Microorganisms: Viruses

Question 1

What is the study of viruses?

Answer 1

Microbiology: virology

Question 2

What is a virus?

Answer 2

A small infectious agent that replicates in living cells only (hosts).

Question 3

Are viruses alive? What can they infect?

Answer 3

No, they are complex biochemical molecules. They can infect all organisms (archaea, bacteria, animals, plants etc.) They form their own category.

Question 4

Why can’t they be classified as living?

Answer 4

They do not respire, feed, excrete, move, grow or respond to stimuli, and they are not made of cells.

CAN ONLY: replicate and perform metabolism when inside host cell

Question 5

Are viruses pathogenic? Define this term and explain why they can be described as such.

Answer 5

Yes; organisms that cause disease.
They are transmitted from host to host and disrupt normal cell activities (with harmful effects on the organism).

Question 6

What happens when a virus enters a host cell?

Answer 6

It goes from inactive (isolated0 to a highly active genetic program that takes over the nuclear machinery of the host cell.

Question 7

How big are viruses?

Answer 7

20-300 nm (only seen using an electron microscope). About 1/100 the size of a bacterium.

Question 8

Describe the structure of viruses.

Answer 8

They are acellular.
3 MAIN COMPONENTS:

1) genetic material of RNA or DNA never both (molecules that carry hereditary information)
2) The capsid (the protein coat). This surrounds and protects the genetic information. Made of capsomeres or protein subunits (vary in number according to the virus.)
3) Sometimes an envelope of lipids surrounds the protein coat when the virus is outside host cell.

Question 9

How are they classified?

Answer 9

According to what they infect eg. Animal virus

Question 10

What genetic information do bacteriophages/phages contain?

Answer 10

Most of the time DNA although a few contain RNA.

Question 11

What genetic information do plant viruses contain?

Answer 11

Most of the time RNA although a few have DNA.

Question 12

What genetic information do animal viruses contain?

Answer 12

Either DNA or RNA (never both)

Question 13

What is viral replication?

Answer 13

The formation of viruses during the infection process (active stage) to allow for the production and survival of its kind. (Essentially copies of genome in virus packaging)

Question 14

How do viruses replicate? (FLOW DIAGRAM)

Answer 14

ATTACHMENT: 1. The viruses attach to the cell membrane of the cell.
PENETRATION: 2. It injects RNA or DNA
into the cell of the host cell (phages)— OR the cell membrane invaginates, engulfing the virus, enclosing it in a pinocytic vesicle.
UNCOATING: 3. Host cell enzymes strip off the virus protein coat (if engulfed). This releases the virus genome. The nucleic acid is released.

DNA viruses assemble in the host cell nucleus. RNA viruses in the cytoplasm alone.
Once in the host, it can either enter a lytic or lysogenic cycle.

Question 15

FLOW diagram showing the lytic cycle of a virus

Answer 15

1. Virus attaches to the host cell
2. Viral RNA/DNA is released into the host cell
3. DNA or RNA is replicated
4. Viral proteins are produced through transcription and transaltion in protein synthesis.
5. All the components/viral particles are assembled
6. The host cell lyses and the viral particles are released

Eg, rhinovirus

Question 16

FLOW diagram showing the lysogenic cycle of a virus

Answer 16

1. Virus attaches to the host cell
2. The Viral DNA/RNA is released into the host cell
3. The viral DNA is incorporated into the host genome (DNA). It is now called a provirus/prophage.
4. The cell divides. The virus is passed to the daughter cells.
5. It is possible for the viral DNA to be excised from the host cell and enter a lytic cycle

eg. HIV or Herpes simplex

Question 17

What are retroviruses?

Answer 17

Viruses possess RNA which is used to make DNA. (Reversal of the genetic process in which DNA is made from RNA.)

Question 18

What type of virus is HIV?

Answer 18

Lysogenic, retrovirus. H: humans, I: immunodeficiency (attacks immune system. Less and less able to fight infections: AIDS). V: Virus
AIDS: Acquired Immune Deficiency Syndrome

Question 19

What is required for a person to contract HIV? Two things.

Answer 19

Body fluids from an infected person, containing a large enough quantity of the virus (high viral load) must enter the body of the uninfected person. This must be long enough for the virus to enter the blood stream.

Question 20

How does this happen? (HIV transfer through tissue)

Answer 20

Happens through a cut, tear or wound. Can be absorbed through mucous membranes: lining of the mouth, lining of the vagina, surface of the eyeball.
Fluids usually are: blood, semen, vaginal, breast milk.

Question 21

What are the four ways a person can contract HIV?

Answer 21

1. Sexual intercourse: unprotected vaginal/oral. Membranes torn; virus enters
2. Monocytes in semen pass through intact mucous membranes
3. MTCT (pregnancy/labour: 30% chance). Breast feeding: further 10-20%
4. Not as common, but intravenous drug usage or sharing of sharp objects.

