Define Gene in Biology | Structure and Functions of Gene | Genetics

Define Gene in Biology

A gene is a fundamental unit of heredity in all living organisms. It is a segment of DNA (or RNA in some viruses) that contains the necessary information to produce a specific protein or functional RNA molecule. Genes determine the physical and functional traits of an organism, passing genetic instructions from one generation to the next.

Structure of Gene

A gene consists of several distinct regions, each playing a specific role in transcription, translation, and regulation of gene expression. These regions can be broadly categorized into

1. Regulatory Regions (Control Elements)
2. Coding Region (Exons and Introns)
3. Termination Region

1. Regulatory Regions (Control Elements)

Regulatory sequences control when, where, and how much a gene is expressed. These regions are often located upstream (before) or downstream (after) the coding sequence.

a. Promoter

The promoter is a DNA sequence located at the beginning of a gene that serves as the binding site for RNA polymerase and transcription factors. It determines the efficiency and rate of transcription. The key promoter elements include:

• TATA Box: A conserved sequence (TATAAA) found in eukaryotes that helps position RNA polymerase.
• GC Box: Rich in guanine (G) and cytosine (C) and involved in regulating transcription.
• CAAT Box: Enhances transcription efficiency.

Prokaryotic promoters have two conserved regions:

• -35 Region: It is recognized by RNA polymerase (TTGACA sequence).
• -10 Region (Pribnow Box): It helps in transcription initiation (TATAAT sequence).

b. Enhancers and Silencers

Enhancers are DNA sequences that increase transcription by providing binding sites for activator proteins. While silencers are DNA sequences that repress transcription by binding to repressor proteins. These sequences can be located upstream, downstream, or even within the gene.

c. Operators

They are found mainly in prokaryotic genes, operators are sequences where repressors bind to block RNA polymerase, thereby inhibiting transcription. Example includes Lac Operon in E. coli.

2. Coding Region (Exons and Introns)

The coding region is the part of the gene that contains the actual instructions for protein synthesis. It consists of exons and introns.

a. Exons

Exons are sequences that contain the actual genetic code used to synthesize proteins. They are expressed and remain in the final mature mRNA after RNA splicing. The sequence of exons determines the amino acid sequence of the protein.

b. Introns

Introns are non-coding sequences present within a gene that are removed during RNA processing (splicing). They are found only in eukaryotic genes. Though they do not encode proteins, they may have regulatory roles and influence alternative splicing.

3. Termination Region

This region signals the end of transcription and ensures proper termination of RNA synthesis.

a. Terminator Sequence

A DNA sequence that marks the end of a gene. In prokaryotes, termination occurs by:

• Intrinsic Termination: A hairpin loop forms in the RNA, causing RNA polymerase to detach.
• Rho-Dependent Termination: The Rho protein helps stop transcription.

In eukaryotes, transcription terminates after the addition of a polyadenylation signal (AAUAAA).

Additional Elements in Gene Structure

Besides the core elements, some additional structures help in gene expression and regulation:

1. 5' Untranslated Region (5' UTR)

A sequence located before the start codon (AUG). It regulates translation efficiency and stability of mRNA. It contains ribosome binding sites (Shine-Dalgarno sequence in prokaryotes; Kozak sequence in eukaryotes).

2. Start Codon

The first codon in an mRNA sequence that signals the beginning of translation. AUG (Methionine) is the universal start codon.

3. 3' Untranslated Region (3' UTR)

A sequence after the stop codon that helps in mRNA stability and regulation. It contains signals for polyadenylation (Poly-A tail addition) in eukaryotes.

4. Stop Codon

It signals the termination of translation. Three stop codons exist: UAA, UAG, UGA.

5. Poly-A Tail (Eukaryotes Only)

It is a string of adenine nucleotides added to the 3' end of mRNA. Protects mRNA from degradation and helps in nuclear export.

Functions of Gene

Genes have various essential functions. Some of them are listed below.

1. Protein Synthesis: They provide instructions to synthesize proteins via transcription (DNA to RNA) and translation (RNA to protein).
2. Regulation of Biological Processes: Genes control growth, development, metabolism, and other physiological functions.
3. Heredity: They carry genetic information from parents to offspring.
4. Mutation and Evolution: Mutations in genes can lead to genetic variations, contributing to evolution and species diversity.

Types of Gene

Genes can be classified based on their function and location. Different types of genes are:

1. Structural Genes: It encode proteins that form cellular structures and enzymes.
2. Regulatory Genes: It control gene expression by regulating transcription factors.
3. Housekeeping Genes: It is essential for maintaining basic cellular functions and are expressed in all cells.
4. Developmental Genes: It direct the formation and differentiation of tissues and organs.
5. Oncogenes and Tumor Suppressor Genes: It is involved in cell cycle control and cancer development.

Mutations and Genetic Disorders

Changes in a gene's DNA sequence (mutations) can lead to genetic disorders or contribute to evolution. Different types of mutations include:

1. Point Mutations: Single nucleotide changes.
2. Insertions/Deletions: Addition or removal of nucleotides.
3. Chromosomal Mutations: Large-scale alterations affecting multiple genes.

Examples of genetic disorders caused by mutations include:

1. Cystic Fibrosis (mutation in the CFTR gene)
2. Sickle Cell Anemia (mutation in the HBB gene)
3. Huntington’s Disease (expansion of CAG repeats in the HTT gene)

Summary

Genes are the fundamental units of heredity that carry the instructions for life. They encode proteins and functional RNA molecules, playing a crucial role in determining traits, regulating biological processes, and ensuring proper growth and development. Located on chromosomes, genes are passed from one generation to the next, influencing both inherited characteristics and evolutionary changes. 

Understanding genes has led to significant advancements in medicine, biotechnology, and genetic research, helping diagnose genetic disorders and develop innovative treatments. As scientific knowledge progresses, the study of genes continues to unlock new possibilities for improving human health and understanding life at a molecular level.

Some Short Questions and Answers

1. What is a gene in biology?

A. A gene is a segment of DNA that contains the instructions for making proteins or functional RNA molecules, playing a key role in heredity and biological functions.

2. What is the function of a gene?

A. Genes control the production of proteins, regulate biological processes, influence traits, and are responsible for passing genetic information from parents to offspring.

3. Where are genes located?

A. Genes are located on chromosomes inside the nucleus of a cell, and in prokaryotes, they are found in the cytoplasm within a circular DNA molecule.

4. What are the main components of a gene?

A. A gene consists of regulatory regions (promoters, enhancers), coding regions (exons and introns), and termination sequences that help control gene expression and protein synthesis.

5. How do genes influence traits?

A. Genes determine traits by coding for proteins that affect physical characteristics, metabolism, and even behavior. Traits can be inherited based on dominant and recessive alleles.