Lecture 6 - DNA is the genetic material

Question 1

What was Garrod’s ‘inborn errors of metabolism’?

Answer 1

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

Question 2

Why did Beadle and Tatum use red bread mould in their experiment?

Answer 2

Grows rapidly on a very simple medium containing only salts, C and N

Question 3

What was Beadle and Tatum ‘one gene-one enzyme’ experiment?

Answer 3

Hereditary diseases are ‘inborn errors of metabolism’ is correct

1. Mutagenesis: took haploid and illuminate it
2. Grow all survivors in complete medium
3. Identify mutants: every survivor transferred to minimal medium, failure to grow identified a potential nutritional requirement
4. Identify nutritional requirement: growth on minimal medium containing amino acids identifies a requirement for an amino acid
5. Identify arginine auxotrophs

Question 4

What is a auxotroph?

Answer 4

A mutant that requires a particular additional nutrient

Question 5

What is a prototroph?

Answer 5

The normal strain which does not require that nutritional supplement

Question 6

What was Beadle and Tatum’s key steps to their experiment in 1941?

Answer 6

1. Illuminate and breed sample
2. Transfer some of complete to minimal medium
3. Work out whether it has required amino acids
4. Test each one for the specific amino acid

Question 7

What did Beadle and Tatum identify when testing the arginine auxotrophs?

Answer 7

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

Question 8

How does complementation work in Beadle and Tatum’s experiment?

Answer 8

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

Question 9

What other functions do enzymes perform?

Answer 9

Structural and immunological functions

Question 10

Why did the one gene-one enzyme hypothesis change and what did it change to?

Answer 10

One gene-one protein

Some proteins are not enzymes but perform other functions (structural, immunological)

Multiple genes for a pathway

Question 11

What did Friedrick Miescher discover in 1869?

Answer 11

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

Question 12

Who discovered transformation in 1928?

Answer 12

Frederick Griffith

Showed bacterial transformation (bacteria changes its form and function through the action of a transforming principle or transforming factor)

Question 13

What is the Griffith experiment (1928) - first control?

Answer 13

1. Mouse injected with living R cells
2. Mouse remains healthy
3. 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

Question 14

What is the Griffith experiment (1928) - second control?

Answer 14

1. Mouse injected with living S cells
2. Mouse contracts pneumonia
3. Living S cells can be recovered from mouse heart tissue

= S cells survive in mice, and cause pneumonia, S cells are pathogenic

Question 15

What is the Griffith experiment (1928) - third control?

Answer 15

1. Mouse injected with heat-killed S cells
2. Mouse remains healthy
3. No living streptococci can be recovered from mouse heart tissue

Only living cells are causing disease

= Dead S cells do not cause pneumonia

Question 16

In the Griffith experiment, what is the experimental arm?

Answer 16

1. Mouse injected with mixture of heat-killed S cells and living R cells
2. Mouse dies
3. 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

Question 17

Who did work finding the transforming principle and when?

Answer 17

Avery, MacLeod and McCarthy

1944

Question 18

What was the Avery, MacLeod and McCathy experiments?

Answer 18

Formal evidence that DNA is a transforming factor

1. Living S cells in a flask and boiled to kill cells
2. 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

Question 19

What is another piece of evidence conducted in 1944 with the Avery, MacLeod and McCarthy experiment?

Answer 19

Purifying DNA from S cells resulted in transformation of some R cells to S cells

Definitive evidence that DNA is the hereditary material

Question 20

Who was A. Hershey and what did he do?

Answer 20

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

Question 21

What is the structure of a T2 phage?

Answer 21

DNA - hereditary information, tightly packed

Inside icosahedral head, attached to a core

Surrounded by a sheath

Attached to base plate with tail fibres emerging

Question 22

What is the state of phage research from 1948-1952?

Answer 22

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

Question 23

What did Martha Chase experiment with the state of phage research in 1952? (Hershey-Chase experiments)

Answer 23

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