Question 22

How do you reduce MTCT of HIV?

Answer 22

• HIV treatment during pregnancy
• Caesarean delivery
• Avoid breastfeeding
• Taking antiretrovirals days before birth

Question 23

Describe AIDS

Answer 23

A collection of many signs and symptoms because immune system is weak.
This person is highly infectious (viral load very high)

Question 24

How do you prevent HIV?

Answer 24

Avoid contact with fluids, do not have unprotected sex, wash drug injection needles, ensure pregnant HIV positive women receive antiretroviral treatments.

Question 25

How are antivirals different to antibiotics?

Answer 25

They do not kill the virus, they inhibit its development, unlike antibiotics. Humans have natural immunity to viruses. They assist in immunity. No medecines can kill viruses.

Question 26

What are examples of controlling virus production?

Answer 26

Virus resistant plant species (prevention). Removal of the source of infection: disinfectants. Protection of susceptible plants and animals (by vaccines.)

Question 27

What are other examples (other than antivirals) of treatments against viruses — not including vaccines, which are not treatment anyway?

Answer 27

Interferons: signalling proteins which cause nearby cells to heighten defenses, activate immune cells (natural killer cells and macrophages). Flu-like symptoms are caused by interferons.

VITAMIN C concentrations in plasma and leukocytes decline during infections: supplementation improve anti microbial and natural killer cell activities and lymphocyte proliferation. Cytokine increase.

ZINC (lack of decreases killer cell activity and phagocytosis). They both play a role in immune function and reduce the risk, severity and duration of the infectious disease.

Question 28

Explain how different antiretrovirals work.

Answer 28

1. Reverse transcriptase inhibitor: inhibits viral DNA production
2. Protease inhibitor (stops viral assembly)
3. Fusion/entry inhibitors (stops gp120 from binding to CD4)
4. Intregrase inhibitor (Stops viral DNA from integrating into host DNA)

Question 29

What three proteins (enzymes) are found in HIV viruses? What are their functions?

Answer 29

1) Reverse Transcriptase is an enzyme which reverse transcribes RNA to DNA.
2) Integrase: integrates the viral DNA into the host genome
3) Protease makes viral proteins and is involved in the assembly of the proteins

Question 30

why do RNA viruses have their own enzymes? Can they be lysogenic?

w

Answer 30

We can’t transcribe RNA from RNA; needs a seperate process. Lysogenic virus can be RNA, but there is an extra step, in which the RNA is reversed into DNA – a process in retroviruses.

Question 31

What is the point of no return in CD4 count?

Answer 31

200 cells per mm cubed - have a window period from 400.

Question 32

What are antibodies?

Answer 32

Proteins which respond to specific viruses.

Question 33

How are antibodies manufactured? how do they work?

Answer 33

Once exposed to a disease, memory cells (B Lymphoctyes) respond to the infection by producing large amounts of antibodies, enabling your body to fight the infectiom easily/shorten the length of disease,

Question 34

what is a memory cell?

Answer 34

A long-lived lymphocyte capable of responding to a particular antigen on its reintroduction,long after the exposure that prompted its production.

Question 35

Are vaccines a form of active or passive immunity?

Answer 35

active immunity – antibodies are produced by memory cells. previosuly caught the infection

Question 36

What is a vaccine?

Answer 36

Vaccines contain agents that resembles a disease-causing microorganism and is often made from weakened or attenuated virus, viral capsid with no DNA/RNA, portion of the capsid with spike proteins, or mRNA for the spike proteins. The agent stimulates the body to recognise it as a foreign object, destroy it using WBCs, (antibodies to the antigens) remember it – so it can remember how to do so in future.

Question 37

Draw a graph of the comparison between the number of anitbodies produced during a vaccine, and once infected.

Answer 37

Infected should be significantly higher, stay for longer, spike more quickly.

Question 38

Describe who made the first vaccine and how.

Answer 38

Edward Jenner – 200 years ago. inoculated a boy with a cut with cowpox, then smallpox. did not get sick.

Question 39

Compare bacteriophages to HIV

Answer 39

Bacteriophage: DNA virus, tail fibres attach, no envelope, targets bacteria
HIV: Retrovirus (RNA virus), gp120 protein attaches, envelope, attacks WBCs)

Question 40

When, in relation to their system, does a person with HIV have a higher likelihood of infecting someone else?

Answer 40

When their viral load is high – infectious states

Question 41

What effect does HIV have on the body?

v

Answer 41

• T4 Lymphocytes are destroyed therefore the immune system is weakened
• The body responds to the virus by increasing the number of natural killer cells, the production of interferons and macrophages
• ## AIDS (if too severe) can lead to susceptibility to many diseases