Difference Between Gene and Cistron

The main difference between gene and cistron is that a gene is a nucleotide sequence responsible for the synthesis of an RNA molecule whereas a cistron is a nucleotide sequence responsible for the synthesis of a polypeptide sequence of a functional protein. Furthermore, a gene consists of both coding sequence and regulatory sequences while a cistron consists only of a coding sequence.

Gene and cistron are two structural units of the genome of a particular organism, which are mainly involved in the protein synthesis.

Key Areas Covered

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

Key Terms

Cistron, Gene, Monocistronic, Polycistronic, Polypeptide

What is a Gene

A gene is a region (locus) of a chromosome that encodes a particular protein. It is considered as the molecular unit of heredity based on studies on the inheritance of characteristics by Gregor Mendel in 1860s. The human genome consists of more than 20,000 genes. The main function of a gene is to produce a functional protein in a process known as protein synthesis. First, the gene is transcribed into an mRNA and the mRNA is decoded in order to synthesize an amino acid sequence of a functional protein. Some genes encode other RNA molecules, especially tRNA and rRNA. These genes are called RNA genes.

Figure 1: Gene Structure

The two components of a gene are the coding sequence and the regulatory sequence. Coding sequence consists of the codon sequence, which encodes the sequence of amino acids in a functional protein. The presence of introns interrupts the coding sequence of eukaryotic genes. Due to the presence of large introns, eukaryotic genes are larger than prokaryotic genes. The regulatory sequence of the gene contains promoter region, enhancers, and inhibitors. The main function of the regulatory sequence is to regulate the initiation of gene expression.

What is a Cistron

A cistron is the nucleotide sequence that carries the information required by the production of the polypeptide sequence of a protein. Therefore, it resembles the coding sequence of a gene, which codes for a single protein. The term, cistron was proposed by Seymour Benzer in 1957 during his studies on the fine structure of a genetic region in bacteriophage. His approach is called the cis-trans test. A gene is called a cistron during the cis-trans test.

Figure 2: Polycistron

A group of functionally-related genes in prokaryotes forms an operon, which consists of multiple protein-coding sequences transcribed together. Only one promoter is responsible for the initiation of the transcription of an operon. Therefore, a single mRNA molecule is produced called polycistronic mRNA that can synthesize several, functionally-related proteins. However, eukaryotic mRNA consists of a single protein coding region. Hence, it is monocistronic.   

Similarities Between Gene and Cistron

• Gene and cistron are two types of structural units found in the genome of both prokaryotic and eukaryotic organisms.
• Both help in the protein synthesis.
• Eukaryotes are monocistronic while prokaryotes are polycistronic.

Difference Between Gene and Cistron

Definition

A gene refers to a distinct sequence of nucleotides forming part of a chromosome, the order of which determines the order of monomers in a polypeptide or nucleic acid molecule which a cell may synthesize while a cistron refers to a section of a DNA or RNA molecule that codes for a specific polypeptide in protein synthesis.

Made up of

A gene is always made up of DNA while a cistron can be made up of either DNA or RNA.

Type of Sequence

A gene consists of both coding and regulatory sequences while a cistron consists of only the coding sequence.

Function

A gene is transcribed into an RNA molecule while a cistron is transcribed and/or translated into a polypeptide sequence of a gene.

Number of Proteins

A gene may be encoded for several proteins while a cistron can produce a single protein.

Conclusion

A gene is a part of the chromosome responsible for the synthesis of a functional protein. It consists of both coding and regulatory sequences. The coding sequence is the nucleotide sequence that decodes into a polypeptide sequence. Hence, it is called a cistron. As prokaryotes have operons, they are polycistronic while eukaryotes are monocistronic. The main difference between gene and cistron is the role of each structural unit during protein synthesis.

Reference:

1. Lodish, Harvey. “Molecular Definition of a Gene.” Advances in Pediatrics., U.S. National Library of Medicine, 1 Jan. 1970, Available Here

Image Courtesy:

1. “Gene structure 2 annotated” By Thomas Shafee – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Gene structure prokaryote 2 annotated” By Thomas Shafee – Shafee T, Lowe R (2017). “Eukaryotic and prokaryotic gene structure”. WikiJournal of Medicine 4 (1). DOI:10.15347/wjm/2017.002. ISSN 20024436. (CC BY 4.0) via Commons Wikimedia