Histones are basic proteins found in eukaryotic cell nuclei that are responsible for DNA folding and chromatin formation. There are two main classes of histones: core histones and linker histones. Core histones include the H2A, H2B, H3, and H4 families, while linker histones include H1 and H5. Histone modification, including acetylation, methylation, and phosphorylation, can impact gene expression by altering the accessibility of DNA. Acetylation reduces the positive charge of histone tails, weakening their interaction with DNA and making it more accessible for transcription. Methylation and phosphorylation can also influence chromatin structure and cellular activity.

1. HISTONE MODIFICATION
-By Bansari M. Patel
Roll no-3
M.Sc. Bioinformatics Sem-3

2. HISTONE PROTEINS
Histones are a special group of proteins found
in the nuclei of eukaryotic cells responsible
for DNA folding and chromatin formation.

3. HISTONES PROTEINS
 Chemically they are-
 Highly alkaline basic proteins.
 Histones are positively charged.

4. CORE OF 8 HISTONES

5. CLASSES OF HISTONES
 There are two main classes of histones:-
 Core histones
 Linker Histones

6. LINKER HISTONES
 Linker histones included:
 H1 and H5- both are highest lysine/arginine
ratio
 The linker histone protein H1 binds the
nucleosome at starting and ending site of the
DNA , thus locking the DNA into place and
help in the formation of higher order
structure.
 H5 histones are individual proteins involve in
the packaging of specific region of DNA.

7. LINKER HISTONE

8. CORE HISTONES
 In core histones following families are
included.
 H2A} contain more lysine
 H2B}contain more lysine
 H3} contain more arginine
 H4} contain more arginine

9. FUNCTIONS OF THE HISTONE PROTEIN IN A
CHROMOSOMES
 The DNA is housed in chrosomes in the form
of nucleosomes.
 It is basic unit of chromosome or chromatin
fiber.
 It is DNA duplex coiled around a core of eight
histone proteins.
 Positively charged histones are linked with
negative charged phosphate groups of DNA.

10. HISTONE MODIFICATION
 Acetylation (Acetyl functional groups)
 Methylation(Methyle groups)
 Phosphorylation:-
 It has been proposed that these
modifications results in a ‘code’ which can
be read by proteins involved in gene
expression and other DNA translations.

11. HISTONE ACETYLATION
 Histone Acetylation:-
 Adds acetyl groups to histone tails.
 Reduces positive charge and weakens
interaction of histones with DNA.
 Facilitates transcription by making DNA more
accessible to RNA polymerase II.

12. HISTONE DEACETYLATION
 Histone deacetylation removes acetyl groups
from histone tails.
 Increases interactions of DNA and histones.
 Represses transcription.

13. ACETYLATION
 It is the introduction of an Acetyl functional
group to the Lysine amino acid of the histone
tail.
 These reactions are catalyzed by enzymes
with histone acetyltransferase (HAT) or
histone deacetylase” (HDAC) activity.

14. ACETYLATION AND DEACETYLATION

15. EFFECTS OF ACETYLATION
 -ve charge of histone.
 Reduces affinity of tail for adjacent
nucleosomes.
 Creating a transcription permissive
environment.
 Increase the access of transcription factors.

16. HISTONE METHYLATION
 Histone methyl transferases (HMTs)
 Histone lysine methyl transferases (HKMTs) Methyl
lys (K) residues.
 Protein argenin methyl transferase (PRMTs) Methyl
transferase(PRMTs) methylate arge(R) residues.
 Methylation can result in activation or repression of
expression .
 Trimethylation of histone H3 at lysine 4 (H3K4) is an
active mark for transcription.
 Dimethylation of histone H3 at lysine 9 (H3K9) ,
signal for transcriptional silencing.

17. METHYLATION
 It is the introduction of an Methyl functional
group to lysine or arginine of the histone tail.
 These reactions are catalyzed by enzymes
with histone methyltransferase.
 Arg can be methylated once or twice, and lys
once , twice or thrice.

18. PHOSPHORYLATION
Phosohorylation make the histone more
negative which weakens the packing ability
of histones .
Phosphorylation of histone H3 and H1 has
been associated with the formation of
metaphase chromosomes.
Although information about phosphorylation is
limited , however it is clear they can influence
chromatin structure and have a significant
impact on cellular activity.