DNA replication (3.1.5.2)

Question 1

what model of DNA replication does DNA follow?

Answer 1

semi-conservative replication

Question 2

what does semi-conservative replication mean?

Answer 2

the new DNA molecule after replication has one original polynucleotide strand and one new, complementary strand

Question 3

what is the importance of DNA replicating by semi-conservative replication?

Answer 3

it ensures genetic continuity between generations of cells

Question 4

what is DNA helicase?

Answer 4

an enzyme that breaks hydrogen bonds between DNA strands

Question 5

what is DNA polymerase?

Answer 5

an enzyme which forms phosphodiester bonds between DNA nucleotides

Question 6

which direction does DNA polymerase work in?

Answer 6

5’ to 3’

Question 7

what are activated nucleotides?

Answer 7

the free nucleotides which bind to the bases on template strands - they contain three phosphate groups instead of one, because the extra two provide energy for the formation of phosphodiester bonds between the nucleotides by DNA polymerase (lost in reaction)

Question 8

what are the stages of the semi-conservative replication of DNA?

Answer 8

1. DNA helicase attaches to the DNA molecule and causes the hydrogen bonds between complementary bases to break (unwinding of double helix)
2. the two polynucleotide strands separate from each other
3. free nucleotides line up with their complementary bases (base pairing) on the DNA strands (template strands), and are only held in place between hydrogen bonds between bases
4. DNA polymerase catalyses the formation of phosphodiester binds between the activated nucleotides (condensation reaction)
5. this produces the replicated DNA molecule

Question 9

how do mutations occur in DNA replication?

Answer 9

an incorrect base may be inserted into the polynucleotide strand, so the DNA sequence changes

Question 10

what are the features of mutations?

Answer 10

they are random and occur spontaneously

Question 11

what would happen if DNA replicated by conservative replication?

Answer 11

the replicated DNA molecule would form with two new strands, so it would contain none of the original DNA

Question 12

what are the isotopes of nitrogen and what are their features?

Answer 12

14N - lighter
15N - heavier

Question 13

what facts did scientists rely on when proving that DNA replicates by semi-conservative replication?

Answer 13

1. all DNA bases contain nitrogen
2. baceira will incorporate nitrogen from their growing medium into any new DNA they make

Question 14

how did scientists prove that DNA replicated by semi-conservative replication?

Answer 14

1. started growing bacteria in a medium containing 15N and allowed them to replicate once
2. extracted the DNA from these bacteria and centrifuged it
3. continued growing the first generation bacteria but this time in a 14N medium (replication cycle 1)
4. centrifuged the DNA from these bacteria
5. continued growing the bacteria in a 14N medium (replication cycle 2)
6. centrifuged the DNA from these bacteria
7. continued growing the bacteria in the 14N medium (replication cycle 3)
8. centrifuged the DNA from these bacteria

Question 15

how did the results of the experiment prove DNA replicated by semi-conservative replication?

Answer 15

DNA of original bacteria - formed band near bottom of tube because only contained heavy 15N
DNA from replication cycle 1 - formed band in middle of tube because 14N was now being incorporated so both DNA strands contained light 14N and heavy 15N
DNA from replication cycle 2 - formed band near top of tube because 2 strands contained only 14N, formed band in middle of tube because 2 strands contained 14N and 15N
DNA from replication cycle 3 - formed band near top of tube because 6 strands contained only 14N, formed band in middle of tube because 2 strands contained 14N and 15N

Question 16

what would have happened in the experiment if DNA replicated by conservative replication?

Answer 16

no DNA strands containing both 14N and 15N would have been produced, and most strands would only contain 14N