Lecture 15: DNA Structure & Function

Question 1

What was the significance of Chargaff’s work and give two significant findings:

Answer 1

• He disproved Levene’s hypothesis
• He looked at a few different species and analysed the DNA composition (how many As, Ts, Cs, Gs)
• The As and Ts + Gs and Cs were approx. the same within species but there was variation between species

Question 2

What were Chargaff’s two rules?

Answer 2

First Rule: A = T & G = C

Second Rule: The composition of DNA varies between species

Question 3

What was Chargaff’s conclusion?

Answer 3

Since there is DNA variation between species, this shows that DNA is responsible for genetic material

Question 4

What are the two strains of S.P that Griffith discovered?

Answer 4

• [S]mooth Strain

- [R]ough Strain

Question 5

What is the difference between S and R strains in streptococcus pneumoniae?

Answer 5

• S has a capsule that makes it undetectable to the host cells

Question 6

How did Griffith investigate DNA transformations?

Answer 6

• Living S Strain (virulent) = Dead Mouse
• Living R Strain (non virulent) = Alive Mouse
• Dead (by heating) S Strain (virulent) = Alive Mouse

*** - Dead S Strain (virulent) + Alive R Strain (non virulent) = Dead Mouse (living S Strain had been found in heart)

Question 7

What was Griffith’s conclusion?

Answer 7

• A chemical substance is the bearer of genetic information
• Transforming live R Cells into S Cells
• Info that determines a bacteria’s strain + virulence must be encoded in a non-living chemical, as this information can be transferred from dead to living bacteria

Question 8

What happened in Avery’s experiments?

Answer 8

• Dead S Strain - RNA + R Strain = S + R Present
• Dead S Strain - Protein + R Strain = S + R Present
• Dead S Strain - DNA + R Strain = Only nonvirulent R Strain

Question 9

What was Avery’s conclusion?

Answer 9

• DNA is the chemical substance that causes transformation / is the genetic material

Question 10

What is the structure of DNA?

Answer 10

• Double stranded helical, phosphate - sugar back bone, base points in to middle, constant diameter, 3 prime hydroxyl reacts with 5 prime phosphate of another nucleotide and they link
• ALWAYS synthesised in the 5 prime to 3 prime direction
  - ———>
• One is always “3,5” prime, need to go in opposite direction to each other = antiparallel

Question 11

What’s the difference between pyrimidines and purines?

Answer 11

Pyrimidines: G + C = 3 hydrogen bonds
Purines: A + T = 2 hydrogen bonds