Viruses

Central Dogma of Biology

describes the process which expresses genes into proteins. 

Dictates that DNA can be converted into RNA and RNA into protein, or RNA into DNA, but proteins can never code for genetic information. 

What makes up a virus

1. Core; genetic material DNA OR RNA (not both)

2. Capsid; Protective protein coat surrounding core, made up of many capsomeres, common shape is an icosahedron 

3. Nucleocapsid: core + capsid 

4. Envelope; some virus’s have a lipoprotein layer around capsid derived from the host cells membrane 

Reproduction

obligate endoparasites → dont have ribosomes or enzymes to synthesize protein coats or DNA

highly specific → only attacks certain cells or affects a certain species

all viruses are agents of disease

Living or Non-living (TANTI)

• No cell structure

• require a host cell to replicate

• dont respond to changes in the environment

• non living organisms

Lytic Cycle

1. Absorption

   1. adhesins bind to host receptors

2. Penetration

   1. viral genome is transported through the sheath to enter the hosts cytoplasm

3. Biosynthesis

   1. viral DNA is replicated using DNA replication enzymes of host cell

   2. Viral DNA takes over protein-synthesizing machinery

4. Assembly or Maturation

   1. replicated genome and new capsids assemble forming new virions

5. Lysis or Release

   1. Host cell ruptures - viral DNA instructs production of enzyme

(eg T4 bacteriophage) 

Lysogenic Cycle 

steps of lytic 

However before biosynthesis there is a period of integration

• where the virus is latent and each time the host cell divides the virus’s genome is copied 

• Dormant stage is called the provirus

  • induction event - activates virus (eg changes in temp)

Phage Lambda

Retroviruses 

• genome comprises RNA

• reverse transcriptase - turns RNA into DNA

• integrase - integrates DNA into host' cells DNA

---

Human Immunodeficiency Virus 

virus approaches a T-helper lymphocyte 

undergoes the lysogenic cycle 

after a latency period virus activates 

rna and viral proteins are then assembled to continue to produce new viruses 

How do mutations occur

• In retroviruses reverse transcriptase makes many mistakes leading to incorrect copying of RNA to DNA

• this leads to mutations

Life Cycle of a Retrovirus

1. virus approaches a helper t-lymphocyte

2. viral glycoproteins attach to the cell surface membrane

3. enter via a endocytosis

4. RNA + reverse transcriptase are released into host cell

5. RNA is synthesized into DNA

6. DNA moves into the nucleus and integrates with the hosts DNA

7. every time the cell divides, the viral dna divides with it, increasing the number of host cells infected

8. after a latency period (5yrs) the virus becomes active again

9. using the hosts protein synthesizing material creating RNA and viral proteins assembling new viral particles

10. through exocytosis the viruses are released

label.

a. glycoprotein - targeting and binding with the host cell.

b. envelope

c. viral genome

d. nucleocapsid/capsid/protein coat

label + name virus 

A. Polyhedral Head/Capsid 

B. Base Plate 

C. Tail Fiber 

→ this is a bacteriophage