Cell Theory Flashcards
Cell theory was developed in 1939. At that time, all living things were classified as either animals or plants. What 2 organisms had not been discovered yet?
Bacteria and Viruses
In the late 1700s, what did Dr Edward Jenner, the man who discovered the first vaccine against smallpox, use the term ‘virus’ to describe?
The contents of pustules in people suffering from smallpox.
What did Dr Edward Jenner do wrong when using the term ‘virus’?
He used it in its Latin meaning: poison
By 1855 Louis Pasteur had observed small single-celled entities that he called germs, which could cause certain diseases. In fact, what were these actually?
Bacteria
What experiment did Louis Pasteur do to conclude that the infecting agent which caused rabies was smaller than bacteria?
Pasteur used a very fine mesh to filter the spinal fluid of rabbits infected with rabies. This mesh had very small holes that prevented bacteria from passing through. However, this filtered spinal fluid, when injected into healthy rabbits, gave them rabies. By doing this, he concluded that the infecting agent which caused rabies was smaller than bacteria.
Rabies is caused by a virus belonging to a group of viruses called ____________.
Lyssavirus
Viruses have the following 5 properties:
- They do not have a cell structure - they have no cytoplasm, membranes or organelles.
- They have genetic material - either DNA or RNA, but not both.
- Their genome is enclosed in a protein coat made of capsomeres.
- Some have a lipid membrane, around the protein coat, derived from the membrane of cells that they infect.
- They can only reproduce when inside a living host cell.
Describe how viruses genetically modify other living organisms
Viruses infect other cells by inserting their genomes into the host cell’s genome. Some viruses, e.g. influenza, infect more than one species of host and can carry genes from one species to another in the process. They are nature’s genetic engineers.
Describe why viruses may be considered as not alive
Cell theory states that all living organisms are made up of cells. We know that viruses do not have a cell structure. They are described as akaryotes. So, on that basis, we should say they are not alive.
What is some evidence that suggests that viruses are alive?
- They have DNA or RNA and can replicate - albeit with help from a host cell. This is also true of two groups of very small bacteria, Chlamydia and Rickettsia, which can only reproduce when inside another living cell, but they are classed as living organisms.
- Also bear in mind that cell theory was developed in the 1830s, before viruses were discovered. Perhaps, in time, cell theory will have to be modified so that viruses can better fit into the big picture of life on Earth.
