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The difference between exon and cistron is often being confused as they are closely related terms. Gene is defined as the entire nucleic acid sequence that is essential for the synthesis of a functional gene product (polypeptide or RNA). A gene includes nucleotides encoding an amino acid sequence or a functional RNA, which is known as a coding region. The coding region is part of the gene that will be transcribed and translated into protein, the sum total of its exons. Cistron is the DNA segment that codes for a specific polypeptide in protein synthesis while Exons are the segments of DNA and RNA that contains information coding for a protein or peptide sequence. Let’s discuss the major differences between exon and cistron along with some important questions.
| Table of Content |
Key terms: Cistron, Exon, Splicing, Transcription, Translation, Central dogma, Gene, DNA, RNA, Amino acid, Protein, Protein synthesis
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Central Dogma
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Francis Crick proposed the Central, which states that the genetic information flows from DNA to RNA to Protein.
DNA-Replication Transcription RNA Translation Protein
The process of copying genetic information from one strand of the DNA into RNA is termed transcription, while translation refers to the process of polymerization of amino acids to form a polypeptide.
What is Exon?
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When DNA is copied into messenger RNA, some information from the DNA which is not required for the final product is included, which forms a type of immature RNA. This information needs to be removed to create a functional product. RNA splicing is the process by which additional information is removed, and an mRNA is formed, which is considered mature mRNA.
Exons are the nucleotide sequences that remain in a mature mRNA while Introns are the names to the additional information (gene sequence) regions that are removed (spliced out). The term 'exon' refers to both the DNA sequence within a gene and to the corresponding sequence in mRNA (also known as transcripts). During RNA splicing, introns are removed and exons are covalently joined to one another. An Exon is a portion of a gene that codes for amino acids. The parts of the gene sequence that are expressed in the protein are called exons.
Exons – are coding sequence, Introns – non- coding, spliced gene sequence.
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What is Cistron?
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Cistron is a genetic unit encoding a single polypeptide. S. Benzer in 1957 termed functional gene as cistron. It is an alternative to genes. It is the DNA segment that codes for a polypeptide during protein synthesis.
Cistron is used in combined form like polycistronic and monocistronic. Prokaryotic mRNA is polycistronic i.e., mRNA encodes more than one protein, while eukaryotic mRNA is monocistronic i.e., each mRNA molecule encodes a single protein. This difference between polycistronic and monocistronic mRNAs correlates with a difference in their translation. Cistron is used in combined form like polycistronic and monocistronic.
Difference between Exon and Cistron
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The major differences between exon and cistron are as follows:
| Cistron | Exon |
|---|---|
| DNA segment that directs the synthesis of a peptide sequence. | Coding regions of DNA, i.e., an exon has several cistrons. |
| Discovered by S. Benzer | Discovered by R Roberts |
| Always made of up DNA | processed RNA |
| Consists of both coding and regulatory sequence | Coding Sequence |
| Transcribed to RNA molecule | Translated to Amino Acid (Proteins) |
| Monocistronic and Polycistronic forms | No form |
| Monocistronic – Eukaryotic mRNA Polycistronic - Prokaryotic mRNA | Introns and Exons are formed by Splicing. |
Things to Remember
- Cistron is a genetic unit encoding a single polypeptide.
- An alternative term for gene.
- S. Benzer discovered cistron.
- DNA segment that codes for a polypeptide during protein synthesis.
- Forms like polycistronic and Monocistronic.
- Prokaryotic mRNA -Polycistronic, Eukaryotic mRNA is Monocistronic.
- Exons are the nucleotide sequences that remain in a mature mRNA.
- R. Roberts discovered exons.
- Exon is a portion of a gene that codes for amino acids.
- RNA splicing is the process by which additional information (introns) are removed.
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Sample Questions
Ques. Differentiate between cistron and exon. (4 marks)
Ans.
| Cistron | Exon |
|---|---|
| DNA segment that directs the synthesis of a peptide sequence. | Coding regions of DNA, i.e., an exon has several cistrons. |
| Discovered by S. Benzer | Discovered by R. Roberts |
| Consists of both coding and regulatory sequence | Coding Sequence |
| Transcribed to RNA molecule | Translated to Amino Acid (Proteins) |
Ques. Explain the following: (3 marks)
(a) Polycistrons
(b) mRNA
(c) Exons
Ans. (a) Polycistrons – are mRNA that encodes for more than one protein in Prokaryotes.
(b) mRNA – messenger RNA is a type of RNA molecule carry the genetic information needed to make proteins.
(c) Exons - Exons are the nucleotide sequences that remain in a mature mRNA.
Ques. Write and Explain about Cistrons. (2 marks)
Ans. Cistron is a genetic unit that encodes a single polypeptide during protein synthesis. It was discovered by S. Benzer in 1957. It is an alternative term for genes. Cistron is used in combined forms like polycistronic and Monocistronic. Polycistronic in prokaryotic mRNA which encodes more than one protein, while monocistronic in eukaryotic encodes a single protein.
Ques. Enlist the processes which take place in the central dogma of molecular biology. (2 marks)
Ans. The process which takes place in the central dogma of molecular biology is as follows -
- Replication of DNA
- Transcription of DNA to RNA
- Translation of RNA to Protein.
Central Dogma of Molecular Biology was given by Francis Crick.
Ques. Define a Coding Sequence. (1 mark)
Ans. Coding Sequence or Coding region is a gene that includes nucleotides encoding an amino acid sequence or a functional RNA.
Ques. Mention the enzymes involved in DNA replication. (3 marks)
Ans. Enzymes that are involved in DNA Replication are as follows –
- DNA-dependent DNA polymerase - which catalyzes deoxynucleotides of DNA
- DNA ligase – joins the Okazaki fragments
- Topoisomerase - which removes helical twists by cutting a DNA strand and then resealing the cut.
- Helicases - enzymes that are able to unwind DNA by the use of the energy-equivalent ATP.
Ques. State the criteria on which a molecule must fulfill to act as genetic material. (2 marks)
Ans. The following criteria on which a molecule should fulfill as a genetic material are as follows –
- should be able to generate its replica.
- should be chemically and structurally stable.
- should be able to express itself in the form of Mendelian characters.
- should provide the scope for slow changes, mutations which is necessary for evolution.
Ques. Why both the strands are not copied during transcription? Give reason. (2 marks)
Ans. Both the strands are not copied during transcription is -
- If both the strands code for RNA, two different RNA molecules & two different proteins are formed, which will lead to complications.
- Two RNA molecules produced would be complementary to each other, they would wind together to form ds-RNA double-stranded).
Ques. Where do transcription & translation take place in a prokaryotic cell? Mention the three steps involved in translation? (3 marks)
Ans. Transcription and Translation take place in the cytoplasm of prokaryotic cells.
Steps involved in translation are –
- Activation of Amino Acids
- Binding of Activated Amino Acids with tRNA
- Initiation of Polypeptide Chain
- Elongation of Polypeptide Chain
Ques. Describe the termination of a polypeptide chain. (2 marks)
Ans. Termination of polypeptide synthesis -
When one of the termination codons (UAA, UAG, UGA) comes at the A-site, it does not code for any amino acid and there is no tRNA molecule for it, which leads to the stopping of polypeptide synthesis or elongation of the polypeptide. Polypeptide synthesized is released from the ribosome, by the catalysis action of the release factor.
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