Chromosome, Chromatin, and DNA

We all know that our body is made up of millions of cells, each cell makes up the structural and functional unit of life. Each cell consists of many cellular organelles such as nucleus, mitochondria, lysosome, etc.

Chromosomes are one of the important components which are present in the nucleus of the cell.

It contains an organized package of DNA, which is very much important in carrying out the body’s metabolic and enzymatic activities and also has a vital role in carrying out the hereditary characters.

Chromatin is one of the complexes of the DNA and proteins which forms the chromosomes.

What is Chromatin?

Chromosomes are made up of complex substances named as chromatin which is composed of DNA and proteins. When it is observed under a microscope, chromatin, looks like a bead present on a string.

The bead like structure are the nucleosomes. Which are composed of DNA, wrapped around with eight proteins which are called as histones.

These nucleosomes are then further wrapped into a spiral coil of about 30nm which is known as solenoid.

Where the additional histone proteins support the structure of chromatin.

During the process of cell division, the structure of chromosomes and chromatin are clearly visible under a light microscope, and the shape gets change when the DNA is duplicated or separated into two cells.

Chromatin, a complex of DNA and proteins which forms the chromosomes with the nucleus of the eukaryotic cells, where the Nuclear DNA does not appear in the form of free strands, rather it is highly condensed and winded around the nuclear proteins to get fit inside the nucleus.

Chromatin generally occurs in two forms, namely euchromatin and heterochromatin. Euchromatin is less condensed and can be transcribed, where as the heterochromatin is highly condensed and is not typically transcribed.

Structure of Chromatin

Usually, chromatins are the complex macromolecules found inside the nucleus and it is composed of DNA and proteins and in few cases, it also has RNA.

It is found inside the nucleus of the eukaryotic cells. Chromatin mostly exists in two forms as mentioned above, as euchromatin and heterochromatin where euchromatin is found in the extended form and heterochromatin as condensed form.

Chromatin is composed of primary proteins called as histones which helps in organizing the DNA into a structure of beads known as nucleosomes, which provides a base for the DNA to get wrapped around.

Commonly a nucleosome consists of 147 base pairs of DNAs, which is wrapped around a set of eight proteins; named as octomer.

This nucleosome can be further folded to produce the fibers of chromatin. Chromatin fibers are coiled and then condensed to form chromosomes.

Chromatin helps in numerous cell functions such as DNA replication, transcription, DNA repair, genetic recombination of DNA, cell division etc.

Function of Chromatin

Chromatin undergoes various processes like Packaging of DNA, regulation of transcription, and also in the process of repairing the DNA.

These functions are discussed below.

I. Chromatin and DNA Packaging

Packaging of DNA, is considered as one of the important and fundamental function of Chromatin. Here the long strands of DNA are compressed.

Usually the length of DNA, in the nucleus is very large compared with the other DNAs and it is far greater than the compartment of the cell structure.

To fit this in a particular compartment, packaging of DNA is being done, it must be packed in the perfect ratio, it cannot be stuffed such as , it is present in a structure of chromatin.

There is a certain procedure and several hierarchies of organisation. The first step involved in packing is winding the DNA around the nucleosome, it gives a packing ratio of about six.

This structure is invariant in both euchromatin and in heterochromatin in all the chromosomes. The second level involved in packaging is winding the beads which has fibre of size 30nm which is found in both interphase chromatin and in mitotic chromosomes.

Thus this structure increases the packing ratio to forty. The final step involved in this process is the fibres are organised in the form of loops and scaffolds or in the form of domains which gives a final packing ratio of about 1000 in the interphase chromatin and about 10,000 in mitotic chromosomes.

II. Chromatin and Transcription Regulation

Generally, transcription is the process in which the genetic information stored in a DNA, are read by the proteins and are transcribed into the functional proteins.

If the chromatin gets strengthened and restricts the access for the proteins to being read, there will be no transcription. Where as the euchromatin which is in the extended form conducts the transcription; on the other case the heterochromatin which is in the condensed form pack the DNA so tightly for the proteins to be read.

The fluctuations which occur between these closed and open chromatin structures leads to the discontinuity of transcription.

Other factors such as association and disassociation of the transcription factor with the chromatins.

This phenomenon opposes the simple models of transcription which accounts for the high variability in expressing the genes which occurs between the cells in case of isogenic population.

III. Chromatin and DNA Repair

Packaging the DNA into the chromatin results as a barrier for all the functioning’s of the DNA-based processes. Due to this highly dynamic arrangement of proteins and DNA in the chromatin; leads to the change in structure and shape of it.

At this time chromatin relaxation occurs Rapidly at the site of DNA damaging, which allows the repaired proteins to bind with the DNA and repairs it.

Differences Between Chromatin, Chromosomes and Chromatids

We often feel that all these three terms; chromatin, chromosomes and chromatids are similar, because we come across all these terms when we study the same chapters ahead.

However, these three are the structures which composes our genetic material, DNA and the other proteins and which are present inside the nucleus.

Each of these terms can be defined in a unique manner. As said above, chromatin is composed of DNA and histones which are packed as a thin, stringy fibers, Chromatin often undergoes condensation to form the chromosome.

So, it can be said that chromatin serves as a basic unit for chromosomes and it as well forms the lower order in organization of DNA and chromosomes are consider as the higher order in the DNA organization.

Chromosomes are the group of single stranded condensed chromatin. During the process of cell division chromosomes replicates and forms a new daughter cells such that each cell receives the same number of chromosomes.

This duplicated form of chromosome is double stranded and it is of X – shaped, where the two strands are identical to each other and are connected to a central region known as centromere.

Chromatid is the name of the strand in a singular form of the replicated chromosome.

Chromatids are often called ad sister chromatids as they are similar in structure and they are connected by the centromere. At the end of the cell division, sister chromatids separate and become as daughter chromosomes in the newly formed daughter cells.