DNA experiments

Question 1

Describe the Mendelian laws of inheritance

Answer 1

• Segregation: genes come in pairs, and individuals only pass on one of these to their offspring
• Independent assortment: different genes are passed on separately from each other → inheritance of one does not depend on that of another
• dominance: an individual with two alleles of a gene will express the dominant form

Question 2

Describe segregation

Answer 2

genes come in pairs, and individuals only pass on one of these to their offspring

Question 3

Describe independent assortment

Answer 3

different genes are passed on separately from each other → inheritance of one does not depend on that of another

Question 4

Describe dominance

Answer 4

an individual with two alleles of a gene will express the dominant form

Question 5

Which experiment provided a physical basis for Mendel’s independent assortment?

Answer 5

Sutton-Boveri theory of chromosomal inheritance (1902)

Question 6

Describe Griffith’s 1928 ‘Transforming principle’ experiment?

Answer 6

Can rough bacteria be transformed into smooth ones?
Smooth = Pathogenic
Rough = Not pathogenic
1. Kill S bacteria by heating them to high temps.
2. Inject S bacteria = no infection
3. Inject S strain + R strain = infection, mouse dies → Live S strain is recovered

Question 7

What were the results of Griffith’s 1928 ‘transforming principle’ experiment?

Answer 7

• Inoculation with dead S bacteria and live R bacteria = infection with a polysaccharide coat
• R cells have undergone a ‘transformation’ → hereditary material has passed from the S bacteria to the R bacteria, changing the genotype.

Question 8

Describe how Avery, Macleod & Mccarthy (1944) tested which molecules are responsible for the transformation Griffith observed

Answer 8

• They systematically destroyed each component of the S strain using enzymes (that are specific to each type of molecule) then combined with R strain to test for transformation

= In order for bacteria to be virulent, they need the DNA encoding for the polysaccharide coat but not the polysaccharides themselves

Question 9

Outline Hershey & Chase’s bacteriophage experiment

Answer 9

• Used bacteriophage T2
  1. Label bacteriophage DNA or protein by growing them in radioactive medium (containing phosphorous-32 / sulfur-35)
  2. Infect unlabelled bacteria with radioactive phage
  3. Separate phage ghosts from infected bacteria using a blender
  4. Centrifuge to separate phage ghosts (supernatant) and bacteria (pellet) by weight
  5. Test radioactivity with a Geiger counter

Question 10

What were the results of Hershey & Chase’s bacteriophage experiment?

Answer 10

Radioactivity found in phage ghosts (labelled protein)
Radioactivity found in bacteria (labelled DNA)
= Radioactive DNA is carried over from gen to gen