Question 1

Which experimental result from Frederick Griffith's work with Streptococcus pneumoniae provided the first strong evidence that a 'transforming principle' could transfer genetic information between bacteria?

Accepted Answer

Mice injected with a mixture of heat-killed S cells and living R cells died, and living S cells were recovered from their blood.

Question 2

In the Hershey-Chase experiment, which radioactive isotope was used to label DNA, and what was the key finding regarding the location of radioactivity after phage infection of E. coli?

Accepted Answer

Radioactive phosphorus ($$^{32}P$$); radioactivity was found inside the bacterial cells.

Question 3

Which of the following best explains why proteins were originally considered more likely than DNA to be the genetic material?

Accepted Answer

Proteins are composed of 20 different amino acids, allowing for greater heterogeneity and specificity.

Question 4

According to Chargaff’s rules, if a DNA sample from a newly discovered organism contains 22% adenine, what percentage of guanine would you expect to find?

Accepted Answer

$$28\%$$

Question 5

What structural feature of DNA did Rosalind Franklin’s X-ray diffraction photograph reveal, which was critical for Watson and Crick’s model?

Accepted Answer

That DNA is a double helix with a uniform diameter.

Question 6

Why does pairing a purine with a pyrimidine in the DNA double helix result in a uniform diameter, as observed in X-ray diffraction studies?

Accepted Answer

Pairing two purines would be too wide, and two pyrimidines would be too narrow; only purine-pyrimidine pairs fit the observed width.

Question 7

Which of the following best describes the directionality of a DNA strand?

Accepted Answer

It runs from the 5' phosphate end to the 3' hydroxyl end.

Question 8

A scientist isolates DNA from a human cell and finds that 30.4% of the bases are adenine. According to Chargaff’s rules, what percentage of the bases should be cytosine?

Accepted Answer

$$19.6\%$$

Question 9

Which of the following statements about the sugar-phosphate backbone of DNA is correct?

Accepted Answer

The backbone is composed of alternating deoxyribose and phosphate groups, joined by phosphodiester bonds.

Question 10

In the Hershey-Chase experiment, what would have been the result if proteins, not DNA, were the genetic material of phages?

Accepted Answer

Radioactive sulfur would have been found inside the bacterial cells.

Question 11

Which of the following best explains the phenomenon of transformation observed by Griffith in his experiments with S. pneumoniae?

Accepted Answer

A heritable substance from dead S cells was taken up by living R cells, changing their genotype and phenotype.

Question 12

Which chemical group is found at the 3' end of a DNA strand?

Accepted Answer

A hydroxyl group ($$-OH)$$

Question 13

If a DNA molecule is 20% thymine, what is the percentage of guanine in the molecule?

Accepted Answer

$$30\%$$

Question 14

Which of the following is a direct consequence of the antiparallel arrangement of DNA strands in the double helix?

Accepted Answer

The sugar-phosphate backbones run in opposite directions, allowing complementary base pairing.

Question 15

Which of the following best describes the chemical difference between DNA and RNA nucleotides?

Accepted Answer

DNA contains deoxyribose sugar, while RNA contains ribose sugar.

Question 16

A DNA double helix makes one full turn every $$3.4\ \mathrm{nm}$$ and has $$0.34\ \mathrm{nm}$$ between stacked base pairs. How many base pairs are there per turn of the helix?

Accepted Answer

$$10$$

Question 17

Which of the following is a real-world application of the concept of transformation as demonstrated by Griffith’s experiment?

Accepted Answer

The genetic engineering of bacteria to produce human insulin by introducing recombinant DNA.

Question 18

Which of the following statements about hydrogen bonding in DNA is correct?

Accepted Answer

Guanine forms three hydrogen bonds with cytosine.

Question 19

If the distance between nucleotides in a DNA strand is $$0.34\ \mathrm{nm}$$, what is the approximate length of a human haploid genome with $$3.2 	imes 10^9$ nucleotides?

Accepted Answer

$$1.1\ \mathrm{m}$$

Question 20

Which of the following best explains why the sequence of bases in DNA can encode genetic information?

Accepted Answer

The sequence of bases determines the order of amino acids in proteins.

The Discovery and Structure of DNA as the Genetic Material

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