Viruses

Question 1

Order

Answer 1

The highly ordered structure that typifies life

Question 2

Growth and Development

Answer 2

Consistent growth and development controlled by inherited DNA

Question 3

Energy processing

Answer 3

The use of chemical energy to power an organism’s activities and growth

Question 4

Response to the environment

Answer 4

An ability to respond to environmental stimuli

Question 5

Regulation

Answer 5

An ability to control an organism’s internal environment within limits that sustain life

Question 6

Evolutionary adaptation

Answer 6

Adaptations evolve over many generations as individuals with traits best suited to their environments have greater reproductive success and pass their traits to offspring

Question 7

Reproduction

Answer 7

The ability of organisms to reproduce their own kind

Question 8

Viruses

Answer 8

“Borrowed life”
- An infectious particle consisting of genes packaged in a protein coat
- Cannot reproduce or carry out metabolism outside of a host cell
- exist in a shady area between life-forms and chemicals, leading a borrowed life

Question 9

What was one of the first observation of viruses?

Answer 9

In the late 1800’s, researchers hypothesized that unusually small bacteria or a secreted toxin might be responsible for Tobacco mosaic disease (caused a mosaic coloration to the leaves)
- Later work suggested that the infectious agent did not share features with bacteria
- In 1935, Wendell Stanley confirmed this latter hypothesis by crystallizing the infectious particle

Question 10

What is the main structure of viruses?

Answer 10

Very small infectious particle consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope

Question 11

Capsid

Answer 11

The protein shell that encloses the viral genome built from subunits called capsomeres

Question 12

Viral envelopes

Answer 12

• Derived form membranes of host cells
• Surround the capsids of some viruses (influenza, animals)
• Contain a combination of viral and host cell molecules

Question 13

What does the DNA of viruses look like?

Answer 13

Classified as DNA viruses or RNA viruses
- double- or single-stranded DNA or
- double- or single-stranded RNA
The genome is either a single linear or circular molecule of the nucleic acid
Have between three and 2,000 genes in their genome

Question 14

What are two key variables used to classify viruses that infect animals?

Answer 14

• An RNA or DNA genome, either single-stranded or double-stranded
• THe presence or absence of a membranous envelope (anvelope is usually derived from the host cell’s plasma membrane as the viral capsids exit)

Question 15

Host range

Answer 15

A limited number of host cells that a virus can infect
- a function of inability of the virus to successfully absorb and/or enter cells because of an incompatibility between virus capsid/envelope proteins and the host receptor molecule

Question 16

What does SARS-CoV-2 target when infecting a human?

Answer 16

• Infects the nasal cavity
• Infection extends to lungs
• Lung tissue is destroyed & large immune response
• Enters blood stream and infects blood vessel linings (endothelium) around the body
  - explains cardiovascular and clotting symptoms,
  and risks to people with pre-existing conditions

Question 17

Replicative cycle

Answer 17

• The viral genome enters the host cell in a variety of ways
• Once a viral genome has entered a cell, the cell begins to manufacture viral proteins
• The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, membranes, and other molecules
• Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses
  (more detailed from Covid replication slide)
• Following entry, two large ORFs are translated, and processed to form viral replication and transcription complex
• Synthesis of structural proteins and genomic RNA, translocated through ER and golgi, and new virions secreted

Question 18

How does Remdesivir and Molnupiravir stop viruses from replicating?

Answer 18

Remdesivir uses one of its proteins as a stop codon, causes the replicase complex to terminate transcription
Molnupiravir inserts one of its proteins into the viral DNA, virus will insert the wrong base when it tries to copy templates containing Molnupiravir

Question 19

Hersey-Chase experiment

Answer 19

Phages were labeled with radioactive sulfur to detect proteins or radioactive phosphorus ot detect DNA

Question 20

Bacteriophages

Answer 20

(also known as phages)
- Viruses that infect bacteria
- Have an elongated capsid head that encloses their DNA
- A protein tail-piece attached the phage to the host and injects the phage DNA inside

Question 21

How do phages replicate?

Answer 21

Phages have two alternative reproductive mechanisms: the lytic cycle and the lysogenic cycle

Question 22

The Lytic cycle

Answer 22

A phage replicative cycle that culminates in the death of the host cell
- produces new phages and lyses (breaks open) the host’s cell wall, releasing the progeny viruses
- a phage that reproduces only by the lytic cycle is called a virulent phage

Question 23

The Lysogenic cycle

Answer 23

Replicates the phage genome without destroying the host
- the viral DNA molecule is incorporated into the host cell’s chromosome
- integrated viral DNA is known as a prophage
- every time the host divides, it copies the phage DNA and passes the copies to daughter cells
- Phages that use both the lytic and lysogenic cycles are called temperate phages 1

Question 24

What is bacteria defense against phages?

Answer 24

Natural selection favors bacterial mutants with surface proteins that cannot be recognized as receptors by a particular type of phage
- foreign DNA can be identified as such and cut up by cellular enzymes called restriction enzymes
- the bacterium’s own DNA is protected from the restriction enzymes by being methylated

Question 25

What defense system can be used by both bacteria and archaea?

Answer 25

The CRISPR-Cas9 system

Question 26

HIV

Answer 26

Human immunodeficiency virus
- the retrovirus that causes AIDS (acquired immunodeficiency syndrome)
- releases viral proteins and RNA
- reverse transcriptase enzymes make DNA strands complementary to viral RNA
- the viral DNA is integrated into the host genome as a provirus
- unlike a prophage, a provirus remains permanent resident of the host cell

Question 27

Retroviruses

Answer 27

Use reverse transcriptase to copy RNA genomes into DNA

Question 28

What are proviral genes transcribed into?

Answer 28

• RNA as genomes for new progeny viruses
• mRNA for translation into capsid proteins

Question 29

Virus-first hypothesis

Answer 29

Could they be relics of the pre-cellular world - products of the first protocells? Many have RNA genomes
- viruses do not fit our definition of living organisms

Question 30

Progressive/escape hypothesis

Answer 30

Since viruses can replicate only within cells, they probably evolved as mobile pieces of nucleic acid that could move between cells

Question 31

Regression hypothesis

Answer 31

Could they be relics of symbiotic which over time became parasitic relationships in early cells

Question 32

Mimiviruses

Answer 32

Large viruses - the size of a small bacterium
- large genome of 1,000 genes encodes protein involved in translation, DNA repair, protein folding, and polysaccharide synthesis
- lack any genes for ribosomal proteins, making mimivirus dependent on a host cell for protein translation
Originated as intracellular parasites that lost ability to live independently?

Question 33

What are prions?

Answer 33

• incorrectly folded proteins, can be transmitted in food, act slowly, and are virtually indestructible
• are somehow able to convert a normal form of the protein into the misfolded version
• cause brain diseases in mammals (like Creutzfeldt-Jakob disease in humans)