MOLECULAR ORGANTIZATION OF EUKARYOTIC CHROMOSOME AND STRUCTURE OF NUCLEOSOME PARTICLES

INTRODUCTION

The large human chromosome of humans contains 300 million base pairs. If it is in normal confirmation, it constitutes a chromosome over 10 cm in length.

How can I fit all 46 chromosomes to a single nucleus? At the same time as being 10 µm in diameter, it keeps DNA accessible to enzymes and regulatory proteins.

Each eukaryotic chromosome is composed of a single, very long DNA molecule. It takes tremendous packing and folding for all of this DNA to fit in the nucleus. Eukaryotic DNA packing is not static, but it changes regularly depending on the cellular process.

The intracellular DNA of eukaryotes is closely related to proteins. This combination of DNA and protein is called chromatin. There are two types of proteins related to DNA. Histone and non-histone proteins.

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MOLECULAR ORGANIZATION  OF EUKARYOTIC CHROMOSOMES

• Molecular organization of eukaryotic chromosome is composed of repeatating subunits called nucleosome. Majority of nucleosome is packed into nucleosomes.
• Nucleosome is made up of eight histone proteins also called histone octamer and DNA wrapped around it. Histone octamer consists of two copies of each histone H2A, H2B, H3, H4 ( Core histones).
• The complete chromatin subunit consists of the nucleosome core, the linker DNA, one molecule of histone H1, and the associated non histone proteins.
• The DNA between each nucleosme is called linker DNA.
• Nucleosome compact the DNA by approximately six folds.
• The DNA most tightly associated with nucleosome , called the core DNA, is wound about 1.65 times around the histone octamer.
• CORE DNA LENGTH= 147bp

LINKER DNA LENGTH= 20-60bp

• In any cell, there are strecthes of DNA that are not packaged into nucleosomes. Typically, these are regions of DNA engaged in gene expression, replication, or recombination.

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HISTONES

• Histones are small, positively charged proteins.
• Histones are most abundant proteins associated with DNA of eukayotes.
• Eukaryotic chromosme most commonly contain five abundant histones: H1, H2A, H2B, H3, and H4.
• H2A, H2B, H3, H4 are core histones.
• H1 is linker histone.
• Core histones are present in equal amounts in the cell. H1 is half as abundant as the other histones.
• HIstones are madeup of highly positivly charged amino acids. As they have to bind to negatively charged DNA molecule
• 20% of the histone is either arginine or lysine.
• Core histones size= 11 to 15kDa.
• H1 is slightly larger at = 21kDa.

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NUCLEOSOME STRUCTURE

Nucleosome : A DNA molecule of 2nm diameter wrapped around nucleosome core particles of 11nm diameter is the lowest level of chromatin organization. Each nucleosome consists of eight histones proteins around which the DNA wraps 1.65 times.

• The structure of nucleosome involves the orderd association of these building blocks with DNA.
• The H3.H4 tetramer binds to DNA; then 2 H2A.H2B dimers join the H3.H4 DNA complex to form the final nucleosome.
• The core histones have an amino-terminal, called a tail as it lacks a defined structure and is accessible within the nucleosome.
• The treatment of nucleosome with trypsin rapidly removes the accessible amino-terminal tails of the histones but cannot cleave the tightly packed histone-fold regions.

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ATOMIC STRUCTURE OF THE NUCLEOSOME

• The high affinity of the nucleosome for DNA, the distortion of the DNA when bound to the nucleosome, and the lack of DNA sequence specificity can each be explained by the nature of the interactions between the histones and the DNA.
• The structure also sheds light on the function and location of the of the amino-terminal tails.
• The interaction between the DNA and the histone octamer provides insight into the dynamic nature of the nucleosome .

Interaction of the histones with nucleosomal DNA

• H3.H4 binds the middle and the ends of the 60bp of DNA .
• H2A.H2B binds 30bp of DNA on one side of the nucleosome.

The histone Amino-Terminal tails stabilize DNA wrapping around the octamer

• The four H2B and H3 tails emerge from between the two DNA helices.
• H2A and H4 amino-terminal tails emerge from either above or below both DNA helices.
• The tails are the sites of extensive post-translational modifications that alter the function of individual nucleosomes. The modifications include phosphorylation, acetylation, and methylation on serine, lysine, and arginine residues.

Wrapping of DNA around the histone protein core stores negative superhelicity

• Each nucleosome added to covalently closed circular template changes the linking number of the associated DNA by approximately -1.2.
• The DNA packed into nucleosomes would become negatively supercoiled if nucleosomes were removed from the DNA.
• Thus, the nucleosome can be viewed as storing or stabilizing negative superhelicity.

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