The key difference between plasmid and chromosome is that the plasmid is a circular double-stranded extra-chromosomal DNA structure of bacteria while the chromosome is a well-organized thread-like structure that contains genomic DNA tightly coiled with proteins.
The bacterial cell contains a chromosome and several extra-chromosomal DNA circles called plasmids. Bacterial chromosome contains the genomic DNA of bacteria. Furthermore, eukaryotic genomic DNA also exists as chromosomes. The human genome has 46 chromosomes. Generally, plasmids are present in bacteria and archaea. Though bacteria have plasmids, plasmids do not contain the genes that are essential for the survival of bacteria and their major functions. However, plasmids contain several genes that provide additional advantages to bacteria such as antibiotic resistance, drought tolerance, herbicide resistance, etc. The purpose of this article is to discuss the difference between plasmid and chromosome in detail.
CONTENTS
1. Overview and Key Difference
2. What is a Plasmid
3. What is a Chromosome
4. Similarities Between Plasmid and Chromosome
5. Side by Side Comparison – Plasmid vs Chromosome in Tabular Form
6. Summary
What is a Plasmid?
Plasmid is a small circular DNA molecule present bacteria and archaea. It is an extra DNA element except for the genomic DNA or chromosomes. The plasmid has the origin of replication. Hence, it has the ability of self-replication, and it possesses only a small number of genes. Its size can vary from less than 1.0 kb to more than 200 kb, but the number of plasmid in a cell is a constant from generation to generation. ‘Copy number’ refers to the average number of plasmid copies in a single bacterial cell. Therefore, the copy number may range from 1 to 50, but this differs among the bacterial species.
Since plasmids do not contain genomic DNA, they are not essential for the function of the bacteria where they reside. But the genes of plasmids provide beneficial effects and extra survival to bacteria. Moreover, some genes code for the antibiotic resistance. For example; some Staphylococci possess plasmid genes that code for penicillinase enzyme to break down penicillin. Hence they acquire resistance against the penicillin antibiotic. In Rhizobium leguminosarum, plasmid genes are responsible for the nitrogen fixation and nodule formation.
Figure 02: Plasmid
Moreover, it is possible to insert the plasmid into other bacteria, and consequently, it functions as a part of host bacteria. This ability is significant in genetic engineering when introducing important genes into host organisms.
What is a Chromosome?
Chromosome is a thread like structure composed of DNA and proteins. Chromosomes are present in all living organisms including bacteria and eukaryotes. There is only one chromosome present in bacteria while there are 46 chromosomes present in humans. In all living organisms, genomic DNA present in chromosomes.
Hence, they are extremely important for the survival and the functioning of the organism. In bacteria, chromosome freely floats in the cytoplasm while in eukaryotic organisms, they reside inside the nucleus. In contrast to prokaryotic chromosomes, eukaryotic chromosomes contain thousands of genes. Furthermore, eukaryotic chromosomes contain histone proteins while prokaryotic chromosomes do not contain histone proteins.
Figure 02: Chromosome
Generally, chromosomes are not visible under the microscope. But when the cell divides, chromosomes are visible as tightly coiled threads during the prophase.
What are the Similarities Between Plasmid and Chromosome?
- Plasmid and chromosome are two components of a bacterial cell.
- Both contain DNA molecules.
- Furthermore, both contain genes.
- Besides, bacterial chromosome and plasmid are double-stranded circular molecules.
What is the Difference Between Plasmid and Chromosome?
The key difference between plasmid and chromosome is that the plasmid does not contain genomic DNA while the chromosome contains genomic DNA. Chromosome is covered with a protein whereas plasmid is not covered with protein. Thus, it is a difference between plasmid and chromosome. Furthermore, another difference between plasmid and chromosome is that the chromosome is linear since it consists of linear DNA whereas the plasmid is circular. Apart from that, chromosomes are essential to cell function being the genetic information carrier, whereas plasmid is not essential to the function of bacteria where they reside, but these genes give extra survival to host bacteria. Therefore, this is an important difference between plasmid and chromosome.
Moreover, chromosomes have several thousand genes, whereas plasmids have a small number of genes. Also, plasmid size can vary from less than 1.0 kb to more than 200 kb, whereas chromosome size is very much larger than a plasmid. For example, the size of a chromosome is expressed in Mega-scale. Also, chromosomes have a centromere and two sister chromatids, whereas plasmid does not have chromatids or centromere. Hence, this is also a significant difference between plasmid and chromosome. An additional difference between plasmid and chromosome is their application. Plasmids are used as gene carriers to an alien cell; hence, applied in genetic engineering, whereas chromosomes are not used as gene carriers.
Below infographic on the difference between plasmid and chromosome tabulates all these differences for quick comparison.
Summary – Plasmid vs Chromosome
Plasmid and chromosome are two structures made from DNA. Both types are present in bacteria while plasmids are not generally present in eukaryotes. Plasmid is an extra-chromosomal DNA loop that is circular and double-stranded DNA. On the other hand, the chromosome is a complex and well-organized structure of DNA and proteins. Due to the ease of packaging inside the nucleus, DNA packages tightly with histone proteins in order to make chromosomes. Prokaryotes have a single circular chromosome while eukaryotes have more than one chromosome. Humans have a total of 46 chromosomes in their genome. Thus, this summarizes the difference between plasmid and chromosome.
Reference:
1. “Plasmid.” NeuroImage, Academic Press. Available here
2. “What Is a Chromosome? – Genetics Home Reference – NIH.” U.S. National Library of Medicine, National Institutes of Health. Available here
Image Courtesy:
1.”Plasmid (english)”By Spaully on English wikipedia – Own work, (CC BY-SA 2.5) via Commons Wikimedia
2.”2817314″ by mcmurryjulie (CC0) via pixabay