PinkMonkey Online Study Guide-Biology
8.1 Packaging of Hereditary Material
Structure of nucleus (A) in a prokaryotic cell and (B) in an eukaryotic
As stated earlier, chromosomes are the carriers of the
hereditary material. In cells, chromosomes are located in the nucleus.
In prokaryotic cells (e.g., bacteria, cyanobacteria, etc.), organized
nuclei are not present. As a result, the hereditary material occurs in
the cell cytoplasm as a nucleoid (Figure 8.1-A) without the nuclear
(a) Eukaryotic nucleus (Figure 8.1-B): In
eukaryotic cells (e.g., all higher plants and animals), a well organized
nucleus is present. It is bounded by a double membrane nuclear envelope
with trilaminar structure. There are pores in the envelope. Internally,
the nucleus is filled with nucleoplasm (nuclear sap) that is acidophilic
and clear. The interphasic nucleus (i.e. the nucleus which is not
in the stage of division) usually shows the presence of one dark,
spherical body called nucleolus. The nucleoplasm also contains nuclear
reticulum or chromatin network. It consists of very fine, long chromatin
fibers. These represent the chromosomes at the interphasic stage.
They become short, thick and distinct during cell division.
(b) Structure of chromatin (Figure 8.2): The chromosomes
are composed of nucleoprotein called chromatin. The chromatin fiber
appears to have a structure like a ‘string of beads.’ According to the
nucleosome-solenoid model proposed by Komberg and Thomas (1974),
the beaded string is made of repeating units called nucleosomes
(Figure 8.2A). Each nucleosome is a bead-like particle about 60 Ao
high and 110 Ao wide.
Click here for enlarge
Figure 8.2. Nucleosome - solenoid model for the structure of chromatin
In the nuclesome, the DNA double helix is wound around
the core of eight histone molecules (octamer). The segments of DNA joining
the ‘beads’ are called linker DNA. Each linker DNA has an average of one
molecule of a histone protein (H1) attached to it.
The thin, long and "beads-on-string" chromatin
fiber is condensed and packed into a short and thick chromosome (metaphase
chromosome) because of further coiling and "super coiling",
wherein (1) the string of beads condenses and coils to produce a 100 Ao
thick nucleosome fiber (2) the nucleosome fibre then supercoils to produce
a 300-350 Ao thick solenoid fiber. A solenoid represents
the structure of chromatin fiber in the metaphase chromosome.