In eukaryotic organisms — plants, animals and fungi — the major portion of DNA is present in the chromosomes which are well-organised structures and quite different from the prokaryotic counterparts. Besides the chromosomes, mitochondria of both plants and animals and the chloroplasts of green plants also contain DNA. Interesting is the fact that the organization and the nature of the DNA of these cell-organelles are similar to those of bacterial DNA. In both these organelles, the DNA is a covalently closed circular molecule.
In eukaryotic cells, the chromosomes are present in a distinct, double-membrane bound structure, the nucleus which occupies on the average about 10% of the cell volume. The membrane is continuous with the endoplasmic reticulum and is provided with pores. The number of chromosome is variable, but fixed for a biological species. Chromosomes change in their physical characteristics during cell division. All these features are absent in the prokaryotic cells.
An individual eukaryotic chromosome contains a single enormously large linear ds-DNA molecule. For example, a diploid human cell containing 46 chromosomes (22 pairs of autosomes and one pair of sex chromosomes) has a total of 6 x 109 base-pairs.
The length of the DNA molecules of individual human chromosomes varies from 1.5 cm to 8.7 cm. These large molecules have to be packed into chromosomes, generally measuring a few microns in length and breadth. This is accomplished by binding DNA to proteins. The protein-DNA complex of eukaryotic cells is known as chromatin.
The DNA-binding proteins are distinguished into two main types — the histones and non-histone proteins. The histones are basic proteins, rich in basic amino acids, like lysine and arginine. Histones have large amount of positive charges and can bind tightly the negatively charged DNA molecules.
These binding results in the formation of the characteristic structural units called the nucleosomes. The long ds-DNA molecule of each chromosome is folded in a very orderly way around the histones to form the nucleosomes.
The nucleosomes are bead-like structures connected to each other by linker DNA. Each nucleosome consists of a histone core composed of 8 subunits (octamer) of 4 different histones — H2A, H2B, H3 and H4 with two molecules of each. The protein core is wrapped by two turns of ds-DNA molecule to form a nucleosome.
The DNA molecule runs as a continuous thread from one nucleosome to another. The intervening portion of DNA between two nucleosomes is the linker. Width of a nucleosome is 11 mm and, on the average, nucleosomes are repeated at intervals of 200 nucleotide pairs of DNA. The length of the linker between two nucleosome is variable.
These features of eukaryotic DNA are shown diagrammatically in Fig. 9.10:
The nucleosomes are basic structures from which chromatin is made. They are further organized into closely packed 30 nm fibres of chromatin. These fibres are visible under high resolution electron microscope.
The 30 nm fibres shown in Fig. 9.11, are organized into higher orders of increasing complexity, like 300 nm fibres and 700 nm fibres to produce chromosomes:
The eukaryotic chromosomes are characterized by the presence of three types of specialized nucleotide sequences in their DNA. These sequences serve as origin of replication, as centromere which helps the daughter chromosomes to move to opposite poles, and telomere which has a number of repeating sequences functioning as template for RNA-primer in DNA synthesis.