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MBBCh 1- APES1001A > Viruses > Flashcards

Viruses Flashcards

1
Q

What is a virus?

A

> A virus is an infectious particle that reproduces by “commandeering” a host cell and using its machinery to make more viruses.

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2
Q

What is a virus made up of?

A

> A virus is made up of a DNA or RNA genome inside a protein shell called a capsid.

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3
Q

> Some viruses have an external membrane _____.

A

envelope

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4
Q

How are viruses diverse?

A

> Viruses are very diverse. They come in different shapes and structures, have different kinds of genomes, and infect different hosts.

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5
Q

How do viruses reproduce?

A

> Viruses reproduce by infecting their host cells and reprogramming them to become virus-making “factories.”

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6
Q

How complex are viruses?

A

-Viruses are much smaller and simpler than eukaryotic and even prokaryotic cells.
o Lack structures and metabolic machinery found in a cell.

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7
Q

Viruses consist of ____ in a protein coat. (packages of nucleic acid and protein)

A

genes

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8
Q

Are viruses living?

A

-Because they can’t reproduce by themselves (without a host), viruses are not considered living. Considered between life forms and chemicals

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9
Q

How do viruses hijack host cells?

A

• Viruses hijack or invade host cells by injecting their genetic information into a cell, recruiting cellular machinery to manufacture many new viruses and promote further infection.

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10
Q

How are viruses helpful? (3)

A

o Helpful in first understanding genes and genetic material+replication+protein synthesis
o Helpful in understanding diseases
o Gene therapy

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11
Q

What is the structure of a virus? (4)

A
  • A protective protein shell, or capsid
  • A nucleic acid genome made of DNA or RNA, tucked inside of the capsid
  • A layer of membrane called the envelope (some but not all viruses)
  • The structure of a virus consists of a nucleic acid enclosed by a protein coat and sometimes surrounded by a membranous envelope
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12
Q
  • Range from 20nm to several hundred nm in dimeter—>too small for ____ microscope
  • The _____ virus is 1500nm in diameter
A

light

largest

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13
Q

What is the viral envelope?

A

Viral envelope - membranous envelope surrounding the virus derived from membranes of the host cell.

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14
Q

What does the membranous envelope contain? (2)

A

o Contain host cell phospholipids and membrane proteins.

o Contain proteins and glycoproteins of viral origin.

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15
Q

What is the name of the enzymes in the capsids of some viruses?

A

> Some viruses carry enzymes in their capsids. E.g viral polymerase.

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16
Q

Where are the most complex capsids found?

A

> Most complex capsids are found among bacteriophages. (phages)
o Viruses that infect bacteria.

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17
Q
  • they “borrow” a patch from the host membranes on their way out of the cell. _____ do, however, contain proteins that are specified by the virus, which often help viral particles bind to _____ cells.
A

Envelopes

host

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18
Q

What is a capsid?

A

A protein shell enclosing the viral genome is known as a capsid.

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19
Q

What are the different shapes of virus capsids? (3)

A

o Depending on the type of virus the capsid may be :

  • rodshaped
  • polyhedral
  • or more complex (bacteriophage)
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20
Q

What are capsids made of?

A

o Built from a large number of protein subunits called capsomeres.

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21
Q

o Rod-shaped viruses—> helical viruses: _____ mosaic.

A

Tobacco

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22
Q

How are adenoviruses arranged?

A

o Adenoviruses—>arranged in a polyhedral capsid with 20 faces: infect respiratory tract —> ICOSAHEDRAL SHAPE

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23
Q

Where are the most complex capsids found?

A

Most complex capsids are found on bacteriophages which contain an elongated polyhedral head attached to a protein tail with fibres.

24
Q

The genome of viruses may be: (4)

A
  • double-stranded DNA,
  • single-stranded DNA,
  • double-stranded RNA or
  • single-stranded RNA—> depends on the type of virus
25
Q

What are viruses classified as based on their genome?

A

Viruses are classified as either DNA or RNA viruses based on their genome

26
Q 
In either case, the genome is usually organized in a \_\_\_\_\_ linear or \_\_\_\_\_ molecule of
nucleic acid (although some viruses can have multiple molecules)
A

single

circular

27
Q
  • Some viruses have multiple molecules of _____ _____.

- Number of genes ranges from __ in the smallest viruses to _____ in a large virus.

