net positive
charge. This facilitates their binding to thecharge. This facilitates their binding to the
negatively charged DNA.negatively charged DNA.

9. There are five types of histonesThere are five types of histones
 H2A, H2B, H3 and H4H2A, H2B, H3 and H4 are theare the core histonescore histones
• Two of each make up the octamerTwo of each make up the octamer
 H1 is the linker histoneH1 is the linker histone
• Binds to linker DNABinds to linker DNA
• Also binds to nucleosomesAlso binds to nucleosomes
 NonhistonesNonhistones are usually acidic proteins, they have a netare usually acidic proteins, they have a net
negative charge at normal pH, and are likely to bind tonegative charge at normal pH, and are likely to bind to
positively charged histones in the chromatin.positively charged histones in the chromatin.
Two DNA components are involvedTwo DNA components are involved::
a. CoreCore DNADNA is the DNA that is actually associated with theis the DNA that is actually associated with the
histone octamer. This value is invariant and is 146 base pairs.histone octamer. This value is invariant and is 146 base pairs.
The core DNA forms two loops around the octamer.The core DNA forms two loops around the octamer.
b.b. The DNA that is between each histone octamer is called theThe DNA that is between each histone octamer is called the
linker DNAlinker DNA and can vary in length from 8 to 180 base pairs.and can vary in length from 8 to 180 base pairs.

10.  IfIf stretchedstretched end to end, a doploid set of humanend to end, a doploid set of human
chromosomes will be overchromosomes will be over 2 meter2 meter long!long!
 Yet the cell’s nucleus is onlyYet the cell’s nucleus is only 2 to 42 to 4 µµmm in diameterin diameter
• Therefore, the DNA must be tightly compacted to fitTherefore, the DNA must be tightly compacted to fit
 TheThe compaction of linear DNAcompaction of linear DNA in eukaryoticin eukaryotic
chromosomes involves interactions between DNAchromosomes involves interactions between DNA
and various proteinsand various proteins
 Proteins bound to DNA are subject to change during theProteins bound to DNA are subject to change during the
life of the celllife of the cell
• These changes affect the degree of chromatin compactionThese changes affect the degree of chromatin compaction
Eukaryotic Chromatin CompactionEukaryotic Chromatin Compaction

11. LEVELS OF COILINGLEVELS OF COILING
1.NUCLEOSOME1.NUCLEOSOME
(11nm)(11nm)
 DNA is wrapped around the 4 types of coreDNA is wrapped around the 4 types of core
histone molecules.histone molecules.
 TheThe nucleosomenucleosome consists of about 200 bp ofconsists of about 200 bp of
DNA wrapped around a histone octamer.DNA wrapped around a histone octamer.
 The winding of DNA around the histone coreThe winding of DNA around the histone core
makes the chromatin fiber to appear likemakes the chromatin fiber to appear like
beads on a stringbeads on a string..

12. Coiling of DNA aroundCoiling of DNA around
histoneshistones

13.  Overall structure of connected nucleosomes resembles “beads on aOverall structure of connected nucleosomes resembles “beads on a
string”string”
 This structure shortens the DNA length about seven-foldThis structure shortens the DNA length about seven-fold
Vary in length between 20 to 100 bp,
depending on species and cell type
Diameter of the
nucleosome

14.  Nucleosomes’ beads associate with each other inNucleosomes’ beads associate with each other in
helical way to form a more compact structure termedhelical way to form a more compact structure termed
thethe 30 nm fiber30 nm fiber that is found in both interphasethat is found in both interphase
chromatin and mitotic chromosomes. This appears tochromatin and mitotic chromosomes. This appears to
be abe a solenoidsolenoid structure with about 6 nucleosomesstructure with about 6 nucleosomes
per turn.per turn.
 Histone H1 plays a role in this compactionHistone H1 plays a role in this compaction
2.Nucleosomes Join to Form2.Nucleosomes Join to Form
a 30 nm Fibera 30 nm Fiber
(SOLENOID)(SOLENOID)

15. Regular, spiral
configuration
containing six
nucleosomes
per turn
Irregular
configuration where
nucleosomes have
little face-to-face
contact

16. 3. RADIAL LOOP DOMAINS3. RADIAL LOOP DOMAINS
(300nm)(300nm)
 An interphase chromosome may have a diameter ofAn interphase chromosome may have a diameter of
300 nm while a metaphase chromosome may have a300 nm while a metaphase chromosome may have a
diameter of 700 nm.diameter of 700 nm.
4. PROTEIN SCAFFOLD4. PROTEIN SCAFFOLD
 The final packaging occurs when the chromatin fiberThe final packaging occurs when the chromatin fiber
is organized inis organized in loops & scaffolds.loops & scaffolds.
 The condensed piece of chromatin has aThe condensed piece of chromatin has a
characteristic scaffolding structure that can becharacteristic scaffolding structure that can be
detected indetected in metaphase chromosomes(1400nm)metaphase chromosomes(1400nm). This. This
appears to be the result of extensive looping of theappears to be the result of extensive looping of the
DNA in the chromosome.DNA in the chromosome.

17. The levels of compaction leading to a metaphase chromosome

19. The levels of compaction leading
to a metaphase chromosome

20.  The compaction level of interphase chromosomesThe compaction level of interphase chromosomes
is not completely uniformis not completely uniform
 EuchromatinEuchromatin
• Less condensed regions of chromosomesLess condensed regions of chromosomes
• Transcriptionally activeTranscriptionally active
 HeterochromatinHeterochromatin
• Tightly compacted regions of chromosomesTightly compacted regions of chromosomes
• Transcriptionally inactive (in general)Transcriptionally inactive (in general)
HeterochromatinHeterochromatin vsvs EuchromatinEuchromatin