Virology Lecture: Genomes and Genetics

• 0:00:14 Introduction to Viral Genomes and Genetics:
  • Viral genomes consist of either DNA or RNA.
  • The lecture covers different configurations of viral DNA and RNA and how we can study them.
• 0:00:36 Key Experiments in Virology:
  • In the 1950s, experiments showed that viral nucleic acid, not protein, carries genetic information.
    • 0:01:11 Hershey-Chase experiment: Demonstrated that DNA is the genetic material using bacteriophages labeled with radioactive sulfur (proteins) and phosphorus (DNA).
    • 0:03:11 Frankel-Conrad experiment: Showed that RNA carries genetic information in tobacco mosaic virus (TMV) by separating and recombining coat proteins and RNA from different TMV strains.
• 0:04:53 The Baltimore Classification System:
  • Viruses are classified into seven groups based on their genome type (double-stranded DNA, single-stranded DNA, double-stranded RNA, etc.).
  • All viral genomes must produce mRNA that can be translated by host ribosomes.
• 0:06:09 Categorizing Genomes by mRNA Production:
  • mRNA is central to the Baltimore system, with different genome types categorized by how they produce mRNA.
  • Double-stranded DNA can be directly transcribed into mRNA.
  • Single-stranded and gapped DNA must be repaired to double-stranded DNA before transcription.
  • Double-stranded RNA viruses need to produce mRNA from their genomes as the plus strand is inaccessible for translation.
  • Negative-strand RNA viruses must be copied into a positive strand (mRNA) for translation.
  • Positive-strand RNA viruses can sometimes be directly translated or go through DNA intermediates (retroviruses).
• 0:08:35 Definitions:
  • Plus strand: Defined as mRNA or the equivalent polarity in DNA.
  • Negative strand: Complementary to the plus strand.
  • Not all plus RNA is mRNA.
• 0:09:33 Elegance of the Baltimore System:
  • Knowing the genome's nature allows prediction of the steps needed to reach mRNA.
• 0:11:25 Structural Diversity of Genomes:
  • Viral genomes exhibit structural diversity: linear, circular, segmented, proteins attached to ends, etc.
• 0:12:56 Function of Genome Diversity:
  • The diversity likely reflects evolutionary advantages, with different genome types offering unique benefits.
• 0:16:40 What's Encoded in a Viral Genome:
  • Gene products and regulatory signals for protein synthesis, replication, assembly, packaging, regulation of replication, modulation of host defenses, and spread to other cells.
• 0:17:39 Giant Viruses:
  • Relatively recently discovered; some encode components of the translational machinery (e.g., aminoacyl-tRNA synthetases, tRNAs), but not ribosomes.
• 0:18:57 What's Not Encoded in Viral Genomes:
  • Complete protein synthesis machinery, genes for membrane biosynthesis (though recently found in some giant viruses), classical centromeres, or telomeres.
• 0:20:08 Smallest and Largest Viral Genomes:
  • Largest known viral genome: Pandoravirus salinus (2.47 million base pairs, double-stranded DNA).
  • Smallest known viral genomes: Satellites (as small as 220 base pairs, RNA, helper-dependent).
• 0:25:23 Viral DNA Genomes:
  • Double-stranded DNA viruses are common, especially in bacteria.
  • They often emulate host processes but differ in that most are not chromatinized.
  • Some viral DNA genomes encode their own DNA polymerase (e.g., adenoviruses, herpesviruses, poxviruses), while others rely on host enzymes (e.g., polyomaviruses, papillomaviruses).
• 0:29:20 Hepatitis B Virus:
  • Has a unique gapped, partially double-stranded DNA genome with an attached RNA and protein.
  • The host cell repairs the genome, making it fully double-stranded for transcription.
• 0:31:04 Single-Stranded DNA Genomes:
  • Include circoviruses (some of which infect all humans) and parvoviruses.
  • Can package plus or minus DNA, which must be repaired to double-stranded DNA by the host cell for transcription.
• 0:33:01 Viral RNA Genomes:
  • Dominate the eukaryotic virus sphere.
  • Mammalian cells lack enzymes to copy RNA to RNA, so RNA viruses must encode their own RNA-dependent RNA polymerase (RdRp).
• 0:34:36 Double-Stranded RNA Viruses:
  • Example: Rotaviruses (segmented genomes).
  • The plus strand in the double-stranded RNA cannot be accessed by ribosomes; an RNA polymerase in the particle must first make mRNA.
• 0:35:42 Plus-Stranded RNA Viruses:
  • The genome can be directly translated into protein.
  • Examples: Coronaviruses, flaviviruses, picornaviruses, togaviruses.
• 0:37:31 Plus-Stranded RNA Viruses with a DNA Intermediate:
  • Retroviruses (e.g., HIV, HTLV).
  • The plus-stranded RNA genome is copied into DNA, which integrates into the host genome.
• 0:38:36 Negative-Strand RNA Viruses:
  • The genome cannot be translated; it must be copied to mRNA by an RNA polymerase carried within the virus particle.
  • Examples: Influenza viruses (segmented), measles virus, rabies virus.
• 0:40:55 Segmented Genomes and Reassortment:
  • Viruses with segmented genomes (e.g., influenza viruses) can undergo reassortment, where segments mix during co-infection, leading to genetic diversity.
• 0:42:55 Ambisense Genomes:
  • Contain both plus and minus polarity regions (e.g., Lassa virus).
• 0:43:41 RNA Structure:
  • Viral RNAs are highly structured, with stem-loops and long-range interactions that are important for replication.
• 0:46:22 Viral Genetics:
  • Traditionally, viral genetics involved isolating mutants (e.g., small plaque mutants) and studying their phenotypes.
  • Nowadays, infectious DNA clones allow for site-directed mutagenesis of viral genomes.
• 0:48:18 Transfection:
  • Introducing viral DNA or RNA into cells to initiate an infectious cycle.
• 0:50:06 Recovery of the 1918 Influenza Virus:
  • Achieved by reconstituting the genome from historical samples and introducing it into cells using plasmids.
• 0:51:14 Recovery of Horse Pox Virus:
  • Synthesized overlapping DNA fragments based on the extinct virus's genome sequence and introduced them into cells to recover the virus.
• 0:51:56 Ethical Concerns and Gain-of-Function Research:
  • The ability to manipulate viral genomes raises ethical concerns, particularly regarding "gain-of-function" research, where viruses are modified to acquire new properties.
  • Debate over the risks and benefits of such research, especially in the context of the COVID-19 pandemic's origins.
• 0:56:13 Conclusion: The lecture ends with a preview of the next topic: how to build viruses.