Prokaryotic cells have no nucleus. Instead, their main DNA is found in an irregular region of the cytoplasm called the nucleoid. The nucleoid is not surrounded by a membrane, so the DNA is in direct contact with the cytoplasm and its ribosomes.
The nucleoid contains a single, large, circular chromosome made of double-stranded DNA. In Escherichia coli, for example, the chromosome is about 4.6 million base pairs long and carries over 4000 genes. Stretched out, it would be roughly 1000 times longer than the cell itself, so it has to be tightly packed. The DNA is supercoiled (twisted on itself) and folded into loops held by proteins. Prokaryotes do not wrap their DNA around histones in nucleosomes as eukaryotes do.The bacterial chromosome carries the essential genes the cell needs to survive, such as those for enzymes of respiration and for DNA replication and protein synthesis. This is different from plasmids, which carry extra, usually non-essential genes.Prokaryotic genes generally have no introns, and genes with related functions are often grouped together and transcribed together. Because there is no nuclear envelope, transcription and translation can happen at the same time: ribosomes can start translating an mRNA molecule while it is still being made.Before binary fission the chromosome is copied, starting at a single origin of replication, and one copy goes to each daughter cell.
The nucleoid contains a single, large, circular chromosome made of double-stranded DNA. In Escherichia coli, for example, the chromosome is about 4.6 million base pairs long and carries over 4000 genes. Stretched out, it would be roughly 1000 times longer than the cell itself, so it has to be tightly packed. The DNA is supercoiled (twisted on itself) and folded into loops held by proteins. Prokaryotes do not wrap their DNA around histones in nucleosomes as eukaryotes do.The bacterial chromosome carries the essential genes the cell needs to survive, such as those for enzymes of respiration and for DNA replication and protein synthesis. This is different from plasmids, which carry extra, usually non-essential genes.Prokaryotic genes generally have no introns, and genes with related functions are often grouped together and transcribed together. Because there is no nuclear envelope, transcription and translation can happen at the same time: ribosomes can start translating an mRNA molecule while it is still being made.Before binary fission the chromosome is copied, starting at a single origin of replication, and one copy goes to each daughter cell.