Chromatin

Introduction:

Chromatin is a DNA and protein complex that forms chromosomes in eukaryotic cells’ nuclei. Nuclear DNA is not found in free strands; instead, it is highly compressed and wrapped around nuclear proteins to fit inside the nucleus.

There are two types of chromatin:

• Euchromatin is a less condensed form of chromatin that can be transcribed.
• The second type, known as heterochromatin, is highly compressed and rarely transcribed.

In its expanded form, chromatin appears as beads on a string under the microscope. Nucleosomes are the name for the beads. In each nucleosome, DNA is wrapped around eight proteins called histones. Additional histone proteins support the chromatin structure by wrapping the nucleosomes into a 30 nm spiral called a solenoid. The structure of chromatin and chromosomes can be seen under a light microscope during cell division, and they change shape as the DNA is duplicated and separated into two cells.

A brief note:

Chromatin is a DNA-protein complex found in eukaryotic cells. Long DNA molecules are compressed and densified into smaller, more compact structures.

This keeps the strands from getting tangled, as well as reinforcing the DNA during cell division, preventing DNA damage, and controlling gene expression and DNA replication.

In anaphase during mitosis and meiosis, chromatin aids proper chromosome segregation; the distinct shapes of chromosomes visible during this stage are the result of DNA being coiled into highly condensed chromatin.

Histones, which bind to DNA and serve as “anchors” around which the strands are wound, are the most important protein components of chromatin.

There are three levels of chromatin organization in general:

• Nucleosomes and the so-called beads on a string structure are formed when DNA wraps around histone proteins (euchromatin).
• A 30-nanometer fiber made up of nucleosome arrays in their most compact form is wrapped around multiple histones (heterochromatin).
• The metaphase chromosome is created by higher-level DNA supercoiling of the 30-nm fiber (during mitosis and meiosis)

Many organisms, on the other hand, do not adhere to this organizational scheme. Most eukaryotic cells, for example, have more tightly packed chromatin than spermatozoa and avian red blood cells, and trypanosomatid protozoa do not condense their chromatin into visible chromosomes at all.

Prokaryotic cells have completely different DNA organization structures than eukaryotic cells.

The overall structure of the chromatin network is also affected by the cell cycle stage. The chromatin is structurally loose during interphase to allow RNA and DNA polymerases to transcribe and replicate the DNA.

The specific genes present in the DNA determine the local structure of chromatin during interphase. In a structure known as euchromatin, regions of DNA containing actively transcribed genes (“turned on”) are less tightly compacted and closely associated with RNA polymerases, Inactive genes (“turned off”) are generally more condensed and associated with structural proteins, whereas regions containing inactive genes (“turned off”) are generally more condensed and associated with structural proteins.

Epigenetic methylation and acetylation of chromatin structural proteins affects local chromatin structure and, as a result, gene expression.

Structure of Chromatin:

• Inside the nucleus of eukaryotic cells, chromatin is a complex of macromolecules made up of DNA, RNA, and protein. Heterochromatin (condensed) and euchromatin (uncondensed) are the two types of chromatin (extended).
• Histones are the main protein components of chromatin, and they help organize DNA into “bead-like” structures called nucleosomes by providing a base for the DNA to wrap around.
• A nucleosome is made up of 147 base pairs of DNA wrapped around an octamer, which is a set of eight histones. The chromatin fiber is made by folding the nucleosome further. Chromosomes are made up of chromatin fibers that are coiled and condensed.
• Chromatin is involved in a number of cell processes, including DNA replication, transcription, DNA repair, genetic recombination, and cell division.

Chromosomes, Chromatids, and Chromatin:

• Within the nucleus, all three structures are made up of DNA and proteins, and each one is distinct.
• DNA and histones are packaged into thin, stringy fibers that make up chromatin, as previously stated. The chromosome is formed by further condensation of the chromatin. As a result, chromatin is the lower order of DNA organization, whereas chromosomes are the higher order.
• Chromosomes are single-stranded condensed chromatin groupings. Chromosomes replicate during the mitosis and meiosis cell division processes to ensure that each new daughter cell receives the correct number of chromosomes.
• A chromosome that has been duplicated is double-stranded and has the familiar X shape. The two strands are identical and are connected at the centromere, a central region.
• A chromatid is one of the two strands of a replicated chromosome. Sister chromatids are chromatids that are connected by a centromere. Sister chromatids separate at the end of cell division and become daughter chromosomes in newly formed daughter cells.

Chromatin and Its Function:

• 

  DNA Packaging

• Compactification of long DNA strands is the most basic function of chromatin. The size of the compartment in which DNA is stored in the nucleus is far greater than the length of DNA in the nucleus.
• The DNA must be compressed in some way to fit into this compartment. The packing ratio is a measurement of how tightly DNA is packed. DNA is not packaged directly into the structure of chromatin to achieve the overall packing ratio. Instead, it has several organizational hierarchies.
• The winding of DNA around the nucleosome achieves the first level of packing, which results in a packing ratio of about 6. All chromosomes have the same structure in their euchromatin and heterochromatin.
• Beads are wrapped in a 30 nm fiber found in both interphase chromatin and mitotic chromosomes at the second level of packing.
• The packing ratio is increased to around 40 with this structure. When the fiber is organized in loops, scaffolds, and domains, a final packing ratio of about 1,000 in interphase chromatin and about 10,000 in mitotic chromosomes is achieved.

• Transcription Regulation

• Transcription is the process by which proteins read the genetic information stored in DNA and then transcribe it into RNA, which is then translated into functional proteins. There will be no transcription if the chromatin is strengthened and access to the read proteins is restricted.
• The transcription process can be carried out by euchromatin, an extended type of chromatin. While heterochromatin, a condensed form of chromatin, is too densely packed for proteins to read DNA.
• Transcriptional bursting, or the discontinuity of transcription, may be caused by fluctuations between open and closed chromatin. Other factors, such as the association and dissociation of transcription factor complexes with chromatin, are likely to be involved.
• In contrast to simple probabilistic transcription models, this phenomenon can explain the high variability in gene expression observed between cells in an isogenic population.

• DNA Repair and Chromatin

• All DNA-based processes are hampered by the packaging of DNA into chromatin. Chromatin can easily change its shape and structure due to the high dynamic arrangement of proteins and DNA. At the site of a DNA damage, chromatin relaxes quickly, allowing repair proteins to bind to DNA and repair it.

Importance of chapter for neet and board exams:

• Many young medical aspirants fail to balance their preparation for NEET and Class 12 board exams. The pressure to do well in Class 12 boards is enormous in a country like ours, where many people compete for a single mark.
• As a result, candidates preparing for both the NEET and the Class 12 board examinations must be able to juggle the two effectively.
• This article on ‘how to prepare for NEET and Class 12 board exams at the same time’ aims to assist candidates in preparing for both exams at the same time. The NEET 2022 exam is scheduled to take place in the first week of May, while the CBSE Class 12 board exam will begin in February/March.

FAQs:

1. How is chromatin assembled?

Soln.: The process of packaging DNA into nucleosomes is the simplest definition of chromatin assembly. Histone chaperones and ATP-utilizing factors catalyze the deposition of histones on DNA to produce periodic arrays of nucleosomes, which is the basic chromatin assembly process.

2. How do chromosomes relate to chromatin?

Soln.: Histones package DNA into chromatin. Chromosomes are formed when chromatin is condensed and organized further. Chromatin is not paired like chromosomes.

3. Where is chromatin found?

Soln.: Inside the nucleus of eukaryotic cells, chromatin is a complex of macromolecules made up of DNA, RNA, and protein.