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What is a Gene?
A gene is the functional unit of inheritance. It carries the instructions for a specific trait or function.
Gene Location
There is no ambiguity that genes are located on DNA. However, defining a gene based on DNA sequence alone can be complex.
RNA-Coding Genes
DNA segments coding for tRNA or rRNA are also considered genes, even though they don’t code for proteins.
What is a Cistron?
A cistron is a DNA segment that codes for one polypeptide chain. It helps specify structural genes.
Monocistronic vs Polycistronic
Monocistronic genes (in eukaryotes) code for one polypeptide. Polycistronic genes (in prokaryotes) code for multiple polypeptides from a single mRNA.
Split Genes in Eukaryotes
In eukaryotes, genes are split into coding and non-coding regions. This complicates gene definition.
Exons
Exons are the expressed sequences that appear in the mature RNA after processing.
Introns
Introns are intervening sequences that are removed during RNA processing. They do not appear in mature RNA.
Split Gene Complication
This split-gene arrangement makes defining a gene purely in DNA terms more complex, especially in eukaryotes.
Role of Promoters & Regulatory Sequences
Inheritance is also influenced by promoters and regulatory sequences, which control gene expression.
Regulatory Genes
Although they don’t code for proteins or RNAs, regulatory sequences are sometimes loosely called genes due to their critical roles.
💡 Application of Gene Concept
Understanding genes helps in gene therapy, genetic engineering, crop improvement, and diagnosing genetic disorders.
🧬 Gene vs Cistron
Gene: Any functional DNA unit that can code RNA/protein.
Cistron: Specifically codes one polypeptide.
➡ All cistrons are genes, but not all genes are cistrons.
What is a Gene?
A gene is defined as the functional unit of inheritance. While genes are located on DNA, precisely defining a gene in terms of a DNA sequence can be complex.
Gene vs. Cistron
A gene can refer to DNA coding for tRNA or rRNA. A cistron, however, is specifically a segment of DNA that codes for a polypeptide.
Monocistronic vs. Polycistronic
Structural genes in a transcription unit can be monocistronic (one polypeptide per unit, common in eukaryotes) or polycistronic (multiple polypeptides, common in prokaryotes).
Split Genes (Eukaryotes)
Eukaryotic structural genes often have interrupted coding sequences. Exons are the expressed sequences appearing in mature RNA, while introns are non-coding intervening sequences that are removed.
Regulatory Sequences
Inheritance is also influenced by promoter and regulatory sequences. These are sometimes loosely called “regulatory genes” even though they don’t code for any RNA or protein.
🧬 What is a Gene?
A gene is a **functional DNA unit** that may code for proteins, RNAs, or even regulate expression. It’s a **broad, loosely used term**.
🧪 Protein-Coding Gene
If a DNA sequence codes for a protein, it’s a gene. Most structural genes fall in this category.
🔁 RNA-Coding Gene
Genes that form tRNA or rRNA — no proteins involved — are still genes, showing the diversity of function.
⚙️ Regulatory Gene Sequences
Even non-coding promoters and enhancers may be loosely called genes due to their importance in controlling inheritance.
🔍 Why So Confusing?
Because gene includes so many roles, it’s hard to define gene by DNA sequence alone. That’s why molecular biologists prefer more specific terms.
🧩 What is a Cistron?
A cistron is a **precisely defined DNA segment** that codes for **one complete polypeptide** — clear and unambiguous.
🧫 Mono vs Poly-Cistronic
In eukaryotes, genes are mostly monocistronic. In bacteria, they’re often polycistronic, coding for many proteins in one mRNA.
🧩 Exons and Introns
Exons are expressed. Introns are removed. Eukaryotic genes are split genes, adding complexity.
🚫 Gene ≠ Always Protein
Not every gene makes a protein. This is why relying solely on the word “gene” can mislead in molecular contexts.
⚖️ Why Prefer “Cistron”?
Cistron is unambiguous — always refers to coding for one polypeptide. When clarity is key, “cistron” beats “gene”.
🎯 Application of Genes
Gene knowledge powers gene editing (CRISPR), therapy, GM crops, and personalized medicine.
🧠 Gene vs Cistron (Final View)
Gene: Broad; may code RNA, protein, or regulate.
Cistron: Precise; codes exactly **one polypeptide**.
➡️ Use “cistron” when clarity matters.