Living organisms are categorised into prokaryotes and eukaryotes based on their cellular features, especially the nature of membrane bound organelles and genetic material organisation. The genetic materials in prokaryotes as well as in eukaryotes are organised into chromosomes. Although the term chromosome is applicable in eukaryotes only, yet the prokaryotic genetic materials are also described in the same way. The prokaryotic and eukaryotic chromosomes are different and their differences are enlisted in table.

Difference between Prokaryotic and Eukaryotic Chromosomes

Eukaryotes

In eukaryotes, the major part of DNA is found in the chromosomes, which themselves are present within the nucleus (a distinct, double-membrane bound structure that makes up about 10% of the cell volume). The eukaryotic cell membrane has pores and is continuous with the endoplasmic reticulum. The eukaryotes have varied number of chromosomes which is fixed for a biological species. During cell division the physical characteristics of chromosomes vary.

A eukaryotic chromosome has a single, very large, and linear double stranded DNA (ds-DNA) molecule. For example, a diploid human cell that contains 46 chromosomes (22 pairs of autosomes and a pair of sex chromosomes) has a total of 6 ×10⁹ base-pairs. In individual human chromosomes, the length of DNA molecules ranges from 1.5−8.7cm. These large DNA molecules are packed in chromosomes ranging from a few microns in length and breadth. This is achieved by the formation of protein-DNA complex known as chromatin (in eukaryotic cells).

The proteins that bind to DNA are histones and non-histone proteins. Histones are basic in nature due to the presence of basic amino acids (such as lysine and arginine). Since they carry positive charges, they bind to the negatively charged DNA molecules forming characteristic structural units, the nucleosomes. The long double stranded DNA molecule in each chromosome systematically surrounds the histones to form nucleosomes.

The structure of nucleosome resembles beads connected to each other through linker DNA. A nucleosome bears a histone core having 8 sub-units (octamer) of 4 different histones, H₂A, H₂B, H₃, and H₄ with two molecules of each. This histone protein core is encircled by two turns of ds-DNA molecule, thus forming a nucleosome.

The DNA molecule runs as a continuous thread from one nucleosome to another. The dominant part of DNA between the two nucleosomes is the linker. Nucleosome width is 11 mm, and it repeats itself at intervals of 200 nucleotide pairs of DNA. The linker between two nucleosomes may have varied length. These features of eukaryotic DNA are shown in the figure.

The nucleosomes are basic structures that build chromatin. They are organised as closely packed 30nm chromatin fibres, which can be observed under high resolution electron microscope.

Prokaryotes

The internal structure and genomic organisation of prokaryotic cells are simple than that of the eukaryotic cells. The cells of prokaryotes do not have complex compartmentalisation as in the cells of eukaryotes. Some prokaryotic cells show specialised membranes that are plasma membrane infoldings participating in metabolic functions.

The prokaryotic genome is structurally different from eukaryotic genome. On an average it has only about one-thousandth of DNA as in eukaryotic genome. In prokaryotes, mostly the genome has a DNA ring with few proteins linked to it. This genetic material ring is the prokaryotic chromosome. In eukaryotes, the chromosome lies within the nucleus, whereas in prokaryotes, the chromosome is in a nucleoid region (a portion of cytoplasm which in electron micrographs appears lighter than the surrounding cytoplasm).

In prokaryotes and eukaryotes, fundamental similarity is observed in DNA replication, transcription, and translation; however, some differences also exist. For example, ribosomes in prokaryotic cell are smaller than those in eukaryotic cells. Also the protein and RNA content of these ribosomes is different.

These differences are important for some antibiotics (like erythromycin and tetracycline) as they bind to ribosomes and block protein synthesis. This blocking action takes place only in prokaryotes and not in eukaryotes. Hence these antibiotics kill bacteria without harming the human cells.

Prokaryotes (like E. coli) lack a well-defined nucleus. Hence, the negatively charged DNA binds to each other through some positively charged proteins in the nucleoid region. The DNA in this region is arranged within the nucleus as large loops connected through proteins.