Lecture 6 - DNA is the genetic material Flashcards
What was Garrod’s ‘inborn errors of metabolism’?
Each case an inheritable factor for a metabolic step was defective
Albinism: lack of pigment
Alkaptonuria: individuals secrete homogentisic acid into their urine, which goes black following exposure to air
Why did Beadle and Tatum use red bread mould in their experiment?
Grows rapidly on a very simple medium containing only salts, C and N
What was Beadle and Tatum ‘one gene-one enzyme’ experiment?
Hereditary diseases are ‘inborn errors of metabolism’ is correct
- Mutagenesis: took haploid and illuminate it
- Grow all survivors in complete medium
- Identify mutants: every survivor transferred to minimal medium, failure to grow identified a potential nutritional requirement
- Identify nutritional requirement: growth on minimal medium containing amino acids identifies a requirement for an amino acid
- Identify arginine auxotrophs
What is a auxotroph?
A mutant that requires a particular additional nutrient
What is a prototroph?
The normal strain which does not require that nutritional supplement
What was Beadle and Tatum’s key steps to their experiment in 1941?
- Illuminate and breed sample
- Transfer some of complete to minimal medium
- Work out whether it has required amino acids
- Test each one for the specific amino acid
What did Beadle and Tatum identify when testing the arginine auxotrophs?
If the auxotroph came from different asci, they would probably have different mutations
If the auxotroph came from one ascus, they have the same mutation
How does complementation work in Beadle and Tatum’s experiment?
As the heterokaryons both contain nuclei, each cell can perform combine phenotypes - each defect complements the other
Suggests they isolated 3 classes of mutants defective in arginine biosynthesis
What other functions do enzymes perform?
Structural and immunological functions
Why did the one gene-one enzyme hypothesis change and what did it change to?
One gene-one protein
Some proteins are not enzymes but perform other functions (structural, immunological)
Multiple genes for a pathway
What did Friedrick Miescher discover in 1869?
Found the ‘nuclein’ which contianed C, N and H prtiens but was rich in phosphorus with no detectable sulphur
Proposed that the nuclein might be the basis of heredity
Who discovered transformation in 1928?
Frederick Griffith
Showed bacterial transformation (bacteria changes its form and function through the action of a transforming principle or transforming factor)
What is the Griffith experiment (1928) - first control?
- Mouse injected with living R cells
- Mouse remains healthy
- Living R cells can be recovered from mouse heart tissue
= R cells survive in mice, but do not cause pneumonia, R cells are non-pathogenic
What is the Griffith experiment (1928) - second control?
- Mouse injected with living S cells
- Mouse contracts pneumonia
- Living S cells can be recovered from mouse heart tissue
= S cells survive in mice, and cause pneumonia, S cells are pathogenic
What is the Griffith experiment (1928) - third control?
- Mouse injected with heat-killed S cells
- Mouse remains healthy
- No living streptococci can be recovered from mouse heart tissue
Only living cells are causing disease
= Dead S cells do not cause pneumonia
In the Griffith experiment, what is the experimental arm?
- Mouse injected with mixture of heat-killed S cells and living R cells
- Mouse dies
- Both R and S cells can be recovered from mouse heart tissue
= An S cell transforming principle that survives heat treatment has altered some of the R cells
Who did work finding the transforming principle and when?
Avery, MacLeod and McCarthy
1944
What was the Avery, MacLeod and McCathy experiments?
Formal evidence that DNA is a transforming factor
- Living S cells in a flask and boiled to kill cells
- Only soluble extract remains, split into 3 and mixed with protease, RNase or DNase enzyme treatment
Protease: transformation of R cells to S cells - protein is not the hereditary material
RNase: S and R cells = still transformation - cannot be RNase
DNase: no DNA - lost during transformation
= DNase destroys the transforming principle
What is another piece of evidence conducted in 1944 with the Avery, MacLeod and McCarthy experiment?
Purifying DNA from S cells resulted in transformation of some R cells to S cells
Definitive evidence that DNA is the hereditary material
Who was A. Hershey and what did he do?
1952: protein is not the hereditary material
1969: gained Nobel prize for discoveries but did not mention Martha Chase (research assistant) in his acceptance speech
What is the structure of a T2 phage?
DNA - hereditary information, tightly packed
Inside icosahedral head, attached to a core
Surrounded by a sheath
Attached to base plate with tail fibres emerging
What is the state of phage research from 1948-1952?
Taking E.Coli and infecting with T2 causes attachment mediated by the fibres
Phage particles remain attached to the bacterium, heads appear empty forming ‘ghosts’
Bacterium burst open exposing new viruses
What did Martha Chase experiment with the state of phage research in 1952? (Hershey-Chase experiments)
Phage protein labelled radioactively
Sulphur: present in proteins, not DNA
Phosphorus: present in large amounts of nucleic acid
Grow bacteria and infect, when burst spin out dead and non-infected cells leaving a suspension of radioactive phage
Next generation are radioactive - confirms DNA is genetic material, only when labelled with phosphorus
