Chromosome Packaging With Reference to Coiling, M Irfan

8/6/2019 Chromosome Packaging With Reference to Coiling, M Irfan

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Muhammad Irfan

M.Phil Zoology

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Prokaryotes

Prokaryotes dont have nucleus, so not packtheir DNA as chromatin

Their seem some organization to their DNA

This suggest that prokaryotes have proteinsthat act like histones

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Bacterial Chromosome Structure

Prokaryotic cells (bacteria) contain theirchromosome as circular DNA

Usually the entire genome is a single circle butoften there are extra circles called plasmids

The DNA is packed by DNA-binding proteins(HU and H)

The bacterial DNA is packed in loops and this

bundled DNA is called the nucleoid It concentrates the DNA in part of the cell

The DNA does form loops back and forth to aprotein core, attached to the cell wall

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Life, the Science ofBiology, by Purves, Orians, & Heller, 5th ed., 1997

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Why Coiling of DNA is Important inEukaryotes?

The haploid human genome containsapproximately 3 billion base pairs of DNApackaged into 23 chromosomes

Most cells in the body (except forreproductive cells) are diploid with 23 pairsof chromosomes

That makes a total of 6 billion base pairs ofDNA per cell

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Cont..

Because each base pair is around 0.34nanometers long (10-9) each diploid celltherefore contains about 2 meters of DNA

(6 ft long)It is estimated that the human body contains

about 50 trillion cells

Which works out to 50 x 2 = 100 trillionmeters of DNA per human

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There are certain proteins that compact

chromosomal DNA into the microscopic space ofthe eukaryotic nucleus

These proteins are called histones

The resulting DNA-histones complex is calledchromatin

Within the nucleus histones provide the energy

(mainly in the form of electrostatic interactions)to fold DNA

Chromatin can be packaged into a much smaller

volume than DNA

Answer!

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Charge on Histones

nar.oxfordjournals.org biog1105-1106.org/demos

Charge on DNA

Electrostatic Interaction

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Structure of Chromatin

NucleosomeThe basic repeating structural and

functional unit of chromatin is the

nucleosome which contains 9 histoneproteins

The observation by electron microscopistssuggests that chromatin appeared similarto beads on a string provided an earlyclue that nucleosomes exist

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molecularstation.com 13


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http://origins.swau.edu/papers/complexity/trilo/eng/histone.jpg

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Electronic micrograph observations

Beads on a string, the 10nm fiber.

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X-ray diffraction experiments

Studies show thatchromatin has a repeating

pattern with an interval ofapproximately 100

This repeat is different

from the repeatingpattern of DNA itself

dnaftb.org16


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Comparison between X-Ray DiffractionPattern of DNA and Chromatin

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sunclipse.orgdnaftb.org


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Life, the Science ofBiology, byPurves, Orians, & Heller, 5th

ed., 1997 18


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Experiments using nucleases

Digest chromatin with ratliver nuclease at lowconcentration

Electrophoresis of the

digested chromatin materialA regular pattern of bands on

the gel approx. every 200 bp

Histones distributed evenlyon DNA

Histone core protect DNAfrom nuclease digestion

dnaftb.org

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Experiments using nucleases


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Histone

A protein around which DNA is wrapped andformed into structural components callednucleosomes

Histones are the chief protein components ofchromatin

It is the spool (cylinder of histones) aroundwhich DNA is wrapped

DNA wraps around 8 histone (core)molecules

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Structure of HistonesHistone proteins

5 main typesH2A

H2B

H3H4

H1attached to the nucleosomeand involved in furthercompaction of the DNA(conversion of 10 nm chromatin

to 30 nm chromatin)21


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A possible nucleosome structure

Peter J. Russell, iGenetics: Copyright Pearson Education, Inc., publishing as

Benjamin Cummings. 22


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H1 Protein

H1 is called a"linker" histonebecause it bindsthe DNA betweennucleosomes

H1 is thought tohelp in organizing

and compactingthe DNA inchromosomes

Linker DNA

Linker DNA isdouble- strandedDNA between twonucleosome

Linker DNAconnects histone H1

to another histoneH1 sits on thenucleosome core

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L

ife, the Science ofB

iology, by Purves, Orians,& Heller, 5th ed., 199724


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Structure of Chromatin

Histone proteinsAbundant

Histone protein sequence is highlyconserved among eukaryotesconserved

functionProvide the first level of packaging for the

chromosome

DNA is wound around histone proteins toproduce nucleosomes

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- Core DNA = 146 bp

- Linker DNA = 8-114 bp (usually 55bp)

- DNA turns around histone octamer.

Life, the Science ofBiology, byPurves, Orians, & Heller, 5th ed.,

1997 26


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10 nm chromatin is produced in the first level of packaging.


Benjamin Cummings.

Nucleosomes connected together by linker DNA and H1 histone to produce

the beads-on-a-string extended form of chromatin

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DNA is further compacted when the DNAnucleosomes associate with one anotherto produce 30 nm chromatin which is

called solenoidH1 plays a role in compaction (if H1 is

absent then chromatin cannot beconverted from 10 to 30 nm)

Cont..

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Packaging of nucleosomes into the 30-nm chromatin fiber


Benjamin Cummings.

30 nm

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Eukaryotic chromosomal organization

Compaction continues by forming loopeddomains from the 30 nm chromatin whichseems to compact the DNA to 300 nmchromatin

Human chromosomes contain about 2000looped domains

30 nm chromatin is looped and attached to a

nonhistone protein scaffolding

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Model for the organization of 30-nm chromatin fiber into looped domains thatare anchored to a nonhistone protein chromosome scaffold


Benjamin Cummings.

300 nm

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Eukaryotic chromosomal organization

Nonhistone proteins

Other proteins that are associated with thechromosomes

May have role in compaction or be involved inother functions requiring interaction with theDNA

Many are acidic and negatively charged bind tothe histones

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The many different orders of chromatin packing that give rise to the highly condensed metaphase chromosome

Peter J. Russell, iGenetics: Copyright

Pearson Education, Inc., publishing asBenjamin Cummings.34


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DNA compaction

Level of DNA compaction changesthroughout the cell cycle most compact

during M-Phase least compact during S-Phase

2 types of chromatin related to the level of

gene expressionEuchromatindefined originally as areas

that stained lightly

Heterochromatindefined originally asareas that stained darkly

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Euchromatin

Found in regions ofchromosome wherethere are many genes

Unfolded structure

Functions in the activetranscription of DNAto mRNA

Usually areas wheregene expression isoccurring

molecularstation.com37


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Heterochromatin

Found in regions of achromosome wherethere are few to no

geneschromosomal regions

that are condensedthroughout the cell

cycle

molecularstation.com 38


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Thank You!

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Chromosome Packaging With Reference to Coiling, M Irfan

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