DNA replication

Question 1

Step 1 of DNA replication

Answer 1

Enzyme DNA helicase catalysed the separation of a polynucleotide strands in a molecule by breaking hydrogen bonds between complementary base pairs

Question 2

Step 2 of DNA replication

Answer 2

Free activated nucleotides are attracted to its complementary pair on template strands and forms hydrogen bonds to it

Question 3

Step 3 of DNA replication

Answer 3

Enzyme DNA polymerase catalyses the formation of phosphodiester bonds in activated nucleotides in a condensation reaction which releases water

Question 4

Step 4 of DNA replication

Answer 4

Now have 2 identical strands of DNA consisting of the original template strand and a newly synthesises strand

Question 5

Which direction does DNA polymerase catalyse formation of phosphodiester bonds?

Answer 5

only at the carbon 3

Question 6

Why is it called semi conservative replication?

Answer 6

Because 1 molecule of DNA is replicated into 2 identical molecules but each molecule has 1 original strand and 1 newly synthesised strand

Question 7

What would hypothetical conservative replication look like?

Answer 7

1 DNA molecule never separates into strands but replicates another molecule which contains both newly synthesises strands

Question 8

What would hypothetical dispersion replication look like?

Answer 8

Sections of nucleotides are copied so each new strand has newly synthesised sections

Question 9

How did scientists work out which replication method was correct

Answer 9

Using Meslson and Stahls method

Question 10

What is the Meslson and Stahl study?

Answer 10

Showing which method of DNA replication is correct
Using the different isotopes of nitrogen to differentiate the different strands of DNA

Question 11

Background of Meselson and Stahl

Answer 11

2 isotopes of N: N-14 (light) and N-15 (heavy)
In nitrogenous bases, the N14 is present in all DNA of a bacteria

Question 12

Step 1 of Meselson and Stahl

Answer 12

Extract DNA from bacteria with N-14 in bases

Question 13

Step 2 or Meslson and Stahl

Answer 13

Centrifuge N-14 DNA sample at very high speeds:
It will produce a band at the top of the test tube because only light dna is present and moves to the top

Question 14

Step 3 of Meselson and Stahl

Answer 14

Bacteria DNA is grown for several generations in N-15 so all nitrogenous bases contain heavy nitrogen

Question 15

Step 4 of Meselson and Stahl

Answer 15

Centrifuge new N-15 grown sample which produces band at bottom because heavy N moves to bottom and only that’s present
100% of DNA is at heavy band

Question 16

Step 5 of Meselson and stahl

Answer 16

Grown heavy nitrogen sample in N-14 growth medium and allow to only go through one generation of DNA replication
So, half of DNA is heavy template N-15 and half is synthesised N-14

Question 17

Step 6 of Meselson and Stahl

Answer 17

Centrifuge half and half sample in a centrifuge
Produces band in MIDDLE because DNA molecules have 1 light strand and 1 heavy. So falls midway
100% of DNA is at the middle density

Question 18

What rules out conservative replication in Meselson and Stahl?

Answer 18

When 1 molecule with half N-14 and N-15 is centrifuged, the band is produced in middle Because it’s middle heavy so falls to middle
However, in cons replication the molecules wouldn’t have mixture of light and heavy so would be at top and bottom
density would be spread out: 50% at top 50% at bottom

Question 19

Step 7 of Meselson and Stahl

Answer 19

Allow half and half DNA molecule to divide once more in light growth medium

Question 20

Step 8 of Meselson and Stahl

Answer 20

Centrifuge to produce a band at intermediate because half and half molecule present but also band at N14 because a fully light nitrogen base detected

Question 21

How is dispersion method disproved?

Answer 21

After undergoing 2 replications, the centrifuge produces band at intermediate because all molecules contain both isotopes but band at N14 shows how semi conservative occurred

Question 22

Description of half and half molecule dividing

Answer 22

The heavy chain from half and half molecule used as template to synthesise light chain = 2 half and half
Then light chain from half and half used to synthesises another light chain = fully N14

Question 23

Why in the formation of phosphodiester bonds between monomers in DNA replication are the nucleotides only attached to carbon 3 of sugar?

Answer 23

Because DNA polymerase which catalyses the formation of phosphodiester bonds is an enzyme with a tertiary structure of active site specific to the carbon 3 end (complementary)