Virus Structure and transmission L12 Flashcards
what are the features of viruses
- DNA/RNA genome
- can be single or double stranded, circular or linear - capsid protein
- made from multiple capsomeres
- protect the genome and can be used to enter host if virus does not have envelope - envelope
- lipid bilayer formed via budding from membranes - surface proteins
What are the characteristics of Viruses
ALL viruses are OBLIGATE intracellular parasites. They can only grow (ie replicate) in appropriate HOST CELLS.
- carry very few (if any) enzymes for replication, transcription and translation. Need to use host cell enzymes and ribosomes
a virus will usually have a specific host
what is a VIRION
a purified virus particle.
Virions can be seen using electron microscope.
- simplest life form
contain numerous copies of one or more different PROTEINS and one or more NUCLEIC ACID MOLECULES which acts as the virus GENOME.
what are the characteristics of virus genome
The virus GENOME encodes viral PROTEINS.
The VIRUS GENOME is enclosed in a protein shell called a CAPSID made up of many identical CAPSOMERES
NUCLEOCAPSID is the name given to the genome enclosed by the capsid
what are the 5 basic structures of viruses
- Naked icosahedral
- just genome and capsid
e.g. poliovirus - Naked helical
e.g. tobacco mosaic virus. - Enveloped icosahedral
e.g. herpes virus, - Enveloped helical
e.g. measles virus - Complex e.g. poxviruses
why study viruses
- high death rate
- economic hardship
- they can be exploited for our benefit. For example the Oxford/Astra Zeneca COVID-19 vaccine uses a harmless adenovirus vector to deliver the gene for COVID-19 spike protein. Bacteriophage are also useful lab tools.
what is the cycle of a virus
- transmission
- when they are host vulnerable - entry
- Binds to receptor on target, enters cell - primary site replication
- spread within host
- shedding
what are the 3 modes of transmission used by viruses
- Horizontal transmission (person to person)
- Vertical transmission (mother to baby)
- Zoonosis (animal to man)
give 5 examples of horizontal transmission
- respiratory
- faecal-oral
- sexual
- urine
- mechanical
explain respiratory transmission
Droplets may:
- be directly inhaled
- infect conjunctiva
- fall onto surfaces and be transferred on fingers to mouth, nose or conjunctiva
- be dessicated (virus inactivated)
Droplet size is important in determining route of transmission
common
rhinovirus, influenza, measles, coronavirus
explain faecal-oral
Very efficient mode of transmission: viruses grow to high titres
Rotaviruses: vomiting, diarrhoea and fever
Hepatitis A: hepatitis (inflammation of liver) leading to jaundice
Poliovirus: 99% of cases result in no symptoms or mild fever & sore throat; ~1% of cases develop irreversible muscle paralysis (paralytic poliomyelitis)
explain sexual
Few viruses are spread by sexual transmission – but those which are pose a serious health risk
- HIV
- HBV
hepatitis, liver cirrhosis, liver cancer
- HPV
Human papilloma virus (HPV) – particularly types 6,11,16 &18: types 6 & 11 - genital warts; types 16 & 18 – cervical and penile cancer
-HSV
Herpes simplex virus (HSV) – predominantly type 2, but also type1: painful blisters on penis, vulva, vagina and cervix; neonatal transmission can result in fatal infection of brain and liver.
-
explain urine transmission
Urine usually sterile. Viruses shouldn’t cross glomerular barrier.
- rare
cytomegalovirus and poliovirus
explain mechanical transmission
Blood-borne viruses (HIV, Hepatitis B virus, Hepatitis C virus) can be transmitted by any activity which involves transfer of blood from one person to another
eg. intravenous drug abuse, blood transfusion, tattooing, body piercing, surgery, insect bite(?)
when can vertical transmission happen
- Can be primary infection of mother while pregnant, or reactivation of a persistent infection
- Transplacental
eg Rubella (german measles): congenital defects including cardiovascular, hearing and sight - During birth (perinatal transmission)
eg Herpes simplex (both type 1 and type 2 can be transmitted at birth – type 1 is most frequent cause of vertical transmission) - After birth (post natal)
eg. transmission of HIV-1 in breast milk, Hepatitis B Virus in saliva
