topic 1 - DNA replication

Question 1

what does DNA replicate by

Answer 1

semi conservative replication

Question 2

what does semi conservative replication mean

Answer 2

during DNA replication, the two strands of nucleotides separate - both strands then form the template for free nucleotides to bind to to create the two identical daughter strands

Question 3

what does genetic continuity mean

Answer 3

the passing of DNA and the genetic information stored in DNA from one generation to the next

Question 4

describe the first step of the semi conservative process

Answer 4

the enzyme DNA helicase breaks the hydrogen bonds between the two polynucleotide DNA strands - this makes the helix unwind to form two single strands

Question 5

describe the second step of the semi conservative process

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 with T, C with G

Question 6

describe the third step of the semi conservative process

Answer 6

condensation reactions join the nucleotides of the new strands together- catalysed by the enzyme DNA polymerase - hydrogen bonds form between the bases on the original and new strands

Question 7

describe the fourth step of the semi conservative process

Answer 7

each new DNA molecule contains one strand from the original DNA molecule and one new strand

Question 8

how does DNA polymerase move

Answer 8

in the opposite way along antiparallel DNA strands

Question 9

what are the names for each end of a DNA strand

Answer 9

3’ (three prime) and 5’ (five prime)

Question 10

why can the enzyme only add nucleotides to the new strand at the 3’ end

Answer 10

since the active site of DNA polymerase is only complementary to the 3’ and of the newly forming DNA strand

Question 11

what does 3’ and 5’ refer to

Answer 11

the number of carbon atoms in a deoxyribose sugar molecule that a phosphate group binds to

Question 12

3’ 5’ page 18

Question 13

who provided evidence for semi conservative replication

Answer 13

meselson and stahl

Question 14

who came up with the theory of semi conservative DNA replication

Answer 14

watson and crick

Question 15

before meselson and stahl’s experiment what were people unsure of

Answer 15

whether DNA replication was semi conservative or conservative

Question 16

what would happen if the method was conservative

Answer 16

the original DNA strands would stay together and the new DNA molecules would contain two new strands

Question 17

what does meselson and stahl’s experiment use

Answer 17

two isotopes of nitrogen (DNA contains nitrogen) - heavy nitrogen (15N) and light nitrogen (14N)

Question 18

what is the first step to the semi conservative method

Answer 18

two samples of bacteria were grown - one in a nutrient broth containing light nitrogen, one in a broth with heavy nitrogen - as bacteria reproduced they took up nitrogen from the broth to hep make nucleotides for new DNA so the nitrogen gradually became part of the bacteria’s DNA

Question 19

what is the second step to the semi conservative method

Answer 19

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 DNA from the light nitrogen bacteria since it’s heavier

Question 20

what is the third step to the semi conservative method

Answer 20

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

Question 21

what is the fourth step to the semi conservative method

Answer 21

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 22

what is the fifth step to the semi conservative method

Answer 22

if replication was semi conservative the new bacterial DNA molecules would contain one strand of the old DNA containing heavy nitrogen - so the DNA would settle out between where the light nitrogen DNA settles out and where the heavy nitrogen DNA settled out

Question 23

what is the sixth step to the semi conservative method

Answer 23

the DNA settled out in the middle, showing that the DNA molecules contained a mixture of heavy and light nitrogen - the bacterial DNA had replicated semi conservatively in the light nitrogen