Nucleic Acids and DNA Flashcards
Three key experiments to show that DNA is genetic material
- Griffith Experiment
- Avery, MacCleod, McCarty Experiment
- Hershey Chase Experiment (Bacteriophage T2and Radioisotopes)
The central dogma
1) DNA contains instructions
2) RNA transcribes DNA
3) proteins are made from the instructions
Frederick Griffith, 1928
Dr. Griffith described two strains of bacteria Streptococcus pneumoniae:
– lethal strain appeared smooth because it was surrounded by a polysaccharide coat
– non-lethal strain which does not have a polysaccharide coat, giving it a rough appearance.
Griffith’s experiment
Initially: Mice injected with the smooth strain would die, Those injected with therough strain would live.
Heat-killed S Strain mixed with R strain and the mouse dies. Even though the S strain was killed it
was able to “change” the R strain into a virulent strain
What was actually happening in griffith’s experiment?
S strain was heat killed, and the DNA and the outside coat fragmented. some of the DNA from the s strain entered the r strain. That piece of DNA is virulent, it holds the message for making the polysaccharide coat
Avery, Macleod and McCarty, 1944
Follow up on Griffith’s experiment.
– Demonstrated that this could be done in a test tube as well
We know that chromosomes are made up of protein and DNA (also knew about RNA in cells)
Treat the cells (S) with different enzymes to break down components and determine which has the transforming ability
– DNases - degrade DNA into individual subunits
– RNases - degrade RNA into individual subunits
– Proteases - degrade protein into individual subunits
Avery, Macleod and McCarty’s experiment
They added the enzymes to the killed S strain of bacteria, and then subsequently added it to the R strain. The tube with DNase was the only one on which the R strain wasn’t able to transform into the lethal S strain
Avery, Macleod and McCarty’s Conclusions
- DNase destroys ability of heated extract to transform Rstrain into lethal smooth (S) strain
- DNA must contain the transforming agent
- DNA must contain the genetic information
Radioisotopes
Elements are defined by the number of protons they possess
Electrons = protons in non-ionized forms #Neutrons can vary - this gives rise to isotopes
Radioisotopes that are commonly used in Molecular Biology
- P32- primarily used for labeling nucleic acids, can also be used for labeling proteins that have been phosphorylated
- S35- primarily used for labeling proteins
Detecting radioactive decay
Various particles are given off- The nucleus is unstable, and gives of particles to relieve the
tension
There are many detection methods
–geiger counters (detects what is being emitted into the air) typical of what you see in the movies - the beeping sound
– scintillation counters (radioactive emissions are converted to light)usually involves some sort of “swipe” and putting the cloth in scintillation fluid
– autoradiography (X-ray film) radioactivity exposes the film that is laid over
Viral genes entering a cell
1) Start of infection. Virus genes enter host cell. Protein coat does not.
2) Virus genes direct the production of new virus particles
3) End of infection. New generations of virus particles burst from host cell.
Hershey Chase Experiment (Bacteriophage T2 and Radioisotopes)
The experiment was to see if viral genes consist of DNA or protein (what’s the genetic material?)
In a virus, DNA was tagged with P32 and in a different set protein was tagged with S35, and both sets infected a culture of E. coli cells.the cultures were blended to separate the coats from the genes within bacterial cells. then they were it in a centrifuge, the bacterial cells settling at the bottom because they are heavier. In the tagged DNA set the radioactive DNA settled in the bottom with the bacterial cells wile the tagged protein was apparent in the solution with the coats.
Hershey Chase Experiment Conclusion
Viral genes consist of DNA. Viral coats consist of proteins.
Nucleotides
monomers that make up DNA
Nitrogenous base, sugar, phosphate