A
  • nucleic acid

- 3, 2000

28
Q

Notably, DNA and RNA viruses always use the same ____ ____ as living cells. If they didn’t, they would have no way to reprogram their host cells!

A

genetic code

29
Q

General features of Viral Replicative Cycles. (9)

A
  1. A viral infection begins when a virus binds to a host cell and the viral genome makes it way
    inside
  2. The method of genome entry depends on the type of virus and type of host cell
  3. Example bacteriophages use their tails to inject their DNA into a bacterium. Other viruses are taken up by endocytosis or by fusion (in the case of enveloped viruses) of the viral envelope and host membrane
  4. Once inside the genome can reprogram the cell to replicate the viral genome and manufacture viral proteins
  5. The host provides the nucleotides, enzymes, ribosomes, tRNAs, amino acids, ATP and other components needed for viral protein synthesis
  6. DNA viruses use host cells’ DNA polymerases to synthesize new genomes, using the viral DNA as a template
  7. Whilst RNA viruses use virally encoded RNA polymerases that can use RNA as a template
  8. Once the nucleic acids and capsomeres are produced they assemble spontaneously forming
    new viruses
  9. The process ends with new viruses exiting the cell, which usually results in the death of the host cell, the viruses can now spread and infect other host cells
  10. This death and damage accounts for viral symptoms
30
Q

What do viruses lack?

A

• Viruses lack metabolic enzymes and equipment for making proteins, such as ribosomes

31
Q

What is the lytic cycle?

A

The lytic cycle: The phage infects a bacterium, hijacks the bacterium to make lots of phages, and then kills the cell by making it explode (lyse).

32
Q

What is the lysogenic cycle?

A

The lysogenic cycle: The phage infects a bacterium and inserts its DNA into the bacterial chromosome, allowing the phage DNA (now called a prophage) to
be copied and passed on along with the cell’s own DNA.

33
Q

What does the lytic replicative cycle result in?

A

A phage replicative cycle that results in the death of a host cell.

34
Q

What does the term lytic refer to?

A

•The term lytic refers to the last stage of infection, where the bacterium lyses (breaks open) and releases the phages produced within the cell.

35
Q

What is each phage capable of?

A

• Each of the phages can then infect a healthy cell, and few lytic cycles can destroy an entire bacteria population in a few hours

36
Q

What is a virulent phage?

A

A phage that only replicates by a lytic cycle is a virulent phage.

37
Q

Reasons why all bacteria have not been exterminated can be attributed to: (3)

A

oNatural selection which favours mutant bacteria with surface proteins no longer recognizable by phages
oRestriction enzymes cutting up viral DNA upon entering cell
oHowever, phages can also benefit from mutations, thus the parasite-host relationship is in a constant evolutionary flux

38
Q

What is a prophage?

A
  • The integrated phage DNA, called a prophage, is not active: its genes aren’t expressed, and it doesn’t drive production of new phages.
39
Q

What happens each time a host cell divides?

A
  • However, each time a host cell divides, the prophage is copied along with the host DNA, getting a free ride.
40
Q
  • The ________ cycle is less flashy (and less gory) than the ______ cycle, but at the end of the day, it’s just another way for the phage to reproduce.
  • Under the right conditions, the ______ can become active and come back out of the bacterial chromosome, triggering the remaining steps of the lytic cycle (DNA copying and protein synthesis, phage assembly, and lysis)
A

lysogenic
lytic
prophage

41
Q

What does the lysogenic cycle allow for?

A

The lysogenic cycle allows replication of the phage genome without destroying the host.

42
Q

What are temperate phages?

A

Phages capable of using both modes of replication are called temperate phages (also called lambda).

43
Q

When does the infection of bacteria bu this phage begin?

A

Infection of bacteria (E. coli) by this phage begins when the phage binds to the surface of the cell and injects its linear DNA genome.

