L5 DNA replication

Question 1

Which one of the following conclusions is supported by Chargaff’s chromatography data on the base composition of DNA in different species?
a) DNA is helical in structure
b) DNA contains pairings between one purine and one pyrimidine
c) AT base-pairs are joined by two hydrogen bonds
d) DNA strands are anti-parallel
e) Eukaryotic chromosomes are linear

Answer 1

b) DNA contains pairings between one purine and one pyrimidine

Question 2

What are the three types of DNA replication?

Answer 2

1. Conservative
2. Semi - Conservative
3. Dispersive

Question 3

What is conservative replication of DNA?

Answer 3

Conservative replication of DNA is when the parent DNA molecule is completely replicated and forms a new full daughter DNA molecule.

Question 4

What is Semi - conservative DNA replication?

Answer 4

Semi - conservative DNA replication is when a single strand of the DNA molecule is synthesised and replicated, in this we get a product of one parent strand and one newly synthesised strand.

Question 5

What is dispersive DNA replication ?

Answer 5

Dispersive DNA replication is when fragments of DNA are seperated and synthesised in fragments and then combines to form two daughter molecules of a mixture of the old and new DNA fragments.

Question 6

Which of the three types of replication is proven to be appropriate in human beings?

Answer 6

Semi - Conservative.

Question 7

Who was responsible for the proven type of replication?

Answer 7

Matthew Meselson and Franklin Stahl (1958)

Question 8

What did meselson and Stahl use to do their experiment?

Answer 8

Differences between isotopes, they used Nitrogen as DNA contained lot of N2 atoms.

Question 9

How did they use isoptopes of nitrogen to differentiate DNA?

Answer 9

They took advantage of their weight difference between 14N and 15N using it to seperate them.

Question 10

Give a short summary of the Meselson and Stahl experiment.

Answer 10

1. Bacteria grown in 15N will be heavy DNA.
2. They are transferred to media with 14N, the new DNA will be light.
3. The heavy and light molecules are then seperated by ultracentrifugation.

Question 11

What was added into the experiment for creating a density gradient to seperate the DNA molecules?

Answer 11

Caesium chloride gradients.

Question 12

How was the DNA differentiated by weight obtained?

Answer 12

1. DNA is isolated from cells which is then mixed with CsCl solution and placed in a centrifuge.
2. It was then centrifuged for days at a very high speed.

Question 13

What was observed after centrifugation?

Answer 13

1. DNA molecules moved to positions where their density was equal to CsCl solution.
2. There were three segments of DNA seperated.
3. The most heaviest DNA will be at the bottom.
4. The hybrid DNA is at the centre.
5. The light DNA is at the top.

Question 14

How is an origin of replication determined?

Answer 14

By a replication fork

Question 15

What are all the enzymes involved in DNA replication?

Answer 15

1. Primase
2. DNA polymerase
3. DNA ligase
4. Topoisomerase
5. Helicase
6. Single strand binding protein.

Question 16

What are the functions of DNA polymerase?

Answer 16

1. They add nucleotides one at a time in the 5’ - 3’ direction.
2. They use template strands to form H-Bonds.
3. They add tens or hundreds of nucleotides per second.

Question 17

What is the function of Primase?

Answer 17

Primase is responsible for generating a primer and they are made up of usually RNA.

Question 18

What does ligase do?

Answer 18

Ligase joins the loose ends together into a single strand of DNA by base pairing nucleobases.

Question 19

What does topoisomerase do?

Answer 19

Topoisomerase relieves pressure from
overwinding around the replication bubble by
making and resealing breaks in the DNA

Question 20

What does helicase do?

Answer 20

Helicase breaks hydrogen bonds
between the two DNA strands,
separating them

Question 21

What does SSB (Single strand binding) protein do?

Answer 21

SSB binds to the separated strands,
preventing them from reannealing

Question 22

What are leading strands?

Answer 22

They are the strands which get replicated continuously in the 5’ to 3’ direction.

Question 23

What are lagging strands?

Answer 23

Strands that are discontinuous and are replicated in pieces.

Question 24

What are the discontinous pieces of the lagging strand called?

Answer 24

Okazaki fragments

Question 25

How does the erosion of genetic material at ends of linear chromosomes happen?

Answer 25

1. Primer removal at the end of the chromosomes leaves a gap that cannot be filled as there is no DNA polymerase following up.
2. So, every round of replication a little piece is lost.
3. This is a problem for the lagging starnd at each end of the linear DNA.

Question 26

How is this erosion of end chromosomes solved?

Answer 26

Telomeres

Question 27

What are telomeres?

Answer 27

Short DNA sequences that are repeated over and over at the ends of the chromosomes.

Question 28

Which enzyme is responsible for the replinishing of telomeres at the end of each replication?

Answer 28

Telomerase.

Question 29

If DNA replication were conservative, how many DNA bands would
Meselson and Stahl observe after two rounds of replication in the
presence of the “heavy” nitrogen isotope?
a) 0
b) 1
c) 2
d) 3
e) 4

Answer 29

C. 2