Prokaryotes are monoploid that is they have only one set of genes stored in a single chromosome composed of a single molecules of DNA. The bacterium Escherichia coli is a closed circular DNA molecule of length of about 4.6 million base pairs, which resides in the region of the cell called the nucleoid. Since an E. coli cell has a diameter of only 1-2 mm, the large DNA molecule present in each bacterium must exist in a highly condensed form. When E. coli chromosomes are isolated in presence of small basic or positively charged proteins, the chromosome remain in a highly condensed state. This structure, called the folded genome, is the functional state of bacterial chromosome. Within the folded genome, the large DNA molecule is organized into 50-100 loops or domains of 50-100kb in length; each of which is independently negatively supercoiled.
Bacteria carry their genes on a single DNA molecule, which is usually circular and haploid. They have only one type of chromosome but several copies of this chromosome are present per cell. Therefore bacterial cells are often referred to as multinucleate. The main chromosome contains a few thousand genes.
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| E.coli Genome |
Bacteriophage genome
Bacteriophages are composed of an inner nucleic acid genome of single or double stranded DNA or RNA. Usually all genes are present on a single nucleic acid, which may be linear or circular. However, in some RNA phages the genes are present on more than one molecule and these phages are said to be segmented or diploid. Single stranded viral genomes are described as positive sense(Same nucleotide sequence as the mRNA), negative sense or ambi-sense(in which genes are encoded in both senses, often overlapping).
Phage genomes vary greatly in size from just a few kilo base pairs to about 150kbp. The genome of phage contains just 10 genes while the large phage T4 has 150 genes. Phage genes encode proteins required for the construction of capsid and enzymes involved in phage DNA replication. All phages also require at least some host proteins and RNAs. A feature of some phages also seen in eukaryotic viruses, is that they have overlapping genes translated in different reading frames.
