TOB S4 - Viruses and Early Embryonic Development 1 Flashcards
How big are viruses?
- Submicroscopic: 18-350 nm - Can’t be seen with an ordinary light microscope; only with an electron microscope - Can pass through filters that trap even the smallest bacteria
Are viruses living?
No, they are obligate intracellular parasites - Have no genes that encode for proteins that function as the metabolic machinery for energy generation or protein synthesis - May or may not contain the genes that encode enzymes involved in nucleic acid synthesis
How do viruses replicate?
They hijack the host’s mechanism to support their own replication
Describe the biochemistry of viruses
- Have DNA or RNA, not both. RNA viruses less stable - Have no small ions or polysaccharides - May (enveloped viruses) or may not (naked viruses) contain lipids
Give some examples of viral shapes
- Adenovirus - Papillomavirus - Parvovirus - Morbillivirus - Herpesvirus - Parainfluenzavirus - Influenzavirus - Poxvirus - Filovirus - See images ppt slides 15-17
How do the growth curves of bacteria and viruses compare?
Bacteria (A) and viruses (B) have very different growth curves
What is the basic structure of a virus?
- Nucleic acid of RNA or DNA is the genetic information for the virus - Single (SS) or double (DS) stranded - Linear, circular or nicked - Unsegmented or segmented (e.g. influenza; allows reassortment in mixed infections, leading to new strains) - SS RNA can be plus (+) or minus (-) sense: - +RNA: genomic RNA can serve as mRNA and be directly translated into protein - -RNA: genomic RNA cannot serve as mRNA and cannot be translated directly into protein (for example Ebola)
How is specific diagnosis of most viral infections achieved?
By molecular detection of their genomes (e.g. by PCR)
What is a long term survival strategy of viruses?
RNA ➡️ DNA Reverse transcriptase enzyme
What are the two types of SS RNA genomes?
+ve (sense) -ve (anti-sense)
What is a capsid?
- Protein outer coat of a virion (common to all viruses) - Capsid composed of in visual subunits called capsomers (vaccines can be made out of these)
What are the two basic capsid structures?
- Icosahedral - Helical
What is the function of the capsid?
- Protects delicate inner nucleic acid from harsh environmental conditions - May be involved in attachment to host cells
What is a nucleocapsid?
A capsid with a genome
What is bacteriophage T4?
- A complex virus (viruses affecting our cells are simple in comparison) - Involved in transfer of drug resistance - Injects DNA through cell wall of bacteria
What is required for a virus to successfully infect a host cell?
- Cell must contain the receptor that the virus bunds to in the process of initiating an infection. The part of the virus that binds to the receptor is called a ligand. The ligand is on the envelope of enveloped viruses. - Glycoprotein gp 120 (the ligand) on HIV binds to CD4 (receptor) and CXCR4 (co-receptor) on T lymphocytes or CCR5 (co-receptor) on macrophages - Glycoproteins in the envelope of Ebola appear to bind a cholesterol transporter in target cells
What is required for a virus to successfully replicate in a host cell?
- Host must contain not only the receptor for the virus, but also the cellular machinery that the virus needs for replication - If the virus successfully replicates in the host cell, the infection is productive and the host cell is said to be permissive for the virus
What is the host range of a virus?
The spectrum of host cells that the virus can successfully infect and replicate in
How are viruses classified?
Most commonly used classification scheme is the Baltimore scheme - based on the relationship between the viral genome and the mRNA used for translation during expression of the viral genome
Give classifications of viruses by nucleic acid type and envelope as mainly used in medical virology
Enveloped viruses are generally more susceptible to disinfectants than non-enveloped
What are the effects of a virus on the host cell?
- Even though can’t see the virus, you may be able to see the effects that the virus has on the host cell - Death of cell: often occurs on release of virus - Cytopathic effects - Many enveloped viruses produce no direct light microscope observable cytopathic effects e.g. HIV and Ebola - Cancer
What are cytopathic effects?
- Visible effects on the host cell caused by viral replication - Inclusion bodies: site of active virus synthesis (Negri bodies of rabies virus) - Syncytia formation: giant, multinucleated cells formed by the fusion of plasma membranes - Chromosomal damage: stop host cell function and direct towards viral replication - Inhibition of host cell protein, RNA and DNA synthesis
How does a virus cause cancer in the host cell?
- Requires that virus integrates all or part of its genome into the host cell DNA (drugs have no effect on integrated DNA) - Only RNA viruses that are retroviruses can cause cancer: they bring in / turn on cellular oncogenes that cause cells to proliferate uncontrollably. - Many DNA viruses can cause cancer, but usually do it in a non-permissive cell (lacks something required for viral growth). They usually inactivate tumour-suppressor proteins that normally act to keep the cell and going through the cell cycle. Thus cells start going through the cell cycle and proliferating
How does Ebola work?
- Damage in infection is often due to ineffective host responses rather than direct toxicity of the microbe - In Ebola, the virus both reduces the effective immune response and enhances unproductive inflammation - These complex interactions lead to coagulation failure and haemorrhage - Specific immune response is often used as an alternative method of diagnosis to genome detection
What happens on Day 4?
- Compaction - Formation of first cavity - Blastocyst
What has happened by the end of Week 2?
- Conceptus has implanted - Embryo and its two cavities (amniotic cavity and yolk sac) will be suspended (connecting stalk) within a supporting sac (chorionic cavity)
What happens on Day 8?
- Bilaminar disk - Syncytiotrophoblast minimises barriers to diffusion and active transport
