Parasitic Lifecycles

Question 1

Give examples of common parasites.

Answer 1

protists - e.g. amoebas 
platyhelminthes - e.g. tapeworms
nematodes - e.g. threadworms 
arthropods - e.g. ticks
bacteria - e.g. tuberculosis 
viruses – e.g. HIV and Influenza

Question 2

How are ectoparasites such as ticks and lice are usually transmitted from host to host?

Answer 2

via direct contact.

Question 3

How are endoparasites of the main body cavities, such as the gut, be transmitted?

Answer 3

by direct contact, often caused by poor hygiene.

Question 4

How are endoparasites of the gut are transmitted?

Answer 4

through consumption of secondary hosts.

Question 5

How are endoparasites of the body tissues usually transmitted?

Answer 5

by vectors.

Question 6

Describe schistosomiasis.

Answer 6

Schistosomiasis - also known as bilharzias. This disease is caused by flatworms of the Schistosoma species which live in fresh water in sub-tropical and tropical regions of the world. Presenting as a skin rash and cough.

Question 7

Describe malaria.

Answer 7

Malaria - a serious tropical disease spread by mosquitoes that are infected with the malaria parasite Plasmodium of which there are several different species. Sometimes, the parasite can exist in a dormant state, often hiding in the liver cells so is undetected by the body’s immune system, allowing future recurrence. In the Plasmodium falciparum, the parasite has adhesive proteins on its surface, allowing it to stick to the inner wall of the blood vessels. This also allows it to remain undetected by the immune system.

Question 8

What are viruses?

Answer 8

Viruses are infectious agents that can only replicate inside a host cell.

Question 9

What is a bacteriophage?

Answer 9

A bacteriophage is a virus that uses a bacterium as a host cell.

Question 10

Describe viral replication.

Answer 10

1. Virus attaches to the surface of the host cell.
2. Virus injects its DNA or RNA into the host cell.
3. Virus interrupts the host cell’s own metabolism, often entering the host’s genome.
4. Virus uses the host cell’s machinery and raw materials to replicate the DNA.
5. Again, using the host cell’s resources, the DNA is transcribed into mRNA then translated into protein so more viral protein coats are produced.
6. The new DNA then enters the newly formed protein coats, thus producing many new viruses.
7. These then leave the cell to infect new cells and the host cell undergoes lysis, bursting.

Question 11

Describe retroviruses.

Answer 11

Retroviruses are viruses with RNA rather than DNA. E.g. HIV.
On injecting their RNA into the host cell, they also inject the enzyme reverse transcriptase to first synthesise DNA from the single-stranded RNA. This new DNA is then inserted into the genome of the host cell. As part of the host’s DNA, the virus’s genes can then be transcribed, ultimately synthesising new viral particles.