A2.3: Viruses Flashcards
List structural features common to all viruses.
- Nucleic acid as genetic material (RNA/DNA - either single or double stranded)
- Capsid made of protein
- NO Cytoplasm or Plasma Membrane
Explain why viruses are said to be nonliving
- They aren’t made of cells
- They can’t undergo self-replication
- They can’t perform independent metabolism
What is the genetic code said to be in cells and viruses?
- Universal; it is shared between viruses and living organisms
State an alternate name for viruses
Obligate intracellular parasites
Compare the size of viruses to bacterial cells and eukaryotic cells
Viruses are larger than bacterial cells and eukaryotic cells
What microscope can allow for viruses to be viewed
As they are too small to be seen by a light microscope, an electron microscope is used as it has a higher resolution and larger magnification
Outline the diversity of structure in viruses, including genetic material and the presence or absence of envelopes.
- Viruses can contain either RNA or DNA that is either single stranded or double-stranded
- Viral genomes can vary in the size as they have different number of genes
- Some viruses contain an extra layer of protection, outside the capsid called an envelope, of the viral genome and helps disguise the virus from the immune system of the host cell
Where is the envelope acquired from
-The host cell membrane when it is released
Characteristics of non-enveloped viruses?
- More resistant to extreme pH, heat, dryness etc
Characteristics of enveloped viruses?
- More sensitive to extreme pH, heat, dryness etc
Outline what positive-sense RNA is
-It is viral mRNA which can be directly translated into virus proteins
Outline what negative-sense RNA is
-RNA that must be transcribed first before being translated into virus proteins
Outline what Retroviruses are
- Retroviruses have RNA that is first reverse transcribed to make DNA which is transcribed and translated to make virus proteins
Outline what DNA Viruses are
- DNA Viruses are transcribed and translated to make virus proteins
Structure of bacteriophage lambda
- Icosahedral-shaped capsid head
- Double stranded DNA
- Short collar, tail and tail fibres to attach to the cell
- Virus contains enzymes (holin too which helps the bacteriophage to make holes in the cell wall of the bacterium)
How do bacteriophage lambda gain access to host cells?
- Virus binds to receptors on host cell membrane and inserts its genetic material directly into host cell
Structure of Coronavirus?
- ONE single stranded RNA
- No Reverse Transcriptase which converts RNA into DNA
- Helical shaped capsid
- Coronavirus genome is in a nucleocapsid
- Contains spiked proteins
- Spherical in shape
Structure of HIV Virus?
- TWO Single Stranded RNA-
- Reverse Transcriptase which converts RNA into DNA
- Icosahedral shaped capsid
- HIV Genome is in an enveloped capsid
- Contains spiked proteins (allows viruses to bind to specific receptors in order to allow viral entry)
- Spherical in shape
Host for HIV Virus?
- Humans
Outline the phases of the lytic cycle of a bacteriophage lambda virus.
R- Phage attachment to host cell
- Phage DNA entry into host cell
- Phage DNA Replication (using rolling-circle replication)
- Phage protein synthesis (host cell’s RNA polymerase used to transcribe phage DNA into phage RNA, host cell’s ribosomes used to translate phage RNA into phage proteins)
- Assembly of new phage viruses
- Lysis (release of phages into bacteria after weakening the bacterial cell wall)
- Spread (when actively infecting and damaging host cells, viruses are said to be virulent)
Outline the phases of the lysogenic cycle of a virus
- Phage attachment to host cell
- Phage DNA entry into host cell (After this, the bacteriophage lambda genome is now called Prophage)
- Phage DNA is integrated into host genome
- DNA Replication occurs
- Cell Division occurs (daughter cells are produced which also contain the bacteriophage DNA which is integrated into their genome)
[ - if exposed to certain environmental conditions like low nutrient conditions etc, viruses return to the Lytic Cycle -]
Contrast the lysogenic cycle with the lytic cycle
Lysogenic cycle:
- Virus assimilates the genome within the host cell’s genome to achieve replication without killing host
Lytic cycle:
- Virus reproduces and bursts out of the host cell, killing it
In the Lytic cycle, what does the virus rely on the host cell for?
- Energy supply: for viral replication
- Nutrition: for the necessary building blocks
- Protein synthesis: using host cells’ machinery like ribosomes and enzymes
- Transport: to move to different parts of the body or spread to other host cells
Outline evidence that viruses evolved after the origin of cells.
- Viruses may have originated from ancient RNA or DNA molecules that became encapsulated in a protective protein coat that may have evolved and replicated
- Viruses may have evolved from viroids (small infectious agents consisting of a short strand of RNA)
- Viruses may have evolved from ancient cells that lost their ability to live independently and became dependent on other cells for reproduction
Define inductive reasoning and give an example
- Using specific observations to form a general conclusion
E.g: Origin of viruses
Outline the regressive hypotheses for the origin of viruses. Give an example of a virus that supports this too
Regressive:
- viruses arose by a loss of cellular components
Supported by the evidence; All viruses require cells to replicate, use the same genetic code as cells, share more protein folds in common with cells than with each other
- E.g: Mimivirus
Outline the progressive hypotheses (or esacpe hypothesis) for the origin of viruses. Give an example of a virus that supports this too
Progressive:
- viruses arose by taking and modifying cell components
Supported by the evidence; All viruses require cells to replicate, use the same genetic code as cells, share more protein folds in common with cells than with each other
- E.g: Retrovirus
Outline the virus-first hypothesis for the origin of viruses and explain is not supported
- Viruses predate or coevolved with their current cellular hosts (they are self-replicating)
Not supported by the evidence; Viruses require cells to replicate and viruses use the same genetic code as all cells
Outline how convergent evolution could result in the commonality of some structures shared by viruses (4 marks)
- The genetic code is universal meaning viruses share the same genetic code
- Viruses have specific protein spikes
- Viruses have capsids surrounding the genetic material
- Viruses require host cells to replicate as they can’t self-replicate
Explain 2 reasons why this convergent evolution has occurred
- Only a limited ways a virus can effectively infect and replicate within host cells
- Only limited number of shapes and sizes optimal for infecting and replicating within host cells
Outline three reasons for rapid evolution in viruses.
- Viruses have high mutation rates (as many RNA viruses are retroviruses that use reverse transcriptase enzymes and RNA polymerase
- Viruses exchange genetic material rapidly through recombination and horizontal gene transfer
- Viruses have short generation times and high reproductive rates
Describe Recombination
- When a virus exchanges genetical material with other viruses / its host cell hence, acquiring new genes
Describe Reassortment
- when a virus infects a host cell that’s already infected with a different strain of influenza and the 2 viruses exchange genetic material leading to emergence of new strains with diff characteristics
Discuss the consequences of rapid virus evolution on treating diseases caused by HIV virus
- Mutates its genetic material very quickly which can lead to the emergence of new HIV strains
- Carries out recombination which can further increase its adaptability
Discuss the consequences of rapid virus evolution on treating diseases caused by Influenza virus
- High mutation rate in hemagglutinin and neuraminidase surface proteins which make it difficult for host immune system to identify and protect against the virus: Flu vaccines tend to have low effectiveness because of this
- Carries out reassortment leading to emergence of new strains with diff characteristics)
Describe the metabolic functions of reverse transcriptase (3 marks)
- Reverse transcriptase is an enzyme present in HIV
- It uses viral RNA as a template
- Reverse transcriptase synthesises viral RNA to DNA
- DNA gets transcribed by host cell to make virus proteins
