DNA and RNA

Question 1

Griffith’s experiments (1928)

Answer 1

Streptococcus pneumoniae are bacteria that can invade and infect lungs causing pneumonia - Griffiths wanted to develop a vaccine

There are two different strains:
II - no capsule, rough colony and non-pathogenic, mice can fight off with an immune response
III - capsule, smooth colony and pathogenic, mice cannot fight off with an immune response

Discovered ‘The Transforming Principle’ - heat killed smooth type III bacteria can transform the live, rough type II bacteria because DNA is released and taken up/incorporated into their genome

Dawson did this but in vitro so it was easier to identify the transforming principle

Question 2

Avery MacLeod and McCarthy (1944)

Answer 2

Discovered the TP was DNA, but others thought it was too simple to change the phenotype of a bacteria

They used 75L of heat-killed IIIS cells to give a soluble extract containing the TP and successfully isolated different components (lipids/proteins and polysaccharides removed)

DNA alone transformed S. pneumoniae which was treated with an enzyme DNase which breaks down DNA and finally activity was lost

Question 3

Hershey-Chase experiment

Answer 3

Working on bacteriophage T2 (virus that infects bacteria) - genetic material enters the infected cell but most of the bacteriophage does not

Question 4

Maurice Wilkins and Rosalind Franklin

Answer 4

Used X-ray diffraction to study DNA fibres

Discovered that DNA is helical

Used Chargaff’s laws: total pyrimidines (T/C) = total purines (A/G)

Question 5

What is the DNA structure?

Answer 5

Double helical structure of two single strands that wind round each other with one narrower groove and one wider.

Strands are antiparallel and held together by hydrogen bonds - 3 bonds between GC pairs and 2 between AT pairs. Forms a phosphodiester backbone with bases stacked in planar arrangement

Nucleotide: deoxyribose sugar attached to an inorganic phosphate and nitrogenous base

5’ phosphate end = free phosphate group not involved in bond formation

3’ hydroxyl end = free hydroxyl group

Question 6

What are the structures of purines and pyrimidines?

Answer 6

Purines - double nitrogenous ring structures

Pyrimidines - single ring structure

Question 7

What are the differences between DNA and RNA?

Answer 7

The presence of a hydrogen at 2’ carbon in deoxyribose and the presence of a hydroxyl group at 2’ carbon in ribose

Nucleoside = sugar + base
Nucleotide = '...' + phosphate

Question 8

What is the structure of RNA?

Answer 8

Single-stranded molecule

Contains uracil instead of thymine

Contains a ribose sugar instead of deoxyribose

Can fold up on itself to form complicated 3D structures

Question 9

What is mRNA?

Answer 9

Messenger RNA

Copied from DNA and used to make protein

Question 10

What is rRNA?

Answer 10

Ribosomal RNA

Functional RNA component of the ribosome

Question 11

What is tRNA?

Answer 11

Transfer RNA

Involved in translation as it brings amino acids to ribosomes