Chromatin


 Chromatin : 
Chromatin is hereditary  DNA_ protein fibrillar complex which contains DNA, RNA and proteins . It has the ability to get stained with basic dyes. It occurs in the form of fine overlapping and coiled fibres which produce a network called chromatin reticulum. Chromatin fibres are distributed throughout the nucleoplasm and are differentiated into two regions__ Euchromatin which is a narrow lightly stained and diffused fibrous part forming  the bulk of chromatin and Heterochromatin which is wider, darkly stained and condensed granular part attached here and there on the euchromatin.


 



  Generally, portion of euchromatin  associated with acid proteins  takes part in transcription or formation  of RNAs. The chromatin fibres during prophase of nuclear division condense to form a definite number of thread _ like structure  called chromosomes

Structure of chromatin :

The term chromatin¹ means ' coloured material ' and indicates the ability of this material to easy take up of stain for microscopic examination. The chromatin occurs in  a nondividing (interphase) nucleus as fine filaments termed the chromatin fibres. The latter lie crisscross and give the appearance of a diffuse network,often called the nuclear, or chromatin, reticulum. The chromatin fibres are simply greatly extended chromosomes

   During cell division, short, thick, rod__ like organelles, called the chromosomes, are formed by condensing and tight coiling of the chromatin fibres. A  fixed number of chromosomes appear in the cells of each species. A chromosome consists of two identical halves, the chromatids,held together at one point, the centromere.The centromere appears as a narrow region, called primary constriction, of the chromosome.The  chromosomes may have additional constriction termed secondary constriction near their  ends. Part of the chromosomes beyond the secondary constriction is termed satellite. The secondary constrictions are always constant in their satellites are known as set chromosomes, or marker chromosomes. Certain secondary constrictions are the sites for the formation of nucleoli and are known as nucleolar organizers.The chromosomes having nucleolar organizers are called nucleolar chromosomes. A chromatid contains a single fine chromatin fibre, the chromonema.It is very long and greatly coiled to be accommodated in a short chromatid. The chromosomes bear hereditary units, the genes.



What are the main Functions of Chromatin ?

 The main functions of chromatin are:

1. Packaging DNA : Chromatin helps to compact and organize the DNA molecules inside the nucleus. It wraps the DNA around proteins called histones, forming a structure known as nucleosomes. This packaging allows the long DNA molecules to fit within the limited space of the nucleus.

2. Gene regulation : Chromatin plays a crucial role in regulating gene expression. The accessibility of genes to transcription factors and other regulatory proteins is influenced by the structure of chromatin. Depending on the level of compaction, genes can be either accessible (active) or inaccessible (inactive) for transcription.

3. DNA replication  and repairs : Chromatin structure also influences the processes of DNA replication and repair. During DNA replication, chromatin is modified to allow the DNA replication machinery to access and copy the DNA. Chromatin structure can also impact the ability of DNA repair proteins to access damaged DNA and carry out repair processes.

4. Chromosome  formation : Chromatin plays a key role in the formation of chromosomes during cell division. The condensation of chromatin into tightly coiled structures allows for the separation and distribution of genetic material during cell division, ensuring that each daughter cell receives a complete set of chromosomes.



Overall, chromatin is essential for the organization, regulation, and integrity of the genetic material inside the cell nucleus.

Types of chromosomes: 

A chromosome may have either equal arms or unequal arms. This depends upon the position of  centromere. Therefore, the chromosome is of four  : 

A): Metacentric  chromosome:

The chromosome in which the centromere is near the middle and the two arms are almost equal in length, is called a metacentric  chromosome.

B):  Submetacentric Chromosome:

The chromosome in which the centromere is slightly away from the  middle point and consequently its one arm is slightly short and the other arm slightly  long, is called submetacentric  chromosome

C): Acrocentric chromosome:

The chromosome in which the centromere is near  the end and consequently its arm is very short and the other arm very long, is called acrocentric chromosome. 

D): Telocentric Chromosome: 

The chromosome in which the centromere is at the tip of chromosome  and the arms are on one side only, is called Telocentric chromosome. 

Chemical Composition  :

A chromatin fibre is a continuous  linear DNA double strands associated with proteins of two types : basic histones and acid or neutral nonhistones. It also contains some RNA, certain metallic ions and some enzymes,  such as DNA and RNA polymerases. Histones are structural proteins. Nonhistones are functional  proteins. 

   The chromatin of prokaryotic  cells is a circular DNA with very little protein.


