[Biology Class Notes] on Histone Pdf for EXAM

Histones are a type of basic protein that binds to DNA in the nucleus and helps it condense into chromatin. Nuclear DNA does not exist in free linear strands; it is highly condensed and wrapped around histones in order to fit inside the nucleus and participate in chromosome formation.

 

Histones are basic proteins with positive charges that allow them to bind to negatively charged DNA. There are some histones that act as spools around which the thread-like DNA wraps.

 

Chromatin appears as beads on a string under the microscope in its expanded form. The beads are known as nucleosomes. Each nucleosome is made up of eight histone proteins that act like spools and are known as histone octamers. Each histone octamer contains two copies of each histone protein H2A, H2B, H3, and H4. The nucleosome chain is then wrapped into a 30 nm spiral known as a solenoid, where additional H1 histone proteins are associated with each nucleosome to maintain chromosome structure.

Types of Histones

There are five types of histones that have been identified: H1 (or H5), H2A, H2B, H3 and H4, the core histones are H2A, H2B, H3, and H4, and the linker histones are H1 and H5. H1 as well as its homologous protein H5 are involved in higher-order chromatin structures. The other four types of histones form nucleosomes when they bind to DNA. H1 (or H5) has about 220 residues. Other types of histones are smaller, with 100-150 residues.

Histone H2A

Many of the fancy tail modifications that have made H3 and H4 so popular in epigenetics may not be present in H2A. H2A has the most variants, resulting in a dizzying array of nucleosome composition diversity. H2A variants are distinguished primarily by their C-terminus, which is responsible for intra-nucleosome and DNA binding.The acidic patch, which is involved in higher order chromatin organisation, is also altered between variants.

Histone H2B

H2B forms a tetramer with (H2A-H2B)-2. In comparison to H3 and H4, this tetramer and its component dimers are easily exchanged in and out of the nucleosome, implying that the modifications on H2A and H2B are less likely to be maintained in chromatin.

What is Histone Acetylation?

• Histone acetylation is an epigenetic modification that is unmistakably linked to increased gene transcription proclivity. Because gene transcription is a key component of long-term memories, increases in histone acetylation generally favour learning and memory and can be regarded as molecular memory aids.

• In terms of neuronal depolarization and synaptic plasticity, histone acetylation readily responds to neuronal activity. So far, two pathways have been identified as mediating this response: the mitogen-activated protein kinase (MAPK) pathway and the dissociation of histone deacetylase 2 (HDAC2) from chromatin.

Histone Modification

Histone modifications either directly (e.g., acetyl groups that repel negatively charged DNA to create open chromatin conformation) or indirectly (via protein adaptors known as effectors) regulate the physical properties of chromatin and its corresponding transcriptional state.

Effector proteins recognise and bind to specific epigenetic marks, and then recruit molecular machinery to change the structure of chromatin. By translating the histone code into action, these epigenetic readers determine the functional outcome of histone modifications.

Histone Methylation

We can define the term histone methylation as the process by which methyl groups are transferred to the amino acids of histone proteins, and which form nucleosomes around which the DNA double helix wraps to form chromosomes. Depending on which amino acids in the histones are methylated and how many methyl groups are attached, histone methylation can either increase or decrease gene transcription.

Methylation events that weaken the chemical attraction between histone tails and DNA promote transcription by allowing the DNA to uncoil from nucleosomes, allowing transcription factor proteins as well as RNA polymerase to access the DNA.

Histone Deacetylase

Histone deacetylases can be defined as a type of enzyme that removes acetyl groups from a histone’s -N-acetyl lysine amino acid, allowing the histones to wrap the DNA more tightly. This is significant because DNA is wrapped around histones, and acetylation and deacetylation regulate DNA expression.

What is Nucleoplasmin?

The first protein to be described as a molecular chaperone was nucleoplasmin. Nucleoplasmin research has resulted in advancements in two areas of cell biology.

To begin, the pathway of nucleosome assembly in Xenopus oocytes and eggs has been elucidated, and it is the only assembly pathway that is known in detail. Nucleosome assembly generally represents the major chaperoning function of nucleoplasmin.

Second, nucleoplasmin was used to study protein transport into the nucleus, revealing a selective entry mechanism for nuclear proteins, passage through the nuclear pore complex, and a two-step transport mechanism.