19. Discovery of DNA and replication

Question 1

What was the view on genetic material before 1940s?

Answer 1

Did not know if proteins or DNA carried genetic material

Question 2

Explain Frederick Griffith’s transformation experiment on DNA

Answer 2

S cells pathogenic, R non-pathogenic, when injected R and heat killed S together → killed the mice (DNA transferred from killed S to non-pathogenic R) → genetically encoded trait was transferred → further investigation

Question 3

What experiment followed Frederick’s Griffith transformation experiment to prove that DNA is the genetic molecule?

Answer 3

Question 4

Hershey and Chase’s experiment for semi-conservative replication of DNA

Answer 4

When labelled RNA – found in the end in the supernatant (viral cells), when labelled DNA – found in the pellet (bacterial cells)

Question 5

What are the Ervin Chargaff’s rules regarding DNA base sequence?

Answer 5

• Base composition of DNA varies between species - In any species the number of A and T is equal as well as G and C

Question 6

What is the contribution of Rosalind Franklin?

Answer 6

Question 7

Why is the double helix right handed? What is the structure od DNA?

Answer 7

if you would hold the molecule - woudl follow the curve of the right hand

Question 8

What base pairing gives to the DNA structure?

Answer 8

• similarity of shape between A and T and C and G gives constant spacing between the strands - gives equal numbers of A and t and C and G

Question 9

What is the basic principle of DNA copying?

Answer 9

Question 10

Why is DNA replication semi conservative?

Answer 10

Because one old DNA strand persist to the following DNA molecule

Question 11

Explain what are the origins of replication?

Answer 11

• origins of replication - particular sites where DNA strands are separated opening a replication “bubble” - eukaryotic chromosomes have many origins of replication - replication procedes in both directions from the origin until the whole molecule is copied

Question 12

Discribe the replication fork and enzymes in replication

Answer 12

Replication fork - Y shaped region where DNA strands are copied

Helicases - enzymes which breaks the base pairing

Single stranded binding proteins - bind to stabilise single stranded DNA

Topoisomerases - untwists DNA

Primase - adds RNA primers

DNA polymerase - synthesises the new DNA strand complementary to the old 5’ to 3’ direction

Question 13

Explain antiparallel elongation

Answer 13

Replication occurs only in 5’ to 3’ end -> lagging strand replicated in Okazaki fragments - RNA primase adds RNA primers - DNA polymerase synthesises Okazaki fragments in 5’ to 3’ direction - RNA primers are removed and the gaps are filled by DNA polymerase I - ligase seals the gaps between DNA fragments

Question 14

Summary of whole mechanism of DNA replication

Answer 14

1. Uncoils double strand, breaks H bonds - helicase (uses ATP) and DNA gyrase (topoisomerase)- replication fork created
2. Single stranded binding proteins attach - prevent strands from pairing
3. DNA polymerase III (always 5’->3’) attaches to leading strand and continuously synthesises new strand 5’ to 3’ end (free nucleotides)
4. DNA (RNA) primase adds RNA primer to lagging strand - DNA polymerase III synthesises in Okazaki fragments

5. DNA polymerase I removes primers - DNA ligase seals fragments