A gene is a specific segment of DNA that carries instructions for making a particular protein or functional RNA. Understanding gene structure is essential for learning how traits are inherited, how cells function, and how genetic diseases develop. Although genes vary in size and complexity, most follow a common structural pattern consisting of coding and non-coding regions.
Basic Components of a Gene
1. Promoter Region
The promoter is a DNA sequence located at the beginning of the gene. It does not code for proteins but acts as a control switch.
Functions
- Starts transcription
- Helps RNA polymerase bind to DNA
- Controls how much mRNA is made
Key promoter elements include the TATA box and CAAT box.
2. Transcription Start Site (TSS)
This is the point where RNA polymerase begins copying the DNA into RNA. It is usually labeled as the +1 position.
3. Exons
Exons are the coding regions of a gene. They contain information that directly contributes to the final protein structure.
Functions
- Carry codons for amino acids
- Join together after RNA splicing
4. Introns
Introns are non-coding sequences found between exons. They are removed during RNA processing.
Importance
- Allow alternative splicing
- Help regulate gene expression
5. Untranslated Regions (UTRs)
These are non-coding parts at the 5′ and 3′ ends of mRNA.
5′ UTR
- Helps ribosomes bind for translation
- Influences mRNA stability
3′ UTR
- Controls mRNA lifespan
- May contain poly-A tail signals
6. Coding Sequence (CDS)
This sequence contains the actual genetic code for protein formation. It starts with the start codon (AUG) and ends with a stop codon (UAA, UAG, or UGA).
7. Terminator Region
The terminator is a DNA sequence that signals the end of transcription. Once RNA polymerase reaches the terminator, it stops copying the DNA.
Regulatory Elements in Genes
1. Enhancers
Enhancers are DNA sequences that boost gene expression. They can be located far away from the gene they regulate.
Functions
- Increase transcription rate
- Interact with transcription factors
2. Silencers
Silencers are opposite to enhancers. They decrease the rate of transcription.
3. Insulators
These help define boundaries between genes and prevent enhancer interference.
Organization of Genes in DNA
In humans, genes are arranged along chromosomes. They are separated by large non-coding regions known as intergenic DNA. Only a small portion of the human genome (less than 2%) actually codes for proteins.
Gene Variability
Different forms of the same gene are called alleles. They produce variations in characteristics such as eye color or blood type.
Mutations—changes in DNA sequence—can occur in any part of the gene. Depending on the location, mutations can:
- Alter protein structure
- Affect gene regulation
- Cause genetic disorders
Detailed Notes:
For PDF style full-color notes, open the complete study material below:
