Lecture 4

Question 1

Who discovered Nucleic acid?

Answer 1

Friedrich Miescher.

Question 2

What type of experiment did Griffith undertake in 1928?

Using which microorganisms?

Answer 2

Transformation experiments with lethal and non-lethal Streptococcus pneumoniae.

Question 3

What letters represent benign and virulent S. pneumoniae?

Answer 3

R = benign.
S = virulent.

Question 4

What did he find?

Answer 4

Heat killed S form + R form caused mice to die. Blood contains live pathogenic S. pneumoniae.

Question 5

What did Griffith’s experiment mean?

Answer 5

Must be a ‘transforming principle’.

Question 6

What did Avery, McLeod and MacCarty do in 1944?

Answer 6

Establish chemical properties of ‘transforming principle’.

Question 7

What are the chemical properties of the transforming principle?

Answer 7

Resistant to proteases - not protein, lipases, ribonucleases - so not protein, lipid or RNA.
Ethanol insoluble - not carbohydrate.
High molecular weight - like DNA.
Positive reaction to Dische test for DNA.

Question 8

What is Chargaff’s rule, and what does it provide evidence of?

Answer 8

A=T, (1:1)
G=C. (1:1)
Evidence of DNA.

Question 9

What is a bacteriophage?

Answer 9

Type of virus composed of DNA and protein which infects bacterial cells.

Question 10

What did the Hershey-Chase experiment show?

Answer 10

Bacteriophage injects DNA into bacterial cell.

Question 11

How was the Hershey-Chase experiment performed?

Answer 11

1. B’phage protein labelled with 35S, mixed with E.coli.
   Most of the radioactivity ended up in supernatant.
2. B’phage DNA labelled with 32P.
   Most of the radioactivity in the pellet.

Question 12

What happens at the end of a bacteriophage infection?

Answer 12

New generation of virus particles burst from host cell.

Question 13

What method did Watson and Crick use for developing their model for the double Helix?

Answer 13

Trial and error, and 2 purines too wide, and 2 pyrimidines too narrow.

Question 14

How many H bonds in A-T?

Answer 14

2.

Question 15

How many H bonds in G-C?

Answer 15

3.

Question 16

Are DNA strands parallel or anti-parallel?

Answer 16

Anti-parallel.

Question 17

What did Watson and Crick suggest regarding replication? What type of replication did they discover therefore?

Answer 17

Each strand can act as a template for synthesis of a new complementary strand.
Semi-conservative replication.

Question 18

What other scientists supported the semiconservative model besides W&C? What did they do in their experiment?

Answer 18

Matthew Meselson and Franklin Stahl.
Labelled old nucleotides with heavy isotope of Nitrogen. Any new nucleotides labelled with lighter isotope.
1st replication produced band of hybrid DNA, eliminating conservative model.
2nd replication produced both light and hybrid DNA, eliminating dispersive model, supporting semiconservative.

Question 19

What end must DNA polymerase add nucleotides to?

Answer 19

3’ OH end.

Question 20

Where does replication begin on the chromosome?

Answer 20

Many sites simultaneously, on both sides of DNA.

Question 21

What are the front and back of synthesising DNA strands called?

Answer 21

Leading and lagging respectively.

Question 22

What does synthesis of the lagging strand require?

Answer 22

Synthesis of multiple short fragments (Okazaki), then joined together.

Question 23

What synthesises a new primer during DNA replication?

Answer 23

Primase.

Question 24

What 6 proteins and enzymes contribute to DNA replication?

Answer 24

Helicase,
Single-strand binding proteins,
Primase,
DNA polymerase III,
DNA polymerase I,
DNA ligase.

Question 25

What does Helicase do?

Answer 25

Unwind DNA helix.

Question 26

What do Single-strand binding proteins do?

Answer 26

Hold DNA helix open.

Question 27

What does Primase do?

Answer 27

Synthesises RNA primers needed to initiate DNA synthesis.

Question 28

What does DNA polymerase III do?

Answer 28

Extend DNA strand from 3’ end, copying template.

Question 29

What does DNA polymerase I do?

Answer 29

Removes RNA primer, filling in gaps between Okazaki fragments.

Question 30

What does DNA ligase do?

Answer 30

Seals gaps between Okazaki fragments.

Question 31

How common is a mistake during DNA replication?

How common is an error in completed DNA, due to?

Answer 31

1 in 10,000 bases.

1 in 1 billion bases, due to proof reading ability of DNA polymerase.

Question 32

How does DNA polymerase recognise an incorrect match, and what does it do?

Answer 32

Recognised by lack of base pairing.

Uses “3’ -> 5’ exonuclease” activity to excise incorrect nucleotide.