Genetics and Recombinant DNA Technology in Microbiology

Genetics: Flow of Genetic Information

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information in cells: DNA → RNA → Protein. Understanding this flow is essential for grasping how genetic information is stored, expressed, and regulated in all living organisms.

• DNA: Stores genetic instructions.

• RNA: Carries a copy of the instructions from DNA.

• Proteins: Perform cellular functions as the final products of gene expression.

Core Definitions

• Gene: A region of DNA that codes for a protein or functional RNA.

• Genome: The complete set of DNA in a cell, including coding and non-coding regions.

• Phenotype: Observable characteristics resulting from gene expression.

• Genotype: The actual DNA sequence of an organism.

Prokaryotic vs. Eukaryotic DNA

Structural Differences

• Prokaryotes (e.g., bacteria):

  • Circular DNA, no nucleus, DNA in cytoplasm.

  • Often contain plasmids (extra circular DNA).

• Eukaryotes (plants, animals, fungi):

  • Linear DNA, enclosed in a nucleus.

  • DNA wrapped around histone proteins to form chromatin.

Impact: These differences affect DNA replication and gene regulation mechanisms.

DNA Packaging and Gene Control

• Open chromatin: Loosely packed, genes accessible and active.

• Closed chromatin: Tightly packed, genes inaccessible and inactive.

• This packaging is an early level of gene regulation in eukaryotes.

DNA Structure and Components

• Each nucleotide contains:

  • Phosphate group (PO4)

  • 5-carbon sugar (deoxyribose in DNA)

  • Nitrogenous base (A, T, G, C)

• Backbone: Sugar-phosphate chain.

• Base pairing:

  • A pairs with T (2 hydrogen bonds)

  • G pairs with C (3 hydrogen bonds)

  • More G-C pairs = stronger DNA stability.

DNA Replication

Mechanism and Enzymes

DNA replication is semi-conservative: each new DNA molecule contains one old and one new strand.

• Replication proceeds in the 5' to 3' direction only.

• Leading strand: Synthesized continuously toward the replication fork.

• Lagging strand: Synthesized discontinuously in short fragments (Okazaki fragments) away from the fork.

• Fragments are joined by DNA ligase.

Key enzymes:

• Helicase: Unzips DNA.

• Topoisomerase: Relieves supercoiling stress.

• Primase: Synthesizes RNA primers.

• DNA Polymerase III: Main enzyme for new DNA synthesis.

• DNA Polymerase I: Replaces RNA primers with DNA.

• DNA ligase: Seals nicks between fragments.

Energy source: Phosphate bonds of incoming nucleotides.

DNA Methylation

• Prokaryotes: Methylate adenine.

• Eukaryotes: Methylate cytosine.

• Functions: Distinguish self from foreign DNA, regulate gene expression, protect against restriction enzymes.

Transcription (DNA to RNA)

Process and Enzymes

• RNA polymerase synthesizes RNA from a DNA template.

• No primer required; unwinds DNA itself.

• Copies only one DNA strand; uses uracil (U) instead of thymine (T).

• Types of RNA:

  • mRNA: Messenger RNA, carries genetic code.

  • rRNA: Ribosomal RNA, structural component of ribosomes.

  • tRNA: Transfer RNA, brings amino acids during translation.

  • RNA primer: Used in DNA replication.

• Location:

  • Prokaryotes: Cytoplasm

  • Eukaryotes: Nucleus

Eukaryotic mRNA Processing

• Pre-mRNA contains introns (non-coding) and exons (coding).

• Spliceosome removes introns and joins exons.

• Only processed mRNA exits the nucleus for translation.

Translation (RNA to Protein)

Mechanism

• Translation converts mRNA codons into an amino acid sequence.

• Codon: Three-nucleotide sequence coding for one amino acid.

• Start codon: AUG (methionine).

• Stop codons: Signal end of translation.

• Ribosome sites:

  • A site: Accepts incoming tRNA.

  • P site: Holds growing polypeptide chain.

  • E site: Exit site for empty tRNA.

• tRNA: Matches anticodon to codon and delivers amino acids.

• Energy for translation comes from GTP.

Control of Translation

• miRNA: Binds mRNA to prevent translation.

• Riboswitch: mRNA region that changes shape to regulate gene expression in response to environmental signals.

Gene Regulation

• Not all genes are expressed at all times.

• Constitutive genes: Always active (e.g., ribosomal proteins, glycolysis enzymes).

• Other genes: Regulated based on environmental conditions.

Operons (Prokaryotes Only)

• Operon: Group of genes regulated together under one promoter.

• Example: Lac operon (lactose operon)

  • Usually off; turns on when lactose is present.

  • Contains promoter, operator, and structural genes.

Mutation

Types and Effects

• Mutation: Permanent change in DNA sequence.

• Types:

  • Base substitution

  • Insertion

  • Deletion

  • Frameshift

  • Inversion

  • Duplication

  • Transposition

• Effects:

  • Silent: No amino acid change.

