CHROMOSOMAL COILINGCHROMOSOMAL COILINGPRESENTERPRESENTER

MARYAM HAMEEDMARYAM HAMEEDROLL # 13ROLL # 13

Bs. ZOOLOGY (REG.)Bs. ZOOLOGY (REG.)DEPT. OF BIO SCIENCESDEPT. OF BIO SCIENCES

U.O.SU.O.S

OUTLINEOUTLINEIn which form does DNA occur in In which form does DNA occur in cell?cell?Prokaryotic chromosomesProkaryotic chromosomes

DNA Super coilingDNA Super coilingEukaryotic chromosomesEukaryotic chromosomesEukaryotic chromatin compactionEukaryotic chromatin compactionLevels of coilingLevels of coilingHeterochromatin Vs EuchromatinHeterochromatin Vs Euchromatin

In which form does DNA and In 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 genome of From small virus genome to big genome of

a complex organisme. a complex organisme. Proteins associated with DNA play a Proteins associated with DNA play a

significant role in regulation of gene significant role in regulation of gene expression/repression.expression/repression.

In bacteria the chromosome is a circular, double-stranded DNA molecule.In bacteria the chromosome is a circular, double-stranded DNA molecule.

The Prokaryotic e.g. bacterial chromosome is found in a region called the The Prokaryotic e.g. bacterial chromosome is found in a region called the nucleoidnucleoid

The nucleoid is not membrane-boundedThe nucleoid is not membrane-boundedSo the DNA is in direct contact with the cytoplasm So the DNA is in direct contact with the cytoplasm

PROKARYOTIC PROKARYOTIC CHROMOSOMESCHROMOSOMES

DNA supercoilingDNA supercoiling is a important way to compact is a important way to compact the bacterial chromosomethe bacterial chromosome

Supercoiling within loops creates a more compact DNA

Eukaryotic species contain one or more sets of Eukaryotic 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 is The 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 the Chromosomes 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 termed The DNA-protein complex is termed chromatin chromatin

EUKARYOTIC CHROMOSOMESEUKARYOTIC CHROMOSOMES

There are two major types of protein associated There 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 histone The DNA is wrapped around a core of histone molecules, and the nonhistones are somehow molecules, and the nonhistones are somehow associated with that complex.associated with that complex.

Histone proteinsHistone proteins are basic are 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, at The histones are relatively small basic proteins, at normal pH of a cell they have a net positive normal pH of a cell they have a net positive charge. This facilitates their binding to the charge. This facilitates their binding to the negatively charged DNA. negatively charged DNA.

There are five types of histonesThere are five types of histones H2A, H2B, H3 and H4H2A, H2B, H3 and H4 are the are 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 net are usually acidic proteins, they have a net

negative charge at normal pH, and are likely to bind to negative 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. Core Core DNADNA is the DNA that is actually associated with the is 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 the The 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.

If If stretchedstretched end to end, a doploid set of human end to end, a doploid set of human chromosomes will be over chromosomes will be over 2 meter 2 meter long! long! Yet the cell’s nucleus is only Yet the cell’s nucleus is only 2 to 4 2 to 4 mm in diameter in diameter

• Therefore, the DNA must be tightly compacted to fit Therefore, the DNA must be tightly compacted to fit

The The compaction of linear DNAcompaction of linear DNA in eukaryotic in eukaryotic chromosomes involves interactions between DNA chromosomes involves interactions between DNA and various proteins and various proteins Proteins bound to DNA are subject to change during the Proteins bound to DNA are subject to change during the

life of the celllife of the cell• These changes affect the degree of chromatin compaction These changes affect the degree of chromatin compaction

Eukaryotic Chromatin Eukaryotic Chromatin CompactionCompaction

LEVELS OF COILINGLEVELS OF COILING

1.NUCLEOSOME1.NUCLEOSOME(11nm)(11nm)

DNA is wrapped around the 4 types of core DNA is wrapped around the 4 types of core histone molecules.histone molecules.

The The nucleosomenucleosome consists of about 200 bp of consists of about 200 bp of DNA wrapped around a histone octamer.DNA wrapped around a histone octamer.

The winding of DNA around the histone core The winding of DNA around the histone core makes the chromatin fiber to appear like makes the chromatin fiber to appear like beads on a stringbeads on a string. .

Coiling of DNA around Coiling of DNA around histoneshistones

Overall structure of connected nucleosomes resembles “beads on a Overall 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

Nucleosomes’ beads associate with each other in Nucleosomes’ beads associate with each other in helical way to form a more compact structure termed helical way to form a more compact structure termed the the 30 nm fiber30 nm fiber that is found in both interphase that is found in both interphase chromatin and mitotic chromosomes. This appears to chromatin and mitotic chromosomes. This appears to be a be a solenoid solenoid structure with about 6 nucleosomes structure 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 Form 2.Nucleosomes Join to Form a 30 nm Fiber a 30 nm Fiber(SOLENOID)(SOLENOID)

Regular, spiral configuration containing six nucleosomes

per turn

Irregular configuration where nucleosomes have little face-to-face

contact

3. RADIAL LOOP DOMAINS3. RADIAL LOOP DOMAINS

(300nm)(300nm) An interphase chromosome may have a diameter of An interphase chromosome may have a diameter of

300 nm while a metaphase chromosome may have a 300 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 fiber The final packaging occurs when the chromatin fiber

is organized in is organized in loops & scaffolds.loops & scaffolds. The condensed piece of chromatin has a The condensed piece of chromatin has a

characteristic scaffolding structure that can be characteristic scaffolding structure that can be detected in detected in metaphase chromosomes(1400nm)metaphase chromosomes(1400nm). This . This appears to be the result of extensive looping of the appears to be the result of extensive looping of the DNA in the chromosome. DNA in the chromosome.

The levels of compaction leading to a metaphase chromosome

The levels of compaction leading to a metaphase chromosome

The compaction level of interphase chromosomes The 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)

Heterochromatin Heterochromatin vsvs EuchromatinEuchromatin