Genetic Inheritance and Transformation Notes

Genetic Inheritance and Transformation

  • Overview: Understanding genetic inheritance and transformation as mechanisms for transferring information between generations.

Module Information

  • Module 2 (L17 – L20) by Prof. K. A. Gibbs, UC Berkeley Bio1A

Viruses and Molecular Biology

  • Viruses are ubiquitous and offer insights into molecular biology and ecosystem dynamics.
  • They leverage the genetic and cellular processes across all life forms.
  • Associated Reading: Chapter 19 on Viruses and Genetic Inheritance

Learning Objectives

  • Comfort with the following concepts by the end of the lectures:
    • Utilizing genetics to understand connections among life branches and viruses.
    • How genomes evolve both over individual lifespans and through evolutionary processes.
    • The role of viruses as significant drivers of random mutations.

Genetic Modification

  • Genetic Modification Defined:
    • An alternative to traditional cross-breeding.
    • Involves integrating foreign DNA into a host genome via scientific methods.
    • Resulting organism: Genetically Modified Organism (GMO).
  • Importance of Technology:
    • Transformational for various industries, notably agriculture.
  • Mechanisms of DNA Integration:
    • Both bacteria and viruses possess unique methods to incorporate foreign DNA into host genomes.

Genomic Diversity

  • Historical Insight:
    • Human ancestry reflected in genomic signatures.
    • Microbial species dominate the ecosystem.
    • Eukaryotes diverge from Archaea, illustrating life's diversity and evolution through genome sequences as historical records.

Viruses in Evolution

  • Investigative Questions:
    • Why are viral sequences present in organismal genomes?
    • How do viruses facilitate evolutionary processes?

Classification of Viruses

  • Why are viruses typically not considered living?
    • Options for consideration include:
      a) Cause disease.
      b) Do not evolve.
      c) No independent metabolism.
      d) Cannot replicate independently.
      e) Few genes.

Characteristics of Viruses

  • Definition of a Virus:
    • Small infectious particles consisting of nucleic acid in a protein coat, sometimes with a membranous envelope.
    • Simpler than prokaryotic cells; viruses lack cellular structure.
    • Unable to reproduce or perform metabolic functions outside a host cell.
    • Gene count range: 3 to 2,000 genes in their genomes.
  • Types of Infections:
    • Lytic (bacterial) or acute (animal) infections.
    • Lysogenic (bacterial) or latent (animal) infections.

Structural Properties of Viruses

  • Capsid:
    • Protein shell enclosing viral genome.
    • Composed of capsomeres (protein subunits).
    • Structural types: helical or icosahedral.
  • Viral Envelopes:
    • Surround capsids in influenza viruses and many animal viruses.
    • Derived from host cell membranes, containing both viral and host molecules.

Viral Infection Mechanisms

  • General Replication Process:
    • Virus enters host cell, releasing its genome.
    • Host enzymes replicate the viral genome.
    • New viral mRNA is transcribed.
    • Host ribosomes translate the mRNA into viral proteins.
    • New viruses are either self-assembled with the viral genome or the host cell may burst, releasing them.

Bacteriophages

  • Definition:
    • Viruses specifically infecting bacteria known as bacteriophages or phages.
    • Best understood among viruses.
  • Structure:
    • Comprises an elongated capsid head that encases DNA.
    • Protein tail used to attach to hosts and inject phage DNA.
  • Reproductive Cycles:
    • Lytic cycle (virulent) and lysogenic cycle (temperate) exist among phages.

Lytic Cycle: Phage T4

  • Key Stages in the Cycle:
    1. Attachment to host.
    2. Entry of phage DNA and degradation of host DNA.
    3. Phage assembly.
    4. Release of new phages.

Lysogenic Cycle: Phage λ

  • Process Involvement:
    1. Phage injects DNA, which circularizes.
    2. Integrated into bacterial chromosome as a prophage, duplicated during bacterial division.
    3. External stimulus can lead to initiation of the lytic cycle.

