8.1 Packaging of Hereditary Material

Structure of nucleus (A) in a prokaryotic cell and (B) in an eukaryotic cell

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 envelope.

(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.

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.

Table of Contents

8.0 Introduction
8.1 Packaging of Hereditary Material
8.2 The Structure of DNA
8.3 Replication Of DNA In Eukaryotes
8.4 Replicatin of Pokaryotic Chromosome
8.5 Plasmids
8.6 RNA: Structure and Types
8.7 The Genetic Code
8.8 The Central Theme of Protein Synthesis

Chapter 9

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