DNA as the genetic material

Question 1

Gregor Mendel 1866

Answer 1

Mendelian laws of inheritance
- Segregation - genes come in pairs, individuals pass only 1 onto offspring
- Independent assortment - different genes passed on separately to each other, inheritance not dependent on each other
- Dominance - Individual with 2 alleles will express a dominant allele
Must be some physical genetic material for it to be passed on
Qualities of genetic material:
- Replicate information
- Store information
- Express information
- Variation by mutation

Question 2

Sutton-Boveri theory of chromosomal inheritance

Answer 2

1902
Wanted to work out where genetic material is carried
Studied early development in 2 model organisms
Used cytology and microscopy
Sutton worked with grasshoppers
Boveri worked with Ascaris worms
Chose to observe the chromosomes in these animals because they are large and there aren’t many of them so its easy to observe the chromosomes
Grouping in pair and subsequent separation - reduction in chromosome number in gametes
Noted observations in line with Mendel’s laws
Suggests inheritance chromosomes causes variation
Theory gave physical basis for Mendel’s independent assortment
Found out that:
-Chromosomes needed for embryonic development
- Chromosomes carry Mendel’s “Factors”/ gene (Gene coined by Sutton)
- Chromosomes are linear structures with genes along them

Question 3

The transforming principle, Frederick Griffith 1928

Answer 3

Looked at Streptococcus pneumoniase, causes pneumonia in human and mice
Only some strains capable causing infection and illness
Streptococcus can live in nasal passages without causing disease
Colonies look different under microscopes:
- S strain - smooth bacteria - polysaccharide coats - virulent
- R strain - rough bacteria - no polysaccharide coats - not virulent
Polysaccharide coat forms capsule that protects some strains from hosts immune system
Griffith wanted to find out if strains are fixed or if they can transform (rough into smooth)

Question 4

The experiment

Answer 4

1. Kill bacteria by heating
2. Inject dead S bacteria into mice with living R bacteria and see if infection is established
3. If infection established, do bacteria have polysaccharide coats?
   Outcomes:
   a) Living S strain injected - mouse dies
   b) Living R strain injected - mouse lives
   c) Heat treated S strain injected - mouse lives
   d) Living R strain and heat treated S strain injected - mouse dies - living S strain recovered - infection established
   Results - Inoculation d establishes infection. bacteria taken from infected mice have polysaccharide coats

Question 5

Candidate for transforming principle

Answer 5

Protein- popular, complex and abundant molecules
DNA - was said to be too simple
Polysaccharides
Lipids
RNA

Question 6

Oswald Avery, Colin MacLeod and Macleyn McCarthey 1944

Answer 6

Wan to know which molecule responsible for transformation Griffith observed
Systematically destroyed each component of S strain extract using enzymes that specifically digest each types of molecule
Then combined with living R strain to see what happens
Only when DNA destroyed does mouse live and no live S strain recovered
Tells us for bacteria to be virulent, require DNA that encodes for polysaccharide coat not just the coat itself

We now know that a small piece of DNA from the dead S cell is integrated into genome of R cell
Lots of different DNA pieces from S cell extract taken by different individual R cells
Only a few R cells take up gene that codes for polysaccharide capsule and will become virulent the other wont

Question 7

Bacteriophage genetic material - Alfred Hershey and Martha Chase 1952

Answer 7

Need to go back over lecture to get this
Wanted to know:
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Already knew:
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Question 8

What is bacteriophage

Answer 8

Category of viruses
Akk viruses require host to reproduce
Host cell of bacteriophage is bacteria

Question 9

Bacteriophage T2 and the outline of the experiment

Answer 9

This host cell specifically Escherichia coli bacterium
Outline:
1. Label bacteriophage DNA or protein with radioactive isotope
2. Infect unlabelled bacteria with radioactive phage
3. Separate phage ghost from infected bacteria
4. Test bacteria and phage ghost for radioactivity
Know DNA has phosphate group and protein contains sulphur

Question 10

Step by step and results

Answer 10

Step 1
- ^32P and ^35 S unstable isotopes of P and S, can be detected by Geiger counter
- Growing bacteriophage in media containing 32P will yield phage containing DNA with label
Step 2
- Knew only genetic material entered infected cell, the rest of the virus remained attached to the outside of the bacterial host
- Now want to separate what injected into cell with what’s stayed outside
Step 3
- Agitated mixture in a blended to make phage ghost fall into bacterium
Step 4
- Centrifuge to separate phage and bacteria
- Separated by weight, bacteria heavier
- Pellet contains bacteria (including phage genetic material)
- Supernatant contains phage ghosts
- Test both for radioativity
Step 5
- If its a mix of protein and bacteria then 35S will be found
- IF mix of DNa and bacteria then 39P will be found
Results
- Labelled DNA fond in pellet
- Therefore DNA being injected and making new phage’s
Radioactive DNA carried over from generation to generation
Shows DNA is being passed on and replicated