“Prokaryotic chromosomes are circular or linear in shape, contain less repetitive DNA and have only a few genes.”
Life on earth is a mysterious thing, how life was evolved was yet another mystery not solved completely. All the life forms on the earth are divided into two broad categories; Prokaryotes and eukaryotes.
Prokaryotes are the single cell, primitive organisms microscopic in nature whilst the eukaryotes are multicellular and possess a higher level of organization.
Furthermore, unlike the eukaryotic cells, the prokaryotes or prokaryotic organisms don’t have membrane-bounded organelles. Bacteria, some fungus, algae and Archaea are some of the examples of it.
Structurally, the prokaryotic cell doesn’t have the nucleus as well. Then how is the genetic material of a prokaryotic cell organized? Where is it located and how was it inherited?
Let me tell you first, that, much like the eukaryotes the prokaryotic genetic material is located on chromosomes but the structure of their chromosome is different.
In the present article, we are discussing the prokaryotic chromosome, its structure and its function. Before that read this related article that will help you to understand the present topic more clearly.
Prokaryotic DNA vs Eukaryotic DNA.
Key Topics:
What is a prokaryotic chromosome?
The whole-genome or the prokaryotic genetic material is located on a single linear chromosome located in the cytoplasm of a cell. Though structurally it is different from the eukaryotic chromosome, it also transmits the genetic material from one cell to another.
The genome of prokaryotes is not only made up of chromosomes but also a structure called the plasmid plays an important role in the inheritance of characters.
The plasmid is a circular single-stranded DNA that inherited several important genes for a bacteria.
The prokaryotes literally mean “before” nucleus. It originated before the evolution of the nucleus and hence it doesn’t have a distinct nucleus-like structure.
Related article: Chromosome- Definition, Structure, Function And Classification.
Definition:
A single linear or circular DNA molecule present in the cytoplasm of a prokaryote is known as the prokaryotic chromosome.
Structure of prokaryotic chromosome:
Structurally, as we said above, it is located in the cytoplasm and it doesn’t have histone in it. And hence it doesn’t follow the eukaryotic model of DNA packaging.
Packaging of DNA is also important in prokaryotes as well because the DNA of prokaryotes is also larger than its cell.
Here, by following the mechanism called supercoiling, the DNA winds around each other and makes a compact chromosome to fit inside the cell. Note that some prokaryotes have histones to pack their genetic material.
The genetic material in eukaryotic organisms is always DNA but in prokaryotes, either DNA or RNA is present as genetic material.
Structurally, it is made up of nitrogenous bases, sugar and phosphate.
Properties of prokaryotic chromosome:
- A prokaryotic chromosome is circular or linear
- It also contains extrachromosomal DNA as a plasmid.
- The genome of it is haploid.
- A single copy of the gene is present on the chromosome.
- Only one single chromosome is present in prokaryotes.
- The genes of prokaryotic chromosomes possess a special mechanism known as operon using which multiple genes make protein using single machinery.
- Not so important gene sequences are located on the chromosome, instead, it is present on plasmid.
- Also, the chromosome of prokaryotes has less repetitive and junk DNA in it. The genes on a chromosome are located very nearer to one another, only 12% of junk DNA is present in the prokaryotic genome.
- It is very compact in nature and has less junk genetic material.
- Transcription and translation both occur in the cytoplasm.
Functions of prokaryotic chromosome:
Like the eukaryotic chromosome, the prokaryotic chromosome helps to store and transmit biological information to another cell.
It replicates, transcribes and translates to form DNA, RNA and protein, respectively.
Packaging in prokaryotes:
The chromosome in prokaryotes is present in an irregular shape structure known as the nucleoid. Most of the prokaryotes follow the mechanism of supercoiling to make a chromosome.
A single circular chromosome of prokaryote lacks histones. Following the process of twisting and coiling it becomes compact to fit in the nucleoid.
Several proteins and enzymes make it possible, some of which are topoisomerase I and II or gyrase, HU, H0NS and IHF. All these help in regulating and maintaining the supercoiling in prokaryotes.
If the supercoiling occurs in the same direction of the double helix, it is known as negative supercoiling while if it occurs in the opposite direction it is known as positive supercoiling. Prokaryotes, especially bacteria, follow the negative supercoiling.
The class of the enzyme known as topoisomerase regulates the tension generated by the process of supercoiling while replication.
Due to the tight supercoiling of the prokaryotic chromosome, the rate of replication is too slow in comparison to eukaryotes. In addition to this, as it is located in the cytoplasm, the process of transcription and translation occur simultaneously at the same location.
Here one thing should be noted clearly that we have discussed all these things by taking the example of E.Coli. So not all the prokaryotes have circular DNA. Some unicellular possess a linear chromosome.
Moreover, it is also not fixed that only a single chromosome is present in them. There might be two or more or, some of them that are circular and linear.
Take a look at these examples:
| Organism | Number of chromosomes | Type of chromosome | Size of genome |
| E. Coli | Single | Circular | 4.5 to 4.7Mb |
| B Burgdorferi | 11 copies | Linear | 950Kb each chromosome |
| V. cholerae | 2 | Circular | 3.2Mb |
Conclusion:
Conclusively we can say that the prokaryotic chromosome is also a kind of complete DNA network. It is either circular or linear, may or may not have histones, single or multiple in size.
Note that the mechanism known as operon helps them to regulate the translation process by translating multiple genes using a single promoter. Read more on a related topic: trp & lac Operon and Gene Regulation in Bacteria.
FAQs:
Do bacteria or prokaryotes have chromosomes?
Yes.
But not like the eukaryotes. The prokaryotic chromosomes are linear or circular in structure.
How many chromosomes do prokaryotes have?
It depends on the organism. Several organisms have a single circular or linear chromosome, for example, E. Coli while others have more than one chromosome for example V cholerae.
What is the type of supercoiling prokaryotes?
The DNA of prokaryotic organisms are mostly negatively supercoiled.
Where the prokaryotic chromosome is located?
Prokaryotes like bacteria don’t have a nucleus or membrane-bounded organelles hence their chromosome is located in the cytoplasm of the nucleoid.
It is written that a plasmid is a single-stranded molecule. Is it?