Prokaryotic Chromosome Structure: 7 Facts You Should Know

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In this article, we will understand the organization and structure of the prokaryotic chromosome.

Prokaryotic chromosome organization is different from eukaryotic chromosome organization in certain specific ways that are discussed in this article.

7 facts you should know about prokaryotic chromosome structure:

  • Unlike the eukaryotic chromosome, the prokaryotic chromosome is accommodated in the cytoplasm.
  • During chromosome segregation in prokaryotes, a single duplicon is present per chromosome.
  • The circular prokaryotic chromosome makes the rate of replication first.
  • A prokaryotic chromosome is known as a plasmid. Plasmids are circular extra chromosomal double-stranded DNA molecules.
  • Chromosomes in a prokaryotic cell are arranged in nucleoids. Unlike nuclei, these are not membrane-bound structures.
  • Prokaryotes replicate using a single bubble in their chromosome.
  • Prokaryotic chromosomes are double-stranded.  

Are prokaryotic chromosomes circular or linear?

Prokaryotic chromosomes are mostly circular. However, linear chromosomes are identified in occasional lineages.

Circular chromosomes are present in archaea. Circular chromosomes can be tailored to make stable linear chromosomes with the help of hairpin telomeres. Circular chromosomes present in prokaryotes are devoid of centromeres.

The circular prokaryotic chromosome makes the rate of replication first as both the replication forks move at the same pace leading to their termination at the same time.

Why prokaryotic chromosomes are circular?

Circular prokaryotic chromosomes aid in faster replication thereby making them suitable for experimental use as their number can be increased rapidly.

Only 12% of the prokaryotic genome consists of non-coding sequences. In prokaryotes, multiple coding regions are attached to a single promoter and these clusters are called operons. The proteins coded by a single operon work for a common metabolism such as lac operon.

What does a chromosome look like in a prokaryotic cell?

A prokaryotic chromosome is known as a plasmid. Plasmids are circular extra chromosomal double-stranded DNA molecules.

Plasmids consist of their own replicons and contain many essential genes. Plasmids consist of chromosome-like GC content. Prokaryotes accommodate chromosomes larger than their size by supercoiling leading to the formation of a condensed ball. Supercoiling can be of two types: negative supercoiling and positive supercoiling.

How are chromosomes arranged in a prokaryotic cell?

Chromosomes in a prokaryotic cell are arranged in nucleoids. Unlike nuclei, these are not membrane-bound structures.

When the chromosomes are coiled in a direction against that of the double helix, it is called negative supercoiling and when it is coiled in the direction of the double helix, it is called positive supercoiling. During normal growth, bacterial genomes are generally negatively supercoiled.

A large number of proteins participate in folding prokaryotic chromosomes. The nucleoid present in prokaryotes consists of a protein called HU that generates tension in the chromosome and instigates it for negative supercoiling.

prokaryotic chromosome structure
Prokaryotic chromosome structure from Wikimedia

Another protein that aids HU is the enzyme topoisomerase I. Few other proteins such as host factor (HF) aid in supercoiling by binding to specific sequences in the genome. The supercoiled DNA then folds around HU protein as eukaryotic DNA folds around histone proteins.

How does the structure of a prokaryotic chromosome change during mitosis?

Prokaryotes replicate using a single bubble in their chromosome.

During mitosis, small regions project out from the tightly packed nuclei so that proteins can act on them. Unlike eukaryotes, prokaryotes witness transcription and translation together in the cytoplasm as the ribosomes responsible for translation are not membrane-bound.

Is the prokaryotic chromosome double-stranded?

Yes, prokaryotic chromosomes are double-stranded and are negatively supercoiled.

DNA gyrase, DNA topoisomerase I, H-NS, and factor for inversion stimulation (FIS) are the proteins that maintain the supercoiled structure of prokaryotic DNA and also regulate the expression of different genes that respond to environmental stimuli.

Conclusion

Taken together, prokaryotic chromosomes are double-stranded and circular in structure.

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