Basic Processes in Biotechnology for Class 11 NCERT Students

Understanding Mutations: Types and Causes

Mutations are permanent changes in the DNA sequence that can affect how genes function. They can occur spontaneously due to errors during DNA replication or be induced by external mutagens like chemicals and radiation.

Types of Mutations

• Point mutations: Change in a single nucleotide base. Includes substitution, insertion, and deletion.
• Frameshift mutations: Insertions or deletions that shift the reading frame of the gene.
• Chromosomal mutations: Large-scale changes affecting chromosome structure or number.

Example: Base Analog Mutation

5-bromouracil (5-BU) is a base analog that can substitute for thymine. When incorporated into DNA, it can pair incorrectly with guanine instead of adenine, causing an A=T to G≡C substitution over replication cycles.

This example highlights how chemical mutagens can cause base substitutions, leading to genetic variation or diseases.

DNA Repair Mechanisms: Protecting Genetic Integrity

Cells have evolved several DNA repair pathways to correct mutations and maintain genome stability. These mechanisms detect and fix damaged or mismatched DNA bases.

Major DNA Repair Pathways

• Base Excision Repair (BER): Removes damaged bases by DNA glycosylase creating an AP site, followed by excision and gap filling.
• Nucleotide Excision Repair (NER): Excises bulky helix-distorting lesions like thymine dimers caused by UV radiation.
• Mismatch Repair (MMR): Corrects base mismatches after DNA replication using MutS, MutL, and MutH proteins.
• Direct Repair: Reverses specific base modifications without excision.

These repair systems prevent mutations from accumulating, reducing risks of genetic diseases and cancer.

Gene Expression: From DNA to Functional Proteins

Gene expression is the process by which information in a gene is used to synthesize functional products like proteins. It determines the phenotype and controls cellular activities.

Steps of Gene Expression

1. Transcription: DNA sequence of a gene is copied into messenger RNA (mRNA). 2. RNA Processing (in eukaryotes): mRNA undergoes capping, splicing, and polyadenylation. 3. Translation: mRNA is decoded by ribosomes to build a protein.

Gene expression is tightly regulated to ensure proteins are produced only when needed. This regulation can occur at transcriptional, post-transcriptional, translational, or post-translational levels.

Regulation of Gene Expression in Prokaryotes: The lac Operon Model

In prokaryotes like E. coli, gene expression is regulated to conserve energy. The lac operon is a classic example controlling lactose metabolism.

Components of lac Operon

• Structural genes: lacZ, lacY, lacA coding for β-galactosidase, permease, and transacetylase.
• Promoter: RNA polymerase binding site.
• Operator: Repressor binding site.
• Regulatory gene: Produces repressor protein.

Working Mechanism

• Without lactose, the repressor binds to the operator, blocking transcription.
• When lactose is present, it binds the repressor, causing it to detach.
• RNA polymerase transcribes structural genes, enabling lactose metabolism.

This inducible system exemplifies how cells regulate gene expression based on environmental signals.

Proteins Essential for DNA Replication and Their Roles

DNA replication is a complex process requiring many proteins to ensure accurate copying of the genome.

Key Proteins Involved

• DNA Helicase: Unwinds the double helix.
• Single-Strand Binding Proteins (SSB): Stabilize unwound strands.
• Primase: Synthesizes RNA primers.
• DNA Polymerase: Adds nucleotides to synthesize new DNA strands.
• DNA Ligase: Seals nicks between Okazaki fragments.

Effect of Protein Loss

Without these proteins, replication cannot proceed. For example, loss of DNA polymerase halts nucleotide addition, while absence of helicase prevents strand separation. This leads to replication failure and cell death.

Understanding these proteins is crucial for grasping how cells maintain genetic information.

DNA Damage by UV Radiation and Cellular Repair

Ultraviolet (UV) rays damage DNA primarily by inducing thymine dimers—covalent bonds between adjacent thymine bases. This distorts the DNA helix and blocks replication and transcription.

Molecular Basis of UV-Induced Mutation

• Thymine dimers cause errors during replication, leading to mutations.

Repair Mechanism: Nucleotide Excision Repair (NER)

• Damage recognition by UvrA and UvrB proteins.
• UvrC cuts damaged strand on both sides of lesion.
• DNA polymerase fills the gap.
• DNA ligase seals the nick.

This repair restores DNA integrity and prevents mutation accumulation, highlighting the importance of cellular defense against environmental damage.