viruses and plasmids

🦠 Viruses and Plasmids: Natural Gene Modifiers

🔬 What Are Viruses?

• Viruses are non-cellular parasites made of:

  • Nucleic acid (DNA or RNA)

  • Protein coat (capsid)

  • Sometimes a lipid envelope

• They cannot reproduce on their own and must infect a host cell.

• Exist at the boundary between living and nonliving.

• Size range: 10–300 nanometers.

🧩 Virus Structure

• Complete virus particle = virion

• Components:

  • Genome: DNA or RNA (linear or circular)

  • Capsid: protein shell that protects genome and helps infect host

  • Envelope (optional): derived from host cell membrane

• Capsid shapes:

  • Helical

  • Polyhedral

  • Complex

🧪 Examples of Viruses

• Influenza virus

  • RNA virus

  • 8 RNA fragments

  • Helical capsids

  • Infects birds and mammals

• Bacteriophage T4

  • Infects E. coli

  • DNA genome

  • Polyhedral head + tail fibers

  • No envelope

• Adenovirus

  • Double-stranded DNA

  • Polyhedral capsid with 20 faces

  • Glycoprotein spikes for attachment

  • Causes respiratory infections

• Tobacco Mosaic Virus (TMV)

  • RNA virus

  • Rod-shaped

  • Helical capsid

  • Infects plants

🔁 Viral Life Cycles

🧨 Lytic Cycle

• Viral genome enters host

• Takes over host machinery

• Produces many virions

• Host cell lyses (bursts) and dies

• Fast replication

💤 Lysogenic Cycle

• Viral genome integrates into host DNA

• Virus becomes prophage

• Replicates silently with host

• Can later enter lytic cycle

• Long-term, dormant

🧬 Transduction (Gene Transfer by Viruses)

• Lytic cycle: viral genes may integrate into host DNA accidentally

• Lysogenic cycle: faulty removal of prophage can mix viral and host genes

• Result: genetic recombination

🧫 Plasmids

What Are Plasmids?

• Small, circular DNA molecules in bacteria

• Self-replicating

• Not essential for survival but provide advantages:

  • Antibiotic resistance

  • Metabolism of unusual substances

🔗 Conjugation

• Transfer of plasmids between bacteria

• Uses pili (hair-like structures)

🧠 Why Plasmids Are Useful

• Easy to:

  • Isolate

  • Cut

  • Modify

  • Reinsert into bacteria

• Rapid bacterial reproduction = billions of gene copies

🧬 Artificial Gene Modification

🧠 Key Terms

• Transgenic organism: contains genes from another species

• Gene vector: vehicle used to transfer genes (plasmids or viruses)

• Recombinant DNA: DNA from different organisms combined

✂ Restriction Enzymes

• Also called molecular scissors

• Cut DNA at specific restriction sites

• Create:

  • Sticky ends (single-stranded overhangs)

• Bacteria protect their own DNA by chemical modification

🧲 Ligase

• Enzyme that glues DNA fragments together

🧪 Gene Splicing Steps

1. Cut target gene using restriction enzyme

2. Separate gene using electrophoresis

3. Cut plasmid with same enzyme

4. Join gene and plasmid using ligase

5. Insert recombinant plasmid into host cell

💉 Example: Insulin Production

• Human insulin gene inserted into bacteria

• Bacteria replicate rapidly

• Large-scale insulin production

• Cheap, efficient, lifesaving

🦠 Gene Transfer by Viruses

• Viruses naturally inject DNA into cells

• Disease-causing genes are removed

• Safe viral vectors include:

  • Retroviruses

  • Adenoviruses

  • Adeno-associated viruses

  • Herpes simplex viruses

🌱 Applications of Genetic Engineering

• Medicine: insulin, clotting factors, vaccines

• Agriculture: pest resistance, nutrition, shelf life

• Environment: oil spill cleanup, waste processing

• Research: disease modeling, drug testing

⚖ Ethical & Social Issues

• Fate of unused IVF embryos

• Gene editing in unborn children (“designer babies”)

• Safety of genetically modified foods

• Herbicide resistance spreading to weeds

• Risk of genetically engineered bioweapons

🧠 One-line Summary

Viruses and plasmids are natural gene carriers, and scientists harness them as tools for genetic engineering, unlocking massive benefits while raising serious ethical questions.