Lecture 4

Question 1

Friedrich Miescher

Answer 1

Discovered Nucleic Acid

Question 2

How did he discovered nucleic acid?

Answer 2

1. Pus from bandages to obtain white blood cells
2. Nuclei was purifed and extracted
3. A phosphate rich precipitate was found, also rich in nitrogen. Named “Nuclein”
4. Contained two fractions, N-rich protein and a acidic P-rich “nucleic acid”

Question 3

What is the basic structure of nucleic acid?

Answer 3

• Repeating units of 5-carbon sugar, nitrogenous bases and phosphate
• Either Ribose nucleic acid or Deoxyribose nucleic acid

Question 4

Pyramidine bases

Answer 4

Thymine and cytosine

Question 5

Purine bases

Answer 5

Adenine and Guanine

Question 6

Friedrich Griffith experiment

Answer 6

• 1928
• Used two stains of Streptoccus pneunmonia, benign R and virul S
• Genetic material allowed heat-killed S to convert R into active S

Question 7

Oswald Avery experiment

Answer 7

• 1944
• Subjected heat-killed S to treatments to identify the “transformative principle”
• The principle was protease-resitance, lipase-resistant, ribonuclease-resistant, ethanol insoluble and had a high molecule weight with a positive Diesche result.
• It was DNA!

Question 8

Hershey-Chase experiment

Answer 8

• 1952
• Focused on the genetic material that bacteriaphaes inject into their host
• Labelled Protein with 35S but no protein injected into host
• Labelled DNA with 32P, was found in host! this was indeed the genetic material!

Question 9

Chargaffs rule

Answer 9

• Ratio of four bases is not 1:1:1:1 so not a simple structure
• A binds to T (U in RNA)
• C binds to G
• Binds with hydrogen bonds

Question 10

How wide is DNA?

Answer 10

2nm

Question 11

How long is each DNA turn?

Answer 11

3.4nm

Question 12

The distance between bases?

Answer 12

0.34nm

Question 13

How many nucleotides per turn?

Answer 13

10

Question 14

Are strands parallel or anti parallel?

Answer 14

Antiparallel (5->3)

Question 15

What is semi-conservation replication?

Answer 15

Each strand of DNA act as a template for a new complementary strand

Question 16

Meselson-strahl experiment

Answer 16

1. Bacteria cultured in heavy 15N medium
2. Bacteria transferred to lighter 14N medium
3. DNa smaple centrifuged after first replication
4. DNA sample had intermediate density (1 light and 1 heavy strand)

Question 17

DNA Synthesis (long)

Answer 17

1. Unwinding of Double helix by helicase
2. Single-strand binding proteins keeps the helix open
3. Primase, an enzyme, synthasises a short RNA primer complementary to the template DNA, this provides a free 3’ hydroxyl group which DNA polymerase can add nucleotides
4. DNA polymerase lll adds nucleotides to the growing DNA strands. It ultilises all four nucelotide triphospahtes (dNTP) as substrates for DNA synthesis
5. It synthesises DNA 5’ to 3’
6. The leading strand is synthesised continously in the direction of DNA unwinding
7. The lagging strand is synthesised discontinously in short fragments called Okazaki fragments.
8. DNA polymerase “proof-reads” the nucleotides, removing incorrect ones
9. DNA Ligase will join the Okazaki fragments together forming a strand,