  • Missense: One amino acid changes.

  • Frameshift: Major disruption of protein.

• Mutations can be harmful, neutral, or rarely beneficial.

Mutagens

• Radiation:

  • Gamma rays: Break DNA backbone, cause double-strand breaks.

  • X-rays: Cause strand breaks, base changes, and reactive molecules.

  • UV: Causes thymine dimers, leading to DNA distortion and increased mutation risk.

• Chemicals:

  • Benzo[a]pyrene: Causes frameshifts.

  • Aflatoxin: Causes point mutations.

DNA Repair Mechanisms

• Mismatch Repair: Corrects replication errors using methylation to distinguish old and new strands.

• Base Excision Repair: Removes and replaces damaged bases.

• Pyrimidine Dimer Repair:

  • Light Repair (Photoreactivation): DNA photolyase breaks thymine dimers using visible light.

  • Dark Repair (Nucleotide Excision Repair): Damaged segment is excised and replaced.

• SOS Repair: Emergency, error-prone repair system activated under severe DNA damage.

Horizontal Gene Transfer (Prokaryotes)

Mechanisms

• Transformation: Uptake of naked DNA from the environment (e.g., Griffith's experiment with Streptococcus pneumoniae).

• Transduction: Transfer of DNA via bacteriophages (viruses).

  • Generalized: Random DNA transferred.

  • Specialized: Specific genes transferred.

• Conjugation: Direct transfer of DNA through cell-to-cell contact using pili and F plasmid.

Transposons

• Segments of DNA that can move within the genome ("jumping genes").

• Found in both prokaryotes and eukaryotes.

• Can carry antibiotic resistance genes and increase genetic variation.

Recombinant DNA Technology (Genetic Engineering)

Definition and Goals

Recombinant DNA technology uses enzymes and natural DNA processes to manipulate genes for industrial, medical, and agricultural purposes.

• Eliminate unwanted traits (e.g., disease genes).

• Combine genetic traits (e.g., engineered research animals).

• Create organisms that produce useful products (e.g., vaccines, hormones).

Tools of Recombinant DNA Technology

• Polymerase Chain Reaction (PCR): Amplifies specific DNA sequences in vitro.

  • Applications: DNA sequencing, pathogen detection, genetic mapping.

  • Example: Identification of SARS coronavirus in 2003.

• Restriction Enzymes: Cut DNA at specific palindromic sequences.

  • Natural role: Bacterial defense against viruses.

  • Bacterial DNA is protected by methylation.

  • Named after source organism (e.g., HindIII from Haemophilus influenzae).

• Reverse Transcriptase: Synthesizes DNA from RNA templates (cDNA synthesis).

• Synthetic Nucleic Acids: Chemically synthesized DNA for gene design and probes.

• Vectors: DNA carriers (plasmids, viruses) for gene transfer; must replicate in host and often contain selectable markers.

Applications

• Protein Synthesis: Bacteria/yeast produce proteins (e.g., insulin, interferon, growth hormones).

• Vaccines: Production of antigens for safe vaccines (e.g., recombinant hepatitis B vaccine).

• Genetic Screening: Early detection of pathogens or inherited conditions.

• DNA Fingerprinting: Identification based on unique DNA patterns (forensics, paternity testing).

• Gene Therapy: Replacing defective genes; challenges include immune response and delivery.

• Medical Diagnosis: PCR-based detection of pathogens (e.g., HIV, hepatitis viruses).

• Agricultural Applications: Engineering crops for herbicide, salt, freeze, pest resistance, and improved nutrition.

Plasmids

Definition and Types

• Small, circular DNA molecules independent of the bacterial chromosome.

• Replicate independently; carry genes for specific traits.

• Not essential for survival but provide advantages.

Type of Plasmid	Main Function	Example
Resistance (R) Plasmid	Antibiotic/heavy metal resistance	E. coli with ampicillin resistance
Bacteriocin Plasmid	Production of protein toxins (bacteriocins)	Gut bacteria competition
Fertility (F) Plasmid	Conjugation and pili formation	F+ cells transfer DNA
Virulence Plasmid	Pathogenicity factors (toxins, adhesion)	E. coli causing diarrhea

Positive Selection

Principle and Application

• Method to identify mutants by allowing only those with a specific trait to survive.

• Example: Penicillin resistance selection.

• Mutagens can increase the number of resistant mutants, indicating increased mutation rates.

Summary Table: DNA Polymerase vs. RNA Polymerase

Feature	DNA Polymerase	RNA Polymerase
Primer Required?	Yes	No
Template Copied	Entire DNA	One gene
Nucleotides Used	Deoxyribonucleotides	Ribonucleotides
Accuracy	High	Lower

Big Picture Understanding

• Genetics involves storing, copying, expressing, regulating, and changing information.

• Recombinant DNA technology manipulates these processes for practical applications.

• Plasmids, mutations, and gene transfer contribute to genetic diversity and adaptation.

• DNA repair systems maintain genetic stability, balancing mutation and repair.