Viral Impact on Ecosystems

  • Viral Ecology: General Observations
    • Role in global carbon balance and ecosystem dynamics.
  • Ecosystem Functions of Viruses:
    1. Limit host population density without causing extinction.
    2. Promote host diversity and prevent species dominance.
    • Viruses are the most numerous entity in oceanic ecosystems.

Emerging Viruses

  • Definition:
    • Newly identified viruses that suddenly become prominent.
  • Examples of Emerging Viruses:
    • Ebola, Zika, and SARS coronaviruses among significant threats.
  • Contributing Factors:
    • High mutation rates in RNA viruses.
    • Dissemination from isolated populations and animal to human transmission as main routes.

Epidemics and Pandemics

  • Epidemic:
    • An outbreak affecting a large number in a specific region, e.g., 2009 flu-like illness.
  • Pandemic:
    • Global epidemic affecting multiple regions.
  • Flu strains (Type A) classification based on surface proteins: hemagglutinin (HA) and neuraminidase (NA).
    • Example: H1N1 responsible for 2009 flu pandemic and 1918 pandemic causing substantial mortality.

Coronaviruses Overview

  • Human Pathogenic Coronaviruses:
    • List of coronaviruses with their respective hosts, years of discovery, and symptoms:
    • HCoV-229E, 1966, mild respiratory.
    • HCoV-NL63, 2004, mild respiratory.
    • HCoV-OC43, 1967, mild respiratory.
    • HCoV-HKU1, 2005, pneumonia.
    • SARS-CoV, 2003, severe acute respiratory syndrome; ~10% fatality.
    • MERS-CoV, 2012, severe acute respiratory syndrome; 37% fatality.
    • SARS-CoV-2, 2019, severe acute respiratory syndrome; 2.9% fatality; ~6.8 million deaths (as of March 12, 2023).

Replication Cycle of RNA Viruses

  • Enveloped RNA viruses, like SARS-CoV-2, replicate in the cytoplasm utilizing their positive-sense ssRNA genome.
  • Components involved:
    • Capsid, genome, glycoproteins, host ER for protein synthesis, etc.

SARS-CoV-2 Characteristics

  • Description:
    • Enveloped virus with ~30 kb positive-sense ssRNA genome, associated with nucleocapsid and membrane proteins (S, M, E).
  • Entry Mechanism:
    • S protein binds to ACE2 receptor for viral entry.

Viral Influence on Population Genomics

  • Viral pressure can lead to varied population genomics via mechanisms such as:
    • Bottleneck effects due to selective pressures on random mutations, potentially linked to diseases like Celiac.

Retroviruses

  • Definition:
    • Retroviruses like HIV use reverse transcriptase to convert their ssRNA genome into dsDNA.
  • Mechanism of Integration:
    • HIV integrates into host genome as a provirus, making it a permanent resident, unlike prophages.

Summary of HIV Characteristics

  • Transcription of proviral DNA yields mRNA for viral protein synthesis and new virus particles.
  • Annual incidence of new HIV infections in the U.S. approximates ~38,000.
  • Prevention and treatment strategies are critical.

Endogenous Retroviruses

  • Definition:
    • Integrated into germline chromosomes, affecting gene expression patterns in offspring.

Summary Question

  • How do viruses contribute to life on earth?
    • A) Promote host diversity.
    • B) Control host population density.
    • C) Facilitate gene transfer mechanisms.
    • D) All of the above.

Eukaryotic Chromosome Structures

  • Chromatin complex structure containing DNA and proteins:
    • Chromatid (700 nm), nucleosome (10 nm), and 30-nm fiber.
  • Distinction between chromatin types:
    • Heterochromatin: More condensed, typically not expressed.
    • Euchromatin: Open structure, accessible to transcription machinery.

Concept Questions

  • Focus on the specific mechanisms by which HIV targets certain white blood cell types, correlated with receptor presence and other viral replication abilities.