DNA Replication

Question 1

What is the monomer/ repeating unit of DNA?

Answer 1

A nucleotide

Question 2

What is a nucleotide?

Answer 2

the monomer/ repeating unit used in DNA

Question 3

What is the name of the bond between 2 nucleotides adjacently?

Answer 3

Phosphodiester bond

Question 4

What is ribose?

Answer 4

The pentose sugar used in RNA

Question 5

What is the pentose sugar used in:

a) DNA
b) RNA

Answer 5

a) Deoxyribose

b) Ribose

Question 6

What is deoxyribose?

Answer 6

The pentose sugar used in DNA

Question 7

What type of bonds form between the bases of oposite nucleotides (between 2 strands of DNA)?

Answer 7

Hydrogen bonds

Question 8

Hydrogen bonds are used where in DNA?

Answer 8

Between 2 strands of DNA/ the bases of opposite nucleotides

Question 9

DNA strands have a leading and lagging strand. What does this mean the DNA’s strands are?

Answer 9

Anti-parallel strands of DNA

Question 10

DNA has anti-parallel strands. What does this means?

Answer 10

The strands travel in opposite directions - so DNA has a leading strand (forward) & a lagging strand

Question 11

What type of replication does DNA go through? What does this mean?

Answer 11

Semi-conservative replication. This means that from the original 2 parent strands, the 2 new DNA double strands have 1 parent strand each with a newly copied one. Old=new new=old

Question 12

Why doesn’t the DNA unzip completely, and then the free nucleotides attach to each parent strand?

Answer 12

The parent strands would get separated and lost in the nucleoplasm

Question 13

Talk about the:

a) leading strand
b) lagging strand

Answer 13

a) All the bases can be added in order because DNA polymerases’ active site can bind to same end unzips from due to sugar shape & direction, the shape is complementary. Continuous synthesis until it reaches the end of replication
b) The rest of the DNA hasn’t unwound yet, and these bases need to start bonding from the unwound end. So it synthesises in fragments. DNA ligase enzymes join the fragments

Question 14

When does DNA replication occur in the cell cycle?

Answer 14

Interphase

Question 15

Interphase (DNA replication) can only replicate DNA at the same speed in every cell, but the frequency varies. True or false?

Answer 15

True

Question 16

Where can DNA be found?

Answer 16

In the nucleus, also in the mitochondria (only from mother) & in chloroplasts, but scientists not 100% sure why those last 2

Question 17

Explain the stages of semi-conservative replication in DNA

Answer 17

1) DNA unwinds, catalysed by gyrase enzyme. Done a gene at a time, not completely
2) DNA unzips, catalysed by DNA helicase. Breaks the hydrogen bonds between bases, again not completely
3) Free phosphorylated nucleotides in the nucleoplasm (nucleus) bond to the newly exposed bases. Doesn’t need an enzyme, because they are complimentary
3b) If it’s the lagging strand, the joined bases are done in fragments, which are joined by ligase enzymes
4) Strong sugar phosphate back bone is rejoined by DNA polymerase enzyme (phosphodiester bonds, condensation reaction)

Question 18

What do these enzymes do?

a) gyrase
b) DNA helicase
c) ligase
d) DNA polymerase

Answer 18

a) Unwinds the DNA (done a gene at a time)
b) Unzips the DNA - break the H bonds between bases
c) On the lagging strand, joins the completed fragments
d) Rejoins the phosphates, creating the strong sugar phosphate backbone (phosphodiester bonds)

Question 19

What type of reaction creates a phosphodiester bond?

Answer 19

A condensation reaction

Question 20

Who proved that it was semi-conservative replication?

Answer 20

Meselson & Stahl

Question 21

Watson & Krik proved that DNA replicated through semi- conservative rplication. True or false?

Answer 21

False - Meselson & Stahl did. Watson & Krik (after taking Rosalyn Franklin’s images) drew the DNA looking like it did and guessed it was semi-conservative replication, but weren’t able to confirm it.

Question 22

Reorder the stages of Meselson & Stahl’s experiment, and what is did.
1) Now when put in the centrifugation tube, it forms a layer in the middle of the tube, as this is hybrid DNA with equal amounts of heavy & light
2) Using entirely different bacteria & dish, this one only exposed to heavy N. So the bacteria grows with the heavy N int its DNA, and forms a layer at the bottom of the centrifugation tube
3) The new hybrid cells were put back onto a light N plate, and replicate once. Some strands are pure light, some are still half and half (N 15 from parent strand). SO now in the centrifugation tube, 2 layers of DNA are formed - one at the top and one in the middle
4) DNA bases are made from nitrogen and carbon. They used the isotopes N 14 (light nitrogen) and N 15 (heavy nitrogen)
5) They put bacteria in a petri dish, with only access to light nitrogen. The bacteria grew with it, so all the bacteria’s bases are now light nitrogen based
6) They got a scoop of light bacteria on a new petri dish, only exposed to heavy N. They left it for 20 mins, as they only wanted it to replicate ONCE! (the new cells have a strand of N 14 and N 15 in their DNA, light=heavy)
7) In a centrifugation tube, they put in a small sample from each dish. The N 14 bacteria was light so a layer at the top of the test tube
This proves semi-conservative replication because the only way you could get a band in the middle would be if a new hybrid made from light and heavy strands equally were made - exactly what semi-conservative replication means

Answer 22

Proved DNA replicates with semi-conservative replication

4) DNA bases are made from nitrogen and carbon. They used the isotopes N 14 (light nitrogen) and N 15 (heavy nitrogen)
5) They put bacteria in a petri dish, with only access to light nitrogen. The bacteria grew with it, so all the bacteria’s bases are now light nitrogen based
7) In a centrifugation tube, they put in a small sample from each dish. The N 14 bacteria was light so a layer at the top of the test tube
2) Using entirely different bacteria & dish, this one only exposed to heavy N. So the bacteria grows with the heavy N int its DNA, and forms a layer at the bottom of the centrifugation tube
6) They got a scoop of light bacteria on a new petri dish, only exposed to heavy N. They left it for 20 mins, as they only wanted it to replicate ONCE! (the new cells have a strand of N 14 and N 15 in their DNA, light=heavy)
1) Now when put in the centrifugation tube, it forms a layer in the middle of the tube, as this is hybrid DNA with equal amounts of heavy & light
3) The new hybrid cells were put back onto a light N plate, and replicate once. Some strands are pure light, some are still half and half (N 15 from parent strand). SO now in the centrifugation tube, 2 layers of DNA are formed - one at the top and one in the middle

This proves semi-conservative replication because the only way you could get a band in the middle would be if a new hybrid made from light and heavy strands equally were made - exactly what semi-conservative replication means