MICROBIOLOGY - VIRUSES

Question 1

What are viruses?

Answer 1

distinct biological entities that are acellular in nature (thus not in the tree of life). In order to survive and reproduce they must infect a cellular host, making the obligate intracellular parasites.
The genome of a virus enters the host cell and produces the viral components, proteins & nucleic acids needed to assemble new virus particles called virions. Virions transport the viral genome to another host cell to Carry out another round of infection

Question 2

describe characteristics of viruses

Answer 2

• Infectious, acellular pathogens
• obligate intracellular parasites with host and cell type specificity
• DNA or RNA genome
• genome surrounded by protein capsid and sometimes a phospholipid membrane studded with viral glycoproteins
• they lack genes for products needed for reproduction - so it requires exploitation of the host cell genomes in order to reproduce

Question 3

What can viruses infect?

Answer 3

Every type of host cell. Most are only able to infect specific cells of one of a few species of an organism (host range).

Question 4

How are viruses transmitted? and give examples

Answer 4

1. direct contact
2. indirect contact with fomites (materials to carry)
3. Through a vector - animal that transmits pathogens rom 1 host to another
   example 1- arthropods e.g ticks, flies - act as mechanical vectors (physical contact) or biological (bite)
   example 2- zoonoses - viruses that can be transmitted from animal to human cause zoonoses

Question 5

what are the sizes of viruses?

Answer 5

Virions are small and not observed via regular microscope. Smaller than eukaryotes and prokaryotes.
20-900nm
-New giant viral species e.g pandora virus and pithovirus sibericum with sizes similar to bacterial cells.

Question 6

What shapes can viruses have?

Answer 6

1. Helical - forms the shape of tobacco mosaic virus (naked helical virus) and ebola (enveloped helical virus)
2. Polyhedral - forms shapes of poliovirus and rhinovirus - nucleic acid surrounded by a polyhedral capsid in the form of a icosahedron (3D and 20 sided)
3. Complex - certain types of bacteriophages e.g T4 phase and poxvirus have both helical and polyhedral viruses.

Question 7

What are virulent and temperate phages?

Answer 7

Virulent = lead to cell death via cell lysis
Temperate = become part of the host chromosome and replicated within the cell genome until they are induced to make newly assembled viruses/progeny

Question 8

what is a phage. / bacteriophage?

Answer 8

Virus that infects bacteria

Question 9

Describe the lytic cycle of a phage

Answer 9

Bacteriophage takes over the cell, reproduces new phages and destroys the cell e.g T-even phages.

Question 10

Describe the stages of the lytic cycle (phage)

Answer 10

1. attachment - phage attaches to surface of host
2. Penetration - viral DNA enters host cell
3. Biosynthesis - phage DNA replicates and phage proteins are made
4. maturation - new phage particles assmbled
5. Lysis - cell lyses, releasing newly made phages.

Question 11

What is the lysogenic cycle (phage)?

Answer 11

Example- lambda phage. Phage genome integrates into the bacterial chromosome and becomes part of the host. The process by which a bacterium becomes infected by a temperate phage is called lysogeny

Question 12

How do lytic animal viruses differ in the infection stages to phages

Answer 12

Mechanisms of penetration, nucleic acid biosynthesis and release differ
- after binding to host receptors, animal viruses enter via endocytosis (engulfment by host cell) or through membrane fusion (viral envelope with host membrane )

Question 13

what is tissue tropism?

Answer 13

Viruses are host specific - only infect a certain host and certain types of cells within tissues