Histone Protiens

this is a presentation on histone protiens.

1. By: Anjana M adhusoodanan HISTONE PROTEIN

2. Histones or histone proteins These are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosome. Histones are a group of basic protein that associate with DNA and help the DNA to condense it into chromatin.

3. Some Histones function as spools for the thread like DNA to wrap around. Chromatin, under the microscope in its extended form, hooks to beads on a string, called nucleosome. Nucleosome consist of DNA wrapped around eight histone protein called histone octamer.

4. Classification Histone proteins are of two types: -core Histones -linker Histones core # H2A # H2B # H3 # H4 Two of each of these core histone protein assemble to form 1 octameric nucleosome core particle and 147 base pairs of DNA wrap around this core particle.  

5. H2A is important for packaging DNA into chromatin. Since H2A packages DNA molecules into chromatin, the packaging process will effect gene expression.H2A has been correlated with DNA modification. H2A plays a major role in determining the overall structure of chromatin. Inadvertently, H2A has been found to regulate gene expression .

6. H2B is also involved with the structure of the  nucleosomes  of the 'beads on a string' structure. H3 is involved in the structure of  chromatin  in  eukaryotic cells . Featuring a main globular domain and a long  N-terminal tail . H3 is an important protein in the emerging field of  epigenetics , where its sequence variants and variable modification states are thought to play a role in the dynamic and long term regulation of genes.

7. Histone H4 is involved in the structure of  chromatin  in eukaryotic cells.H4 is a structural component of the  nucleosome , and is subject to covalent modification ,including acetylation  and  methylation , which may alter expression of genes located on DNA associated with its parent histone octamer .

8. Linker histone includes- H1 H5 > Histone H1 is one of the five main  histone   protein  families which are components of  chromatin  in  eukaryotic  cells. >Though highly conserved , it is nevertheless the most variable histone in sequence across species.

9. Unlike the other histones, H1 does not make up the  nucleosome  "bead". Instead, it sits on top of the structure, keeping in place the DNA that has wrapped around the nucleosome. H1 is present in half the amount of the other four histones, which contribute two molecules to each nucleosome bead. H5 histones are individual proteins involve in the packaging of specific region of DNA.

10. Packaging of histones In the core of nucleosome, the two dimmers H2A and H2B and two tetramers H3 and H4 are involve and form the tertiary structure. These structure proteins are relatively similar in structure . Histones are highly positively charge and have lysine and argentine residues.

11. Modifications in histones Histone proteins are altered according to the packaging in DNA . There can be many modification that affect how DNA is packaged. There are three types of modifications- >methylation (increases packaging) >acetylation (decrease packaging) >phosporylation (decrease packaging)

12. Normally histones are positively charged but with the modification of methylation it becomes hydrophobic which enable the histone to more highly pack. Acetylation and phosphorylation make the histone more negative which weakens the packing ability of histones due to repelling of negative-negative charges.

13. FUNCTIONS Compacting DNA strand Histones act as spools around which DNA winds. This enables the compaction necessary to fit the large  genomes  of eukaryotes inside cell nuclei: the compacted molecule is 40,000 times shorter than an unpacked molecule . Chromatin regulation Histones undergo modifications  that alter their interaction with  DNA  and nuclear proteins. The H3 and H4 histones have long tails protruding from the nucleosome , which can be  covalently  modified at several places . H istone modifications are also involved in control of transcription . Histone modifications act as Marking sites of DNA damage.

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