Introduction to Viruses-31

Question 1

Viruses are

Answer 1

acellular microorganisms, that must have a host to survive. Infects all cells (animal, plant, bacteria) to obligate intracellular parasites.
Most abundant biological entities on earth.

Question 2

Virus has 2 parts

Answer 2

Genetic material (DNA or RNA) Capsid.
Sometimes 3 an envelope of lipids which surrounds protein coat when outside cell

Question 3

Viral Capsid are made of

Answer 3

Capsomers (protein building block, arranged in precise and highly repetitive pattern around nucleic acid).

Question 4

Capsid/Capsomers can be Arranged in 3 types of symmetry

Draw them

Answer 4

Helical eg TMV (tobacco mosaic virus),

Icosahedral eg Adenovirus, Complex eg Bacteriophage

Question 5

Draw a Helical Capsid

Locate
DNA or RNA
Capsomer of capsid

Answer 5

18x250nm
eg TMV (tobacco mosaic virus)

Question 6

Draw an Icosahedral Capsid

Locate
DNA or RNA
Capsomer of capsid
Protein Spike

Answer 6

20 face polyhedron
each face equilateral triangle
70-90nm
eg Adenovirus,

Question 7

Draw a complex capsid

Locate 
DNA or RNA
Head
Tail Sheath
Tail Fibre

Answer 7

Head has icosahedral
Tail is helical
80x225nm
eg Bacteriophage

Question 8

Draw HIV virus infecting a cell

Answer 8

RNA Enveloped Virus has RNA genome (9 genes 2 identical ss copies of each).

1. HIV fuses with host cell membrane and digestion of capsid. Host cell CD4 T cell.
2. Reverse transcriptase (retro virus-converts RNA to DNA twice).
3. dsDNA incorporated into host genome=Provirus (prophage)
4. RNA genome for the progeny viruses. Viral proteins and envelope glycoproteins.

Question 9

Types of bacteriophages infections in bacteria

Answer 9

Lytic Infection (virulent)= host cell dies

Lysogenic Infection= host cell survives

Question 10

Viral genome is made up

• Size
• Shape

Answer 10

All 4 possible forms of RNA (ss or ds) and DNA (ss or ds).

4000to > 1 million nucleotides (3 genes to 100-1000).

Linear eg poxvirus,
Circular eg hepatitis B, Segmented eg influenza virus.

Question 11

Viruses infect

Answer 11

all cells (eukaryotic and prokaryotic) and 
forms of life (animals, plants, bacteria, fungi, algae).

Question 12

Bacteriophages are

Answer 12

viruses that infect and replicate in bacteria. Had potential for treatment for diseases like typhoid and cholera.

Question 13

Lytic (virulent) cycle of bacteriophage infection (cell death)

Draw

Answer 13

1. ) Attachment of bacteriophage (complex) to host cell. Tail fibres recognise receptors on target cell membrane.
2. ) Entry of phage DNA and degradation of host DNA.
3. ) Synthesis of viral genomes and proteins. (takes over host cells machinery).
4. ) self assembly, complex and highly regulated. Head, tail and tail fibres.
5. ) release. Cell bursts and destroyed. 100-200 particles (burst size)

Question 14

Lysogenic cycle

Draw

Answer 14

1. ) Daughter cell with prophage (bacteriophage genetic material) and phage DNA circularizes.
2. ) Circularized DNA in phage can leave genome and enter lytic cycle or enter lysogenic cycle.
3. ) Phage DNA integrates into bacterial chromosome. Phage genome is silent transcription of prophage genes is inhibited (not kill host).
4. ) Prophage copied with bacterial chromosome. Divided daughter cells have prophage
5. ) two things at same time. i. Many cell divisions create many infected bacteria. Ii. Start cycle again.

Question 15

Summary of HIV life cycle

6 steps

Answer 15

(i) entry and removal of capsid proteins
(ii) synthesis of a DNA strand complementary to the viral RNA
(iii) incorporation of double stranded DNA into the cell’s DNA
(iv) transcription of proviral genes and manufacture of viral proteins
(v) assembly of new virus particles
(vi) exit of new virus particles from the cell