Question 1

Which experimental technique did Alma Howard and Stephen Pelc use to visualize newly synthesized DNA in cells?

Accepted Answer

Autoradiography

Question 2

What was the main finding of Howard and Pelc’s experiment regarding DNA synthesis in eukaryotic cells?

Accepted Answer

DNA synthesis occurs during a specific period in interphase

Question 3

Radioactive thymidine was used in later experiments to confirm DNA synthesis. Why is thymidine a suitable marker for DNA synthesis?

Accepted Answer

It is only incorporated into DNA

Question 4

What is the significance of the S phase in the cell cycle?

Accepted Answer

It is when the cell copies its genetic material

Question 5

Which of the following best describes the cell cycle as defined by Howard and Pelc?

Accepted Answer

The sequence of events from chromosome duplication to cell division

Question 6

If a scientist observes black dots in the autoradiography film of a cell sample, what does this indicate?

Accepted Answer

Active DNA synthesis

Question 7

Why did Howard and Pelc not observe black dots in M-phase cells during their experiment?

Accepted Answer

DNA synthesis does not occur during M phase

Question 8

Which of the following is NOT a correct statement about the S phase?

Accepted Answer

It is when chromosomes are partitioned into daughter cells

Question 9

A researcher wants to track DNA synthesis in a population of cells. Which radioactive molecule should they use for the most specific results?

Accepted Answer

Radioactive thymidine

Question 10

Which of the following equations best represents the relationship between the cell cycle phases, where $$T_{cycle} is $$the total cell cycle time, $$T_S is $$the S phase duration, $$T_M is $$the M phase duration, and $$T_{other} is $$the duration of other phases?

Accepted Answer

$$T_{cycle} = T_S + T_M + T_{other}$$

Question 11

A mutation prevents a cell from entering S phase. What is the most likely consequence for the cell?

Accepted Answer

The cell will not replicate its DNA and cannot divide properly

Question 12

Which of the following best explains why radioactive phosphorus can be used to detect DNA synthesis?

Accepted Answer

Phosphorus is a component of the DNA backbone

Question 13

If a cell is exposed to radioactive thymidine and then immediately enters M phase, what would you expect to see in an autoradiograph?

Accepted Answer

No black dots, because DNA synthesis does not occur in M phase

Question 14

Which of the following statements about interphase is correct?

Accepted Answer

Interphase includes S phase, when DNA is replicated

Question 15

A scientist observes that a certain chemical blocks the incorporation of radioactive thymidine into DNA. What process is most directly affected?

Accepted Answer

DNA replication

The Discovery of S Phase and the Cell Cycle in Eukaryotic Cells

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