Types of Chromatin :

After cell division , the chromosomes change back into chromatin fibres. Most of the fibres become uncoiled, extended and scattered. They form the euchromatin ( true chromatin) of the interphase nucleus. It stains lightly .Some chromatin fibres remain coiled and compacted in the interphase also. They constitute the heterochromatin ( the other chromatin ). It stains deeply. It lies close to the nuclear lamina. 


Nucleosomes : 

Electron microscope shows a chromatin fibre as a chain of similar subunits called nucleosomes. A nucleosome consists of a core particle wrapped by DNA strand. The core particle is an octomer of 8  histone molecules. Each nucleosome is connected  to the next by a short DNA linker. A nucleosome and a linker are together  referred to as a chromatosome. 

Function :

The chromatin forms  chromosomes  during cell division.

The main function of chromatin is to package and organize the DNA inside the nucleus of a cell. It helps to compact the long DNA molecules, making it possible for them to fit within the nucleus. Additionally, chromatin plays a role in regulating gene expression and controlling the replication and repair of DNA. It also helps in the formation of chromosomes during cell division.

Nucleolus ( little Nucleus) :

Discovery  : 

Nucleolus was discovered in 1781 by Fontana,it  was described by Wagner in 1840, and was so named by Bowman in the. same year ( 1840). 

Structure:

Nucleolus is dense, rounded , dark_ staining , granular organelle without a limiting membrane. It consists largely of RNAs and proteins. It also contain DNA which produces precursors RNAs for the formation of ribosomes. Each cell has  a fixed number of nucleoi, usually  1 to 4, may be numerous¹.The nucleoli are formed at specific sites, called  the nucleolar organizers, present on certain chromosomes. Calcium is necessary for maintaining configuration of the nucleolus. The nucleoli are larger and more numerous in the cells which  are actively engaged  in protein synthesis. The nucleus shows four regions with electron microscope. 


Point 1: Upto 1600 in the oocyte of the toad, xenopus.


a):  Matrix : It is amorphous, homogeneous proteinaceous ground substance of the nucleolus. 

b): Fibrillar Region : It contains fine fibrils that represent RNA precursors molecules in early stages of processing. 

c) : Granular Region : It contain spherical granules which represent  ribosome subunits  ( rRNA +  ribosomal proteins).

d): Nucleolar Chromatin : It consists of chromatin fibre that is a part of the nucleolar  chromosome. It synthesises rRNA.


Functions :

Nucleolus serves the following  functions:__

(i) Nucleolus synthesizes and stores RNA.

(ii)It also receives ribosomal  proteins from  the cytoplasm for storage. 

(iii) It forms ribosomal  subunits by wrapping  the rRNAs with ribosomal pproteins.The ribosomal subunits later leave the nucleus through the nuclear pores.

(iv)  Nucleolus also plays a role in cell division. 

Treamission to Daughter  Cells : Of all the components of the nucleus, only the chromosomes are transmitted  from one cell generation to the next.


Which  are the characteristics of chromatin ?

☆ The characteristics of chromatin are as follows:


 1. Structure  : Chromatin is a complex structure of DNA and proteins. It consists of DNA wrapped around histone proteins, forming nucleosomes, which are further condensed into higher-order structures.

2. Packaging  : Chromatin is highly organized and tightly packed to fit within the cell nucleus. It allows for the efficient storage and transport of genetic information.

 3. Chromosomal  material : Chromatin makes up the bulk of a chromosome, providing a physical structure for the organization and segregation of genetic material during cell division.

4. Regulation of gene expression: Chromatin plays a crucial role in regulating gene expression. It can be modified through various chemical modifications, such as DNA methylation and histone acetylation, which determine whether genes are switched on or off.

5. Accessibility  : Chromatin structure can be altered to allow or restrict access to the DNA for transcription factors and other proteins involved in gene expression.

 6. Dynamic : Chromatin is not a static structure but can undergo transitions between open and closed conformations, enabling gene activation or repression in response to various cellular signals and environmental cues.

7. Epigenetic inheritance: Modifications of chromatin, known as epigenetic marks, can be passed on to daughter cells during cell division, leading to the inheritance of gene expression patterns and influencing cell differentiation and development.

What are examples of chromatin ?


☆☆Examples of chromatin include:


 1. Euchromatin : This is a loosely packed form of chromatin that is present in areas of the genome where active genes are expressed. Euchromatin can be seen as light stains when stained with certain dyes.

2. Heterochromatin: This is a tightly packed form of chromatin that is present in areas of the genome where inactive genes are located or regions that are not transcribed. Heterochromatin can be seen as dark stains when stained with certain dyes.

 3. Centromeric chromatin : This is a specialized region of chromatin that forms the centromere of a chromosome. It is critical for proper segregation of chromosomes during cell division.

