A.2.3 Viruses

Based on the 3rd edition biology textbook written by C.J. Clegg, Andrew Davis, Christopher Talbot

virus

non-cellular parasite of animals, plants, and bacteria that consists of nucleic acid surrounded by a capsid

genetic code is shared between viruses and organisms meaning that host cells transcribe their DNA into mRNA and then to a protein

parasite

an organism that lives on or in another organism (its host) for most of its life cycle, deriving nutrients from its host

micro-organism

a living organism that cannot be seen by the naked eye

ex: bacteria, protists, single-celled fungi

capsid

the protein coat of a virus

characteristics of a virus

cellular: no

approximate diameter (μm): 0.02-0.2

nucleic acid: either DNA or RNA, never both

ribosomes: rarely present

nature of outer surface: protein capsid and lipoprotein enveloped (for some)

motility/movement: none

method of replication: viral replication (protein synthesis followed by self-assembly) inside cells

characteristics of a bacteria

cellular: yes

approximate diameter (μm): 1-5

nucleic acid: both DNA and RNA

ribosomes: 70S

nature of outer surface: cell wall containing peptidoglycan

motility/movement: some (via rotating flagella)

method of replication: binary fission (generally outside cells)

common characteristics of viruses

extremely small (20-400 nm in diameter)

nucleic acid (DNA or RNA) as genetic material

capsid made of protein that surrounds nucleic acid

no cytoplasm

few or no enzymes

n

coronavirus

a large and diverse family of RNA viruses that cause respiratory diseases in animals and humans

SARS-CoV-2

severe acute respiratory syndrome coronavirus 2, the virus that causes COVID-19

RNA viruses

nucleic acid replication occurs in the cytoplasm. ex: SARS-CoV-2

nucleic acid replication occurs in the nucleus. ex: influenza viruses SARS-CoV-2

nucleic acid is used as a template for synthesizing DNA in the cytoplasm using a reverse transcriptase (enzyme). ex: HIV

positive-sense RNA

viral RNA that has the same base sequence as mRNA, which allows it to function as a template for protein synthesis during viral replication

ex: SARS-CoV-2

negative-sense RNA

viral RNA that is complementary to mRNA so it cannot directly encode for protein synthesis; it must be replicated to mRNA before protein production can begin

ex: influenza viruses

capsomere

one of the individual proteins that make up a viral capsid

envelope

a membrane typically from the host cell plasma membrane with viral glycoproteins

ex: HIV, influenza viruses, coronaviruses

ex of non: bacteriophages, poliovirus

nucleocapsid

the capsid of a virus with an envelope

virion

an isolated but infectious virus particle found outside the host cell

bacteriophage (phage)

a virus that infects bacteria

lytic cycle

a phage life cycle where a phage attaches to a host bacterium, injecting its DNA which undergoes replication to form new virions, which then lyse/break the cell

these phages are called virulent phages

lysis

breakdown, typically of cells

virulent

the ability of a virus (or bacterium) to cause rapid and severe disease

stages of lytic cycle

1. adsorption - the phage attaches itself to the surface of the bacterium (host)

2. penetration - the phage is absorbed into the bacterium and injects its DNA; the viral DNA enters the bacterium

3. replication - phage DNA is replicated; bacteria DNA is broken down (DNA hydrolysis takes place)

4. maturation - phage proteins are made and assembled to make many new phages; the cell lyses (bursts) and the newly made phages are released

lysogenic cycle

a phage life cycle where a phage attaches to a bacterium and injects its genome, but it does not undergo a full replication cycle

it becomes a resident within the bacterial host where it is maintained in a dormant state

prophage

a bacteriophage in an inactive state in which the genome is typically integrated into the chromosome of the bacterial host

stages of lysogenic cycle

1. adsorption and penetration - the phage infects a cell

2. prophage - the phage becomes incorporated into the host genome; changes the genotype of the bacterium

3. replication - the cell divides, copying the prophage to other cells

4. activate - under stressful conditions, the phage DNA is removed from the host’s DNA entering the lytic cycle

5. replication - the phage replicates

6. maturation - forms new phages and lyses the cell

polyphetic

many independent origins, not one

virus-first hypothesis

viruses evolved before or co-evolved with their current host cells

come from bits of RNA that had self-complementary sequences, folding like proteins leading to a complex shape that could replicate

escaped genes (progressive) hypothesis

viruses arose from genetic elements that gained the ability to move between cells

DNA material copied ad surrounded by a capsid

regressive (reduction) hypothesis

viruses are remnants of cellular organisms

somehow, in the cell, DNA replicated and formed a capsid which allowed self replication

convergent evolution

species occupy similar ecological niches and adapt in similar ways in response to similar selective pressures

analogous structures

traits that arise through convergent evolution

influenza virus (flu)

enveloped virus with a genome made up of negative sense, single-stranded, segmented RNA

infect epithelial cells of the respiratory tract

antigenic drift

small changes (caused by mutations) in viral genes that can lead to changes in the surface proteins of a virus

antigenic shift

an abrupt, major change in the genetic and surface protein structure of a virus done through recombination

recombination

occurs when viruses of two different parent strains co-infect the same host cell and interact during replication to shuffle genes from both parents

human immunodeficiency virus (HIV)

a small group of viruses known as retroviruses

leads to acquired immune deficiency syndrome (AIDS)

retrovirus

a virus that has RNA as its nucleic acid and uses the enzyme reverse transcriptase to copy its genome into the DNA of the host cell’s chromosome

reverse transcriptase

an enzyme that converts RNA into DNA

provirus

a virus that has integrated into a host’s genome and is replicated when the cell replicates its DNA