Meselson and Stahl’s Experiment

Question 1

step 1 : Bacteria are grown in a broth containing the …………… (…..N) nitrogen isotope

DNA contains nitrogen in its bases
As the bacteria replicated, they used nitrogen from the broth to make new DNA ……………….

After some time, the culture of bacteria had DNA containing only ……….. (…N) nitrogen

Answer 1

heavy 15n

nucleotides

heavy 15

Question 2

step 2 A sample of DNA from the 15N culture of bacteria was extracted and spun in a centrifuge

This showed that the DNA containing the ……….. nitrogen settled near the ……………… of the centrifuge tube

Answer 2

heavy

bottom

Question 3

step 3 The bacteria containing only ………N DNA was then taken out of the …….N broth and added to a broth containing only the lighter ……N nitrogen. The bacteria were left for enough time for one round of ……………….. before their DNA was extracted and spun in a centrifuge

Answer 3

15n

14n

DNA replication to occur

Question 4

If conservative DNA replication had occurred,

Answer 4

the original template DNA molecules would only contain the heavier nitrogen and would settle at the bottom of the tube, whilst the new DNA molecules would only contain the lighter nitrogen and would settle at the top of the tube

Question 5

If semi-conservative replication had occurred

Answer 5

all the DNA molecules would now contain both the heavy 15N and light 14N nitrogen and would therefore settle in the middle of the tube (one strand of each DNA molecule would be from the original DNA containing the heavier nitrogen and the other (new) strand would be made using only the lighter nitrogen)

Question 6

Meselson and Stahl confirmed that the bacterial DNA had undergone semi-conservative replication.
The DNA from this second round of centrifugation settled in

Answer 6

in the middle of the tube, showing that each DNA molecule contained a mixture of the heavier and lighter nitrogen isotopes

Question 7

If more rounds of replication were allowed to take place, the ratio of 15N:14N would go from….. after the first round of replication, to ………… after the second and …………….. after the third

Answer 7

1:1

3:1

7:1

Question 8

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