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1

Virus

Obligate intracellular parasite

2

Components of a virus

Head, Tail fibers, genome (nucleic acid)
Capsid (protein shell containing genetic information)

3

Basic steps of Virus

1. Attachment (adsorption)
- specific
- host not yet infected

2. Injection (Penetration)
- host is infected

*Following this, one of 3 life cycles take place*

4

3 different life cycles

Lytic, lysogenic & productive life cycle

5

Lytic Cycle

1. Transcribe & translate the viral genome
Early genes: hydrolase, capsid proteins

2. Replicate the viral genome

3. Lysis of host & release of new viral particles
Late genes: lysozyme

6

Lysogenic Cycle

1. Integrate viral genome with hos genome
2. Normal host activity, including reproduction
3. Excision & lytic cycle

7

Advantages to productive cycle

1. Protection from immune system
2. Easy infection (envelope can fuse with another membrane)
3. Get more virus (long life)

8

Why does virus want to kill the hose genome in lytic cycle?

1. All cellular energy used to work with the viral genome
2. Create a pool of free dNTPs to use in viral genome replication

9

Why does virus want to kill the hose genome in lytic cycle?

1. All cellular energy used to work with the viral genome
2. Create a pool of free dNTPs to use in viral genome replication

10

While transcribing & translating host genome, how does the virus make sure its genome is not expressed in the lysogenic cycle?

Viral genome codes for repressors to make sure it's not expressed

11

Lysogen

When virus incorporated in the host genome, host is referred to as lysogen

12

Provirus/Prophage

Host genome and viral genome are incorporated together

13

Animal virus

Virus with an envelope (productive cycle)

14

Subviral particles: Prions

Proteins (both good ones and mutant ones)
Characteristics:
- no DNA or RNA
- non membranes
- no organelles
- very small
- extremely stable

15

How do mutant (bad) prions come?

1. Spontaneous mutation
- mutation in gene coding for mutant prion and then prions accumulate & lead to cell death

2. Be genetically transmitted
- can have bad prion & pass off to offspring

3. Ingested through diseased tissue
- Healthy cell transcribing & translating normal "good" prion
- Consumption of mutant "bad" prion from diseased tissue
- Mutant "bad" prion changed normal "good" prions to bad ones
- Cell death

16

Subviral particles: Viroids

- virus-like
Characteristics:
- circular RNA
- no capsid
- must be coinfected
- don't code for proteins
- act as miRNAs or siRNAs to block translation

17

Mechanism of viroid

Mechanism 1
1. +RNA circular form as template to make -RNA
2. -RNA linear form
3. Ligase to make a circular -RNA
4. Use -RNA as template to make +RNA
5. + RNA makes more circular +RNA viroids

Mechanism 2
1. +RNA circular form as template to make -RNA
2. -RNA is template to make +RNA linear form
3. Multiple +RNA linear form
4. Circularized to form more viroids

*Both mechanisms equally preferred*

18

How viral genome (+RNA or -RNA) is made

Mechanism 1
1. +RNA (same as mRNA) can either be translated using host ribosomes to form viral proteins OR complementary form using RNA dependent RNA polymerase to make -RNA
2. Use RNA dependent RNA polymerase to make more +RNA
*In this situation, the virus will code for the enzyme on its +RNA genome

Mechanism 2
1. -RNA available and you use RNA dependent RNA polymerase to make +RNA
2. +RNA is now available to translate using host ribosomes to make viral proteins OR to make complementary -RNA using RNA dependent RNA polymerase
*In this situation, the virus will have to carry then code for the enzyme

Mechanism 3 (+RNA lysogenic)
1. +RNA that will create ssDNA using RNA dependent DNA polymerase
2. ssDNA will replicate using host DNA polymerase to make dsDNA
3. dsDNA can be inserted into host cell genome which can then be transcribed and translated using host enzymes to make viral proteins OR transcribed using only hosr RNA polyermases to make +RNA

19

Problems with +RNA lysogenic

1. Very rapid mutation
2. Will permanently be in host genome

Ex. HIV

20

Bacteria: Structure
Shape

1. Round - coccus
2. Elongated - bacillus
3. Spiral - spirochetes

21

Bacterial: Structure
Flagella

____"trichous"
- mono (1 flagella)
- amphi (2 flagella)
- peri (multiple flagella)

Have basal unit connected to cell membrane
- hook is connected to basal unit
- flagella is connected to hook
As hook rotates, the cell moves

22

Chemotaxis

Moving towards favourable chemicals & away from unfavourable chemicals

23

Bacteria: Structure
Cell wall/ cell membrane

Gram positive - stains purple
- has cell membrane surrounded by thick cell wall
Gram negative - stains pink
- has thin inner cell membrane, then cell wall, then outer cell membrane

24

Is gram + or gram - harder to kill with antibiotics?

Gram - because it has antibody degrading enzymes in periplasmic space (inbetween OM and IM)

25

Mesophiles

Bacteria that live in medium temperature

26

Psychrophiles

Bacteria that live in cold temperature

27

Thermophiles

Bacteria that live in hot temperatures

28

Faculative Anaerobe

Oxygen Present - use it to survive
Oxygen Absent - ferment & survive

29

Obligate Aerobe

Oxygen Present - use it to survive
Oxygen Absent - Die

30

Obligate Anaerobe

Oxygen Present - Die
Oxygen Absent - ferment & survive