What is the Difference Between Prokaryotic and Eukaryotic Chromosomes

The main difference between prokaryotic and eukaryotic chromosomes is that the prokaryotic chromosomes are short, circular DNA molecules whereas the eukaryotic chromosomes are long, linear molecules. Furthermore, prokaryotic chromosomes occur in the cytoplasm while eukaryotic chromosomes occur inside the nucleus. Moreover, prokaryotes contain a single chromosome per cell while the number of chromosomes in eukaryotes depends on the species. 

Prokaryotic and eukaryotic chromosomes are two organizations of cellular DNA for packing inside the cell. Double-stranded DNA makes up both types of chromosomes.  

Key Areas Covered 

1. What are Prokaryotic Chromosomes
     – Definition, Structure, Role
2. What are Eukaryotic Chromosomes
     – Definition, Structure, Role
3. What are the Similarities Between Prokaryotic and Eukaryotic Chromosomes
     – Outline of Common Features
4. What is the Difference Between Prokaryotic and Eukaryotic Chromosomes
     – Comparison of Key Differences

Key Terms 

Chromosome Structure, Eukaryotic Chromosomes, Packaging, Prokaryotic Chromosomes, Quantity, Replication

What are Prokaryotic Chromosomes  

Prokaryotic chromosomes are the genetic material of prokaryotes. They make up the total DNA pool in prokaryotes along with the plasmid DNA. Also, only a single chromosome occur per cell in prokaryotes. It is a covalently closed circular DNA (cccDNA) molecule. Moreover, the size of a prokaryotic chromosome can vary from 0.16-12.2 million base pairs. In addition, this DNA molecule is not enclosed by nuclear membranes, and the nucleoid is its cytoplasmic location. Furthermore, the nucleoid-associated proteins help prokaryotic chromosomes to form looped structures for packing. 

Figure 1: Prokaryotic Chromosome during Cell Division

The genes of the prokaryotic chromosomes occur in units called operons. Each operon contains functionally-related genes expressed together. Besides, DNA replication in prokaryotes is relatively a simple process, which occurs at the beginning of the cell division. Here, since the circular chromosome has a single origin of replication, it forms a single replication fork and bubble. Also, the speed of prokaryotic DNA replication is 2000 base pairs for a second. 

What are Eukaryotic Chromosomes  

Eukaryotic chromosomes are the genetic material of eukaryotes. These chromosomes are completely enclosed by two nuclear membranes, forming an organelle called the nucleus. Also, a single cell comprises 10-50 chromosomes. As an example, the human genome consists of 46 chromosomes. And, these chromosomes are linear molecules. Moreover, the size of the human genome is around 3 billion base pairs. And, the center of the chromosome is the centromere while the two ends are telomeres. Furthermore, histones are the type of proteins associated with eukaryotic DNA for dense packing by forming tight coils.  

Figure 2: Eukaryotic Chromosome Structure

Eukaryotic DNA replication is a complex process, which makes use of multiple protein subunits. Each eukaryotic chromosome contains multiple origins of replication; hence, multiple replication forks and bubbles are formed per chromosome. Moreover, the process is slow and its rate is only 100 base pairs per second.  

Similarities Between Prokaryotic and Eukaryotic Chromosomes 

• Prokaryotic and eukaryotic chromosomes are two arrangements of DNA inside the cell. 
• Both the structural organization and the chemical composition of DNA are similar in both types of chromosomes.  
• Here, negatively charged DNA interacts with various positively charged proteins, which help in the packing of DNA. 
• Also, they allow gene expression, which occurs through transcription and translation of genetic information in chromosomes.  
• Furthermore, both types of chromosomal DNA undergo DNA methylation for the structural regulation of gene expression.  

Difference Between Prokaryotic and Eukaryotic Chromosomes 

Definition 

Prokaryotic chromosomes refer to the double-stranded circular DNA in prokaryotes while eukaryotic chromosomes refer to the long strands of DNA containing genetic information in eukaryotes.

Number 

A single chromosome makes up prokaryotic chromosomes while several chromosomes make up eukaryotic chromosomes. This is one difference between prokaryotic and eukaryotic chromosomes.

Occurrence 

Another difference between prokaryotic and eukaryotic chromosomes is that the prokaryotic chromosomes occur in the nucleoid while the eukaryotic chromosomes occur inside the nucleus.  

Homologous Chromosomes 

Moreover, prokaryotic genomes are haploid; hence, no homologous chromosomes occur in them while eukaryotic chromosomes often have a pairing homologous chromosome. 

Attachment to the Cytoplasm 

One more difference between prokaryotic and eukaryotic chromosomes is that the prokaryotic chromosomes are attached to the cytoplasm from the inside while eukaryotic chromosomes have no contact with the cytoplasm.  

General Structure 

Prokaryotic chromosomes are short, circular DNA molecules while eukaryotic chromosomes are long, linear DNA molecules. The main difference between prokaryotic and eukaryotic chromosomes is this structure.

Telomeres/Centromeres 

Furthermore, prokaryotic chromosomes lack telomeres and centromeres while eukaryotic chromosomes have telomeres and centromeres.    

Association with Proteins 

Also, prokaryotic chromosomes associate with nucleoid-associated proteins, which help them for packing while eukaryotic chromosomes form special structures called nucleosomes by associating with histones.  

DNA Replication 

DNA replication is another difference between prokaryotic and eukaryotic chromosomes. Prokaryotic DNA replication occurs at the beginning of the cell division while eukaryotic DNA replication occurs during the S phase of the cell cycle. 

Origin of Replication 

Besides, prokaryotic chromosomes contain a single origin of replication per chromosome while eukaryotic chromosomes contain several origins of replication per chromosome. 

Gene Structure 

Another difference between prokaryotic and eukaryotic chromosomes is that prokaryotic genes are organized into operons while eukaryotic genes are regulated as individual structures. 

Number of Proteins 

Prokaryotic chromosomes encode for a few proteins while eukaryotic chromosomes encode for a large number of proteins.  

Genetic Recombination 

Moreover, prokaryotic chromosomes achieve genetic recombination through horizontal gene transfer while eukaryotic chromosomes achieve genetic recombination through meiosis and fusion of gametes. 

Conclusion 

Prokaryotic chromosomes are short, circular DNA molecules. They possess a single origin of replication per chromosome. On the other hand, eukaryotic chromosomes are long, linear DNA molecules. They are densely packed with histone proteins. Each eukaryotic chromosome contains multiple origins of replication. The main difference between prokaryotic and eukaryotic chromosomes is the structure, quantity, packaging, and replication.

Reference:

1. Kuzminov, A., “The precarious prokaryotic chromosome” Journal of bacteriology vol. 196,10 (2014): 1793-806. Available Here

Image Courtesy:

1. “Figure 10 05 01” By CNX OpenStax –  (CC BY 4.0) via Commons Wikimedia  
2. “Figure 10 01 03” By CNX OpenStax –  (CC BY 4.0) via Commons Wikimedia  

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