TOPIC 1-DNA replication

Question 1

Why does DNA replicate?

Answer 1

DNA copies itself before cell division so that each new cell has the full amount of DNA

Question 2

What is the method of DNA replication called?

Answer 2

Semi-conservative replication

Question 3

Why is it called this?

Answer 3

Only half the strands in each new DNA molecule are from the original DNA molecule which means that there is genetic continuity between generations of cells

Question 4

What is step 1 in DNA replication?

Answer 4

-The enzyme DNA helicase breaks the hydrogen bonds between bases on the two polynucleotide DNA strands
-This makes the helix unwind to form two single strands

Question 5

What is step 2 in DNA replication?

Answer 5

-Each original single strand acts as a template for a new strand
-Complementary base pairing means that free-floating DNA nucleotides are attracted to their complementary exposed bases on each original template strand (A+T and G+C)

Question 6

What is step 3 in DNA replication?

Answer 6

-Condensation reactions join the nucleotides of the new strand together- catalysed by the enzyme DNA polymerase
-Hydrogen bonds form between the bases on the original and new strands
-Each new DNA molecule contains one strand from the original DNA molecule and one new strand

Question 7

What is the difference between the 3’(prime) and 5’ (prime) strand?

Answer 7

-At the 3’ end is a hydroxyl group (OH) that is attached to the pentose sugar of the nucleotide
-At the 5’ end there is a phosphate group

Question 8

Why can DNA polymerase move down the template strand in a 3 to 5 direction?

Answer 8

-During DNA replication the active site of DNA polymerase is only complementary to the 3’ end of the newly forming strand
-This means the enzyme can only add nucleotides to the new strand at the 3’ end

Question 9

What happens because the double helix is antiparallel?

Answer 9

-Because DNA polymerase works on one template strand it moves in the opposite direction the the DNA polymerase works on the other strand

Question 10

Who proved Watson’s and Crick’s theories?

Answer 10

Meselson and stahls experiment

Question 11

What did Watson’s and Crick’s theories prove?

Answer 11

That DNA replication is semi-conservative

Question 12

What is an isotope?

Answer 12

Different forms of the same element

Question 13

What two isotopes did they use in their experiments?

Answer 13

-Heavy nitrogen 15N
-Light nitrogen 14N

Question 14

Why did they use nitrogen?

Answer 14

DNA contains nitrogen

Question 15

What is the first step in their experiment?

Answer 15

-Two samples of bacteria were grown for many generations
-One in a nutrient broth containing light nitrogen and one broth with heavy nitrogen
-As the bacteria reproduced, they took up nitrogen from the broth to help make nucleotides for new DNA
-So the nitrogen gradually became part of the bacteria’s DNA

Question 16

What is the second step in their experiment?

Answer 16

-A sample of DNA was taken from each batch of bacteria and spun in a centrifuge
-The DNA from the heavy nitrogen bacteria settled lower down the centrifuge tube than the lighter nitrogen

Question 17

What is the third step in their experiment?

Answer 17

-The bacteria grown in heavy nitrogen broth were taken out and put in broth containing only light nitrogen
-The bacteria were left for one round of DNA replication and then another sample was spun in the centrifuge

Question 18

What would happen if replication was conservative?

Answer 18

-If replication was conservative the original heavy DNA which would still be together would settle at the bottom and the new light DNA would settle at the top

Question 19

What would happen if replication was semi-conservative

Answer 19

-The new bacterial DNA molecules would contain one strand of the old DNA containing heavy nitrogen and one new strand of light nitrogen
-The DNA would settle out between the light nitrogen DNA and the heavy nitrogen DNA

Question 20

In conclusion, why is DNA semi-conservative?

Answer 20

The DNA settled out in the middle which showed that DNA contained a mixture of heavy and light nitrogen
-The bacterial DNA had replicated semi conservatively in the light nitrogen