DNA is the genetic material in all living cell. The eukaryotic cell has much more DNA content than the prokaryotic cell.

(Just for info: Read about the complexity of the eukaryotic genome)

This large eukaryotic DNA is combined with large amounts of protein and packed within the tiny nucleus. For e.g. a typical human cell contains approximately 2 meters of DNA if stretched lengthwise, which is astonishingly packed within the nucleus only about 6-8 μm in diameter. This amazing packing of DNA into the nucleus is accomplished by a set of proteins which well-orderedly coil and fold the DNA into a very condensed structure called the chromosomes.

The chromosomes undergo structural changes during different stages of the cell cycle. During the interphase, when the cell is not dividing, the chromosome is loosely packed and hence not visible under light microscope. During the cell division or mitosis, the DNA becomes more and more tightly bound around the proteins and assume an X-shaped structure which is visible under light microscope on staining. The chromosome gets strongly stained by some colourful dyes and appear as colourful bodies under the light microscope, hence the name (Greek:- color: chroma; body: soma).

The typical X shape of the chromosome, seen during the mitosis, is because the genetic material is replicated and the cell is about to divide. Hence there are 2 identical copies of the genetic material in a single X-shaped chromosome. Each half of a replicated chromosome is known as the chromatid. The two chromatids of the same chromosome are known as sister chromatids and are held together at the centromere (see fig 1). The joined sister chromatids, separate during anaphase to give two daughter chromosomes.

Mitosis, chromosomes, Chromatids, replication,
Fig 1:Chromosomes: before and after Mitosis.

The centromere divides the chromosome into two sections or ‘arms’, namely p arm and q arm. The location of the centromere is different on different chromosomes and give them characteristic shape. Based on their location on the chromosomes they can be either metacentric, submetacentric, acrocentric or telocentric.

Centromeres not only keep the two sister chromatids together after the replication but also help in proper alignment of the chromosomes during cell division. They also act as a site of assembly of the kinetochores, an apparatus which helps the segregation of chromosomes and attachment to spindle fibres.

Chromosome structure
Fig 2: Chromosome structure

• Chromatin and Chromosomes

The chromatids are made of a substance called chromatin, which is a lesser condensed form of chromosomes made up of long strand of DNA, wrapped around the proteins called histones. Chromatin can be of two types:

1. Euchromatin:

These are the regions where the chromatin are much less densely packed. They stain lightly and occupy most of the nuclear region. The DNA in these regions are genetically active.

Heterochromatin Euchromatin
Fig 3: Heterochromatin and Euchromatin (Image: Sha, K. and Boyer, L. A. The chromatin signature of pluripotent cells (2009) Stem Book, ed. The Stem Cell Research Community)

2. Heterochromatin:

These are the regions of chromatin packed very densely. The packing is as compact as in the chromosomes at mitosis. These regions stain darkly and are genetically inactive or silent. Heterochromatin regions are further classified as constitutive and facultative heterochromatin.

I. Constitutive Heterochromatin:

These are always packed densely or remain condensed. These region contain repetitive sequence, generally not coding for any protein but more involved in the chromosome structure.

II. Facultative heterochromatin:

These regions code for certain proteins only at a particular point of time of development of an organism. Hence these regions may be relaxed at some point of time. Example is the Barr body in mammals (In females, one X-chromsome is active while the other is inactive and in heterochromatin state, which becomes active in the male child)

Hierarchy of organization:

The chromosome is a very condensedly packaged DNA. The packaging is done in a very stepwise manner. Let’s take a look at these “hierarchy of organization” of the chromosome:

1. Nucleosome:

The nucleosome contains around 200 base pairs of DNA, organised by octamer of small, basic proteins called histones. This protein has numerous positive charged amino acids residues, namely lysine and arginine. The negatively charged DNA is wrapped around the protein core, composed of 8 histone proteins, two each of H2A, H2B, H3 and H4. Histone H1 forms the linker between two nucleosomes. These histones wrapped around by DNA give an appearance of a string of beads when viewed using an electron microscope.

Fig 4: Nucleosome

Each octomer binds and wraps approx 1.7 turns of DNA i.e. around 146 base pairs. H1 protein wraps another 20 base pairs, resulting in two full turns around the octamer. The two adjacent nucleosomes are separated by around 20 base pairs long DNA stretch known as linker DNA.

The nucleosome provides the first level of organisation and observed in both euchromatin and heterochromatin. Each nucleosome gives a packing ratio of around 6.

(Just for info: Read more about nucleosome)

DNA packaged into chromosome
Fig 5: Hierarchy of organization (Image:Jansen, An & Verstrepen, Kevin. (2011) MMBR. 75. 301-20)

2. Second level of Organisation:

The second level of organisation involves coiling of the series of nucleosome (can be seen in fig 3) into a helical array to constitute a pipeline structure, called the solenoid fiber. The solenoid literally means ‘pipe-like shape’ (Greek: solen”pipe, channel” + eidos “form, shape).

The diameter of solenoid fibre is around 30nm. This coiling packages the DNA further by a packing ratio of around 40. The formation of solenoid fibre involve proteins other than histones, which are not yet well defined.

3. Third level of Organisation:

The third level of packaging involves the packaging of 30 nm solenoid fibre itself with other proteins to form protein scaffold.

This packages the DNA further by a packing ratio of around 1000 in euchromatin. A even more dense and tightly packed protein scaffold is seen in mitotic chromosomes, having a packing ratio of around 10,000. The two packed systems are interchangeable. Heterochromatin generally have a packing ratio of around hundred 10,000 in both interphase and mitosis.

Hence in the chromosomes, the DNA is condensed by nearly 10,000 times, yet the regions necessary for separation of sister chromatids and other mitotic events are available. Just another fascinating feature of biological world!

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