4. Telomeric chromatin :  This is a specialized region of chromatin found at the ends of chromosomes that protects the genetic material from degradation and fusion.

5. Repressive chromatin : This is a type of chromatin configuration that leads to the silencing of genes. It is characterized by the presence of specific chemical modifications to the histones.

6. Active chromatin :  This is a chromatin structure that is open and allows for the transcription of genes. It is characterized by the presence of specific chemical modifications to the histones.

Differences between Euchromatin and Heterochromatin 


Euchromatin : 

1):  It is narrower ( diameter  30 _ 80 A° ), lightly stained somewhat diffused. 

2): It does not have granules. 

3): It forms the bulk of chromatin. 

4): It possesses  active genes.

5):It takes part  in transcription. 

6): It is affected by a number of factors like pH, temperature and hormones.

7): Crossing  over is quite common. 

8): Euchromatin  replicates early.


Heterochromatin

1): Heterochromatin  is thicker ( diameter  250 A° or more), darkly stained and is condensed. 

2): It has granules of various  sizes.

3): It is present at certain places in the chromatin.

4): It does not possess active genes. 

5): Transcription  is absent in heterochromatin .

6): It is not influenced  by these factors. 

7): It inhibits crossing  over.

8): Heterochromatin replicates late in the S_ phase.

Differences between chromatin  and chromosomes?

chromatin 

1): It is visible  in the interphase nucleus.

2): It is extended, uncondensed form of deoxyribnucleoprotein chains.

3): It appears as fine filaments  lying crisscross, forming  chromatin  reticulum. 

4): It controls  metabolism .

5): Replication of DNA occurs in the chromatin phase.

chromosomes :

1): These are visible in the M phase ( phase of nuclear division). 

2): They are coiled, condensed form of deoxyribnucleoprotein chains. 

3): These appear  as short,  thick, rod_ like organelles. 

4): Chromosomes  distribute genetic  information to the daughter  cells.

5): Replication of DNA   is suspended in chromosome phase. 

What is the history  of  Chromatin? 


The study of chromatin has a rich and evolving history. Here are some key milestones:


1. 1879: German scientist Walther Flemming coined the term "chromatin" to describe the stained material observed in the nucleus during cell division.

2. 1880-1890s: Scientist Friedrich Miescher discovered a substance he called "nuclein" in the nuclei of white blood cells. This later turned out to be DNA, one of the components of chromatin.

3. 1928: German scientist Emil Heitz classified chromatin into two distinct types: euchromatin (light-stained) and heterochromatin (dark-stained), based on their colors when stained with dyes.

4. 1945: Scientists Oswald Avery, Colin MacLeod, and Maclyn McCarty demonstrated that DNA is the genetic material by conducting experiments with bacterial transformation.

5. 1950s-1960s: Scientists Linus Pauling, James Watson, and Francis Crick proposed the double-helix structure of DNA, which provided insights into how DNA is organized within chromatin.

6. 1961: Scientists Fran├žois Jacob and Jacques Monod proposed the operon theory, which described how genes are regulated and expressed in response to specific conditions. This theory laid the foundation for understanding the role of chromatin in gene expression.

7. 1970s-1980s: Scientists developed methods to extract and purify chromatin from cells, allowing for detailed investigation of its structure and components.

8. 1990s-Present: Advancements in molecular techniques, such as chromatin immunoprecipitation (ChIP) and high-throughput sequencing, enabled researchers to uncover the complex organization and regulation of chromatin. This led to the discovery of various chromatin modifications, histone variants, and the identification of specific chromatin remodeling complexes.

Overall, the history of chromatin research is characterized by a series of significant discoveries that have deepened our understanding of its structure, function, and contribution to gene regulation and inheritance.

QNo 1 : The number of DNA strands per chromatids of chromosome is/ are  


1) Many   2)  four  3): Two  4 ):  one 

Ans : 3):    Two 

QNo 2 : Where is chromatin  Location?


 Chromatin is located within the nucleus of eukaryotic cells. It is found inside the nucleus, which serves as the control center of the cell and contains the DNA molecules.

Which cell cycle  has chromatin?


Ans : Chromatin is present throughout all phases of the cell cycle, including interphase (G1, S, and G2 phases) as well as during mitosis and cytokinesis. However, the chromatin structure and organization change during different phases of the cell cycle. In interphase, chromatin is relaxed and dispersed throughout the nucleus, allowing transcription and gene expression. During mitosis, chromatin condenses and forms distinct chromosomes, which are then segregated into daughter cells during cytokinesis.






















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