Biology A2.3 - viruses (HL)

outbreak

unexpected increase in number of people with a specific condition

epidemic

spreads over a large geographical area

pandemic

spreads globally and has exponential growth

phage

infect

parts of a bacteriophage

describe bacteriophage

• small fixed shape

• contain a nucleic acid and either DNA or RNA as genetic material

• enclosed in a boundary - capsid

• no cytoplasm in the capsid

• some possess some enzymes

• if they possess sing or double stranded, they possess very few genes

  • most contain less than 100 genes in their caspid

describe capsid

• made up of proteins

• protein shell

• surrounded by DNA or RNA and protects it

• helps with the attachment to host cells

• delivers genetic material into the host cell

• protects genetic material

  • all viruses have a capsid

describe coronavirus

membrane envelope

• outer layer surrounding some viruses

• made of phospholipid bilayer

• helps the virus enter host cell receptors

• enables entry into the hosts cell

• protects the capsid

• not all viruses have this

non enveloped virus

• no membrane envelope

• made of DNA or RNA and a capsid

  • enters the host cell via endocytosis or injection

  • adenovirus

enveloped virus

• has a membrane envelope surrounding the capsid

• membrane envelope is the outer layer of the cell

• enters the host cell via endocytosis or membrane function

• lentivirus

bacteriophage lambda

• affects the bacteria

  • attaches to specific regions on the bacterium

bacterium lambda key features

• infects E coli

• double stranded DNA genome

• switch between 2 life cycles (lytic and lysogenic)

• uses different sites to infect the host at different locations

• regular genes that control which pathway it chooses

• important model organism in biology

key features in a corona virus

• spherical shape

• single stranded RNA as its genetic material

• envelope outside capsid

• projections of spike proteins at the envelope

examples of past pandemics

• black death

• flu pandemic

• spanish flu

• asian flu

  • AIDS

what all viruses must do to reproduce

1. attach to a site on a specific host cell

2. incorporate their genetic material into the cytoplasm of the host cell

3. use the host cell’s proccesses to produce components of themselves

4. assemble the viral components into new functioning virus particles

5. release the new virus entities into the host cell’s environment

why cant viruses produce their own energy

they do not have a mitochondria and only rarely have enzymes

lytic cycle

1. attachment- phage attaches to the surface of the host'

2. penetration- DNA enters the host cell

3. biosynthesis - phage DNA replicates and phage proteins are made

4. maturation - new phage particles are assembled

   1. lysis - the cell lyses which releases the newly made phages

lysogenic cycle

1. the phage infects the cell

2. the DNA becomes incorporated into the hosts genome

3. the cell divides and the prophage DNA is passed onto daughter cells

4. DNA is made from bacterial chromosomes and enters the lytic cycle

5. DNA replicates and phage proteins are made

6. new phage particles are assembled

7. the cell splits which releases newly made phages

difference between the two cycles

lysogenic cycle:

• host cell remains alive

• virus remains dormant

• DNA integrated into the host’s DNA

• no viruses produced until activated

• cell does not split

lytic cycle:

• host cell is destroyed

• virus immediately takes over

• replicated immediately

• viruses produced quickly

• cell splits

evidence on the origin of the first viruses

• first virus hypothesis: originated before cells

• regressive hypothesis: viruses were once small cells that became parasites of larger cells

• escape hypothesis: parts of genetic material escaped from larger organisms and became surrounded by an outer boundary

convergent evoloution

when different species develop similar features because they live in similar environments. this helps them survive and do the same job, but they evolved independently

features that suggest evidence of convergent evoloution in viruses

• are obligate parasites, none can replicate or carry out the functions of life inidividually

• have a protein outer boundary - the capsid with no cytoplasm

  • have genetic material inside the capsid and the code is shared between viruses

rapid evoloution examples

• influenza

  • HIV

antigenic drift

• small and slow

• small gradual changes in a virus antigens

• caused by random mutations during replication

• happens continuously over time

• common in influenza viruses

  • results in slightly different strains which is why flu vaccines need updating

antigenic shift

• big and sudden

• sudden major change in a virus’s antigens

• caused by mixing of genetic material between different virus strains

• happens quickly

• may lead to pandemics because people have little or no immunity

  • can create a new virus strain