DNA Replication

Question 1

Describe how semi conservative replication happens

Answer 1

1) DNA helicase (enzyme) breaks down hydrogen bonds that join bases together which cause the 2 strands of DNA to separate
2) free DNA nucleotides are attracted to exposed bases and pair up forming hydrogen bonds between complementary bases
3) nucleotides are joined together via DNA polymerase to join adjacent nucleotides together by condensation reaction, forming phosphodiester bonds
4) 2 identical molecules of DNA are formed each molecule retains half of the original strand

Question 2

What was Watson and Crick’s hypothesis

Answer 2

Their is either semi conservative replication or conservative replication

Question 3

Define conservative replication

Answer 3

Original DNA remain intact and 2 new synthesised strands of DNA join together

Question 4

Define semi conservative replication

Answer 4

Each replicated DNA molecule contains 1 strand of the original DNA and 1 strand of the newly synthesised DNA

Question 5

What does Meselon and Stahl’s experiment prove

Answer 5

Semi conservative replication

Question 6

What does it mean when DNA bases are nitrogenous

Answer 6

Contain nitrogen

Question 7

Key facts about semi conservative experiment

Answer 7

• nitrogenous bases contain nitrogen
• Nitrogen has 2 isotopes = 14N 15N
• bacteria takes in nitrogen isotopes to make new DNA nuceolutides
• DNA can be spun in a centrifuge to confirm density

Question 8

What does it mean when bacteria takes in a specific nitrogen isotopes

Answer 8

Bacteria grown in a medium containing 15N will have DNA which only contains this isotope

Question 9

After being spun in then centrifuge what is the density if DNA settles lower

Answer 9

More dense

Question 10

After being spun in then centrifuge what is the density if DNA settles higher

Answer 10

Less dense

Question 11

In G0 what medium is bacteria grown in

Answer 11

15N

Question 12

In G0 where did the DNA settle and what did this mean

Answer 12

DNA settles at the bottom meaning 100% of the DNA is 15N 15N

Question 13

In G1 what medium is bacteria grown in

Answer 13

14N

Question 14

In G1 where does the DNA settle and why

Answer 14

DNA has settled slightly higher means 100% DNA 15N 14N

Question 15

In G2 what medium is the bacteria grown in

Answer 15

14N

Question 16

In G2 where does the DNA settle and why

Answer 16

DNA splits

• 50% 14N 14N = less dense
• 50% 14N 15N = more dense

Question 17

In G3 what medium is DNA grown in

Answer 17

14N

Question 18

What are the results at G3

Answer 18

75% of DNA = 14N 14N

25% of DNA = 15N 14N

Question 19

Describe Meleson and Stahl’s experiment

Answer 19

• growing different bacteria in different growth mediums according to the isotope
• they then spun the DNA sample in the centrifuge
• and they looked at the position that the DNA settled in
• from that they worked out the proportions of 14N and 15N in each DNA sample
• proves semi conservative replication

Question 20

How does the experiment prove semi conservative replication

Answer 20

Each time DNA splits into templates = 1 strand is conserved 1 is newly synthesised