2b. DNA REPLICATION

Question 1

Describe what the 3’ end of a single DNA strand is

Answer 1

The 3′ end (three prime end) of a strand is so named due to it terminating at the hydroxyl group of the third carbon on the pentose sugar.

Question 2

Describe what the 5’ end of a single DNA strand is

Answer 2

The 5’ end (five prime end) of the DNA is the one with the terminal phosphate group on the 5’ carbon of the pentose sugar.

Question 3

In a DNA molecule, the two DNA strands are antiparallel. Describe what this means.

Answer 3

One DNA strand goes in the 3’ to 5’ direction and the other strand goes in the 5’ to 3’ direction. (Run next to each other but in different directions)

Question 4

Describe why DNA replication is ‘semi-conservative’

Answer 4

One strand is old, and one strand is new

Question 5

During which other cellular process does DNA replication occur?

Answer 5

Mitosis

Question 6

Name the two enzymes involved in DNA replication

Answer 6

DNA Helicase and DNA Polymerase

Question 7

Describe the function of DNA Helicase

Answer 7

To unzip the DNA strands by catalysing the breaking of hydrogen bonds between complementary base pairs bases

Question 8

Describe the function of DNA Polymerase

Answer 8

It catalyses the formation of phosphodiester bonds between adjacent nucleotides, forming the new strands

Question 9

Which strands are continuous and discontinuous replication carried out on?

Answer 9

Continuous replication occurs on the leading strand. Discontinuous replication occurs on the lagging strand.

Question 10

Why can continuous replication be carried out on the leading strand?

Answer 10

DNA polymerase has a specific shape and therefore can only bind at the 3’ end. The leading strand has an exposed 3’ end.

Question 11

In continuous replication, what direction does DNA Polymerase run, and in what direction are new nucleotides added?

Answer 11

DNA Polymerase runs 3’ to 5’. The new nucleotides are added from 5’ to 3’

Question 12

Why does discontinuous replication have to be carried out on the lagging strand?

Answer 12

DNA polymerase has a specific shape and can therefore only bind at the 3’ end. The lagging strand has an exposed 5’ end.

Question 13

What are Okazaki fragments?

Answer 13

The short fragments/sequences of DNA formed on the lagging strand in discontinuous replication

Question 14

What happens to Okazaki fragments at the end of DNA replication?

Answer 14

Okazaki fragments are joined together with an enzyme to create a complete strand

Question 15

Recall the two nitrogen isotopes that can be used to form nitrogenous bases. Which is heavy and which is light?

Answer 15

Nitrogen -14 (light) and Nitrogen-15 (heavy)

Question 16

What do we call the original DNA molecule?

Answer 16

Generation 0

Question 17

Generation 0 was produced in N14. Generation 1 was produced in N15. Compare the densities of Gen 0 and Gen 1.

Answer 17

Generation 1 will have a higher density compared to Generation 0

Question 18

Generation 0 was produced in N15. Generation 1 was produced in N14. Compare the densities of Gen 0 and Gen 1.

Answer 18

Generation 1 will have a lower density compared to Generation 0

Question 19

How will you know from the centrifuge tube that the molecules have different densities?

Answer 19

The more dense molecules are lower down and the less dense molecules are higher up

Question 20

What two things are current scientific models based on?

Answer 20

Experimental evidence/observations and current knowledge

Question 21

What will lead scientists to accept or reject an experimental hypothesis?

Answer 21

Using evidence from repeatable experiments

Question 22

What will lead scientists to change a scientific model or theory?

Answer 22

New evidence that disproves old theories and is repeatable.

Question 23

Why were proteins favoured over DNA to be the molecule of inheritance?

Answer 23

Scientists thought that DNA was too simple and has too few components to be the molecule of inheritance. Proteins are more complex.

Question 24

Name the two strains of streptococcus pneumoniae that were used by Griffith in his experiment. Describe if they were virulent or avirulent.

Answer 24

Smooth (virulent) and rough (avirulent)

Question 25

Describe the surprising result in Griffith’s experiment with mice and streptococcus pneumoniae

Answer 25

When mice were injected with a heat-killed version of the smooth strain and the rough strain together, they still died.

Question 26

What did Griffith name the discovery from his experiment with streptococcus pneumoniae?

Answer 26

The transforming principle

Question 27

What is a bacteriophage? What are they made from?

Answer 27

Viruses that infect bacteria. They are made from a protein coat containing DNA

Question 28

Describe which isotopes Hershey and Chase radiolabelled their bacteriophages with.

Answer 28

P32 and S35

Question 29

In a centrifuge tube, what do you call the solid at the bottom, and the liquid?

Answer 29

solid = pellet
liquid = supernatant

Question 30

In Hershey & Chase’s experiment, where were the radiolabelled proteins found?

Answer 30

Sulphur is found in the proteins, but not DNA so it was found in the supernatant

Question 31

In Hershey & Chase’s experiment, where was the radiolabelled DNA found?

Answer 31

Phosphorus is found in DNA, but not protein so it was found in the pellet.