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What are viruses considered and why?
They are the simplest biological entities on earth.
- They are obligate intracellular parasites because they can only replicate within a living cell
- 1) They are inert outside of the cell
- 2) They need to get into the cell, get in, make more copies, get outside, and repeat the process
The virion consists of:
a nucleic acid genome
a protective protein coat (capsid)
some viruses contain a lipid envelope
What do viruses lack?
- 1) They cannot make biological building blocks
- 2) They cannot make their own ATP; they lack the ability to make enzymes
- 3) They cannot make proteins on their own
Virus particles do what?
break down and release their genomes inside the cell (uncoating)
Viral genomes can be?
- RNA or DNA (not both)
- single or double stranded
- circular or linear
Virus replication cycle
- Viral replication is very different from cell division
- In order to make more copies, they have to __
- There is a period of time when there are __. It is simply __
completely break down into component parts—protein and DNA
no viruses present in the infected cell
protein and DNA or RNA
Analysis of viral macromolecules reveals what?
What are the steps?
the detailed pathways of virus replication
- 1) binding to cell receptor
- 2) entry and uncoating
- 3) early gene expression
- 4) replication of viral genomes
- 5) late gene expression
- 6) assembly of virions
- 7) exit
How are viruses very similar?
- They all more or less undergo all of these processes:
- 1) entry-- find and bind to cell, usually with receptor, and get inside
- ----> some are completely engulfed by the host cell
- 2) uncoating: nucleic acid becomes free from virus; naked DNA is present
- 3) As eries of steps that are conserved with a lot of viruses
- ---> make genes or proteins
- ---> replicate viral genome
- ---> make more genes
- ---> assembly of virions
- 4) exit
Why study viruses?
- They cause lots of diseases
- Usually, when a virus affects a cell, that cell ceases to become a functional cell
- --> It is hijacked and the cell just provides a place for replication of the virus
What else has the study of viruses led to?
numerous discoveries in molecular and cell biology such as:
- 1) DNA carries inherited genetic information
- 2) identification of promoters for eukaryotic RNA polymerase
- 3) Enzymes involved in cellular DNA replication
- 4) RNA splicing in eukaryotic cells
- 5) Isolation of numerous cellular oncogenes and the understanding that cancer is caused by their mutation or unregulated expression
Study of tumor viruses led to discoveries in molecular biology and understanding of the __, such as __ and __.
Viruses are now being used to __.
- nature of cancer
- reverse transcriptase
construct vectors to express proteins to destroy tumor cells
Viruses have __.
Viruses are also __. They are __. How so?
very small and simple genomes
- very good vectors; perfect tools for it
- Some have been used as gene therapy or vaccinesSome have been engineered to help fight cancer
What can you do with viruses and your immune system?
- You can infect immune system cells with it and fight off the cancer cells.
- Some have been made to ignore healthy cells, attack cancer cells, and replicate in cancer cells
Viruses were first distinguished from __ in the mid-twentieth century.
other microorganisms by filtration
How did filtration reveal viruses?
People had filters that could track bacteria and other life forms. Only liquid and other things could go through
Whatever caused the diseases was smaller than bacteria--> filterable agents were the culprits
Explain TMV crystallization.
The crystallization of tobacco mosaic virus challenged conventional notions about genes and the nature of living organism
Crystallization led to a picture of a virus for the first time, which ultimately led to the question are viruses alive
What things get crystallized?
Things that get crystallized: DNA and protein, which alive by themselves
Things crystallized are inert, repeating, and have a definite structure
You can't crystallize a cell nor an organism nor lipids; anything that moves or changes shape
What did this ability to crystallize a virus, which is inert, lead to?
Viruses are perched on the cusp of life and non-life
What did scientists do?
The "phage group" stimulated studies of bacteriophages and helped establish the filed of molecular biology
They used techniques, such as mapping phage genes, elucidating phage replication cycles, and developing the plaque assay
Since viruses are obligate intracellular parasites, what can't you do?
you can't grow them like you can with bacteria; you have to grow them in cells
When looking at virus populations, you look at plaque assays.
Take nutrients; grow cells on them and infect cells with the viruses; the viruses infect cells, kill them, and move on to another colony until the absence of bacteria is visible to the naked eye