44
Q

Describe the temperate stage process. (7)

A
  1. Within the host the DNA forms a circle
  2. What happens next depends on the replicative mode (lytic or lysogenic)
  3. During a lytic cycle the viral genes immediately turn the host cell into a lambda-producing factory, and the cell soon lyses
  4. During a lysogenic cycle, the lambda DNA is incorporated into a specific site in E. coli chromosome, in this way the viral DNA is known as a prophage
  5. However, this prophage remains silent in the bacterium until required
  6. Every time the E. coli divides, it replicates the phage DNA as well as its own DNA, such that each daughter cell inherits a copy of the prophage
  7. This gives rise to large population of bacteria carrying the virus in prophage form
  8. The mechanism allows viruses to propagate without destroying host cell on which they depend
45
Q

When are active phages generated?

A
  • This usually occurs when the lambda genome is induced to exit the bacterial chromosome and initiate a lytic cycle
  • An environmental signal usually triggers the switchover from lysogenic to lytic mode
46
Q

What does the replicative cycles of animal viruses depend on?

A

-The replicative cycles of animal viruses depend on the type of viral genome (DNA or RNA) and on the presence of a viral envelope.

47
Q

How does a virus equipped with an envelope use it to enter a host cell? (6)

A
  • Protruding from the outer surface of this envelope are viral glycoproteins that bind to specific receptor molecules on the surface of a host cell (see figure below for cycle)
  • Ribosomes bound to the ER of the host cell make the protein parts of the envelope glycoproteins whilst enzymes in the ER and Golgi body add the sugars
  • These viral glycoproteins, derived from the host cell, are transported to the cell surface
  • In a process similar to exocytosis, new viral capsids are wrapped in membrane as they bud from the cell
  • In other words, the viral envelope is usually derived from the host cell’s plasma membrane
  • The enveloped viruses are now free to infect other cells, this cycle does not necessarily kill the host cell, in contrast to the lytic cycles of phages
48
Q

Where is the RNA genome transcribed?

A

The RNA genome is transcribed into complementary RNA strands, which function both as mRNA and as templates for the synthesis of additional copies of genomic RNA.

49
Q

What do all viruses than use an RNA genome require?

A

All viruses that use an RNA genome as a template for mRNA transcription require RNA—>RNA synthesis. These viruses use a viral enzyme capable of carrying out this process.

50
Q

What is a retrovirus?

A

The RNA animal viruses with the most complicated replicative cycles are retroviruses.

51
Q

What enzyme do retroviruses contain that is vital for their function?

A

These viruses contain an enzyme called reverse transcriptase, which transcribes an RNA template into DNA, providing an RNA—>DNA flow, the opposite to the usual direction.

52
Q

What happens after HIV enters the cell? (2)

A

• After HIV enters a host cell, its reverse transcriptase molecules are released into the cytoplasm, where they catalyse synthesis of viral DNA.
The viral DNA then enters the nucleus and integrates with the DNA chromosome—>this integration is called a provirus, which never leaves the host’s genome
• The Proviral DNA is transcribed and used as mRNA to synthesize proteins as well as genomes for new viruses

53
Q

Describe the cycle of the retrovirus HIV-1. (7)

A
  1. HIV-1 binds to its receptors on host cell
  2. Viral RNA, reverse transcriptase, integrase, and other viral proteins enter the host cell
  3. Viral RNA is reverse transcribed to make DNA
  4. Viral DNA enters the nucleus and integrates it into the host cell’s DNA
  5. New RNA act as viral genomes are translated to make viral proteins
  6. Viral RNA and proteins assemble at the cell surface, forming new HIV-1 particles
  7. A protease processes viral proteins to make a mature HIV-1 particle
54
Q

-They are of special importance because human immunodeficiency virus (HIV), the virus that causes acquired immune deficiency syndrome, or AIDS, is a ____.

A

retrovirus

55
Q
  • Anti-HIV drugs inhibit viral replication at many different phases of the HIV cycle. These drugs include: (4)
A

> Fusion inhibitors, which block fusion of the HIV viral envelope with the plasma membrane of the host cell
Reverse transcriptase inhibitors, which impair the conversion of the RNA genome into double-stranded DNA
Integrase inhibitors, which inhibit the integration of the viral DNA into the host genome
Protease inhibitors, which block processing of viral proteins

56
Q

_____ containing multiple drugs are usually most effective at slowing the progression of the infection and keeping viral levels low.

A

Cocktails

MBBCh 1- APES1001A (38 decks)

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