DNA & RNA

Question 1

What is the background to Griffith’s experiments?

Answer 1

He looked at the streptococcus pneumoniae which contain two twos strains which can be defined by their serotypes

serotype - antibodies to different surface antigens

Seryotype II: no capsule - ‘rough colony’ (R) which is non-pathogenic

Serotype III: capsule - ‘smooth colony’ (S) which is pathogenic

Question 2

How did griffith discover ‘the transforming principle’?

Answer 2

He added different serotypes in different ways to mice and saw what the outcome was.

Adding just the Smooth type III causing the mice to die from pneumonia

Adding just Rough type II does nothing and the mice survives

Adding heat killed Smooth type III allows the mice to survive

Adding heat killed type III and rough type II still causes the mouse to die from pneumonia

Shows that some of the dead, smooth type III bacteria can transform the live, rough type II bacteria

Question 3

What did Dawson discover (1931)?

Answer 3

Heat killed type III smooth cells will ‘transform’ living type II rough cells when in vitro

Question 4

What did Avery discover? (1944)

Answer 4

He discovered that the transforming principle was DNA

he did this by removing lipids, proteins, polysaccharides leaving only DNA remaining and the S, pneumoniae was still transformed

Question 5

What is a bacteriophage’s life cycle?

Answer 5

1. Infection (phage attaches to bacterium and injects DNA which replicates)
2. Early growth phase where heads and tails are made
3. Late growth phase I where DNA is packaged into heads and tails and fibres attached
4. Late growth phase 2
5. Lysis and release of progency phase

Question 6

What is the difference between a nucleotide and a nucleoside?

Answer 6

Nucleoside: sugar + base
Nucleotide: Sugar + base + phosphate

Question 7

How are DNA bases stacked?

Answer 7

Planar arrangement along a right handed helix

Question 8

How many forms of DNA are there?

Answer 8

Forms:

• A DNA
• B DNA
• Z DNA

Question 9

What is A DNA?

Answer 9

Similar to form adopted bu dsRNA

• more compact than B
• smaller distance between bases
• helix diameter is larger

Question 10

What is B DNA?

Answer 10

Common type of DNA founs in cells

• Large major groove
• Some proteins can read the base sequence in the major groove

Question 11

What is Z DNA?

Answer 11

Found in vitro in repeated DNA

• mainly form transiently in vivo during gene transcription
• Left handed curve
• Narrow diameter
• stretched

Question 12

How does the double helix denature and anneal?

Answer 12

Heating will denature the DNA but cooling will anneal and reverse the process