Question 1

Which experimental evidence was most critical in leading Watson and Crick to propose the double helical structure of DNA?

Accepted Answer

Franklin’s X-ray diffraction data showing a helical structure with uniform width

Question 2

According to Chargaff’s rules, if a DNA molecule contains 30% adenine, what percentage of guanine would you expect?

Accepted Answer

$$20\%$$

Question 3

Why does pairing a purine with a pyrimidine in DNA result in a uniform width of the double helix?

Accepted Answer

A purine is larger and a pyrimidine is smaller, so together they fit the helical structure

Question 4

Which of the following best describes the orientation of the two DNA strands in the double helix?

Accepted Answer

The strands run antiparallel: one 5′ to 3′, the other 3′ to 5′

Question 5

What is the distance between adjacent base pairs in the DNA double helix?

Accepted Answer

$$3.4\ 	ext{Å}$$

Question 6

Which feature of DNA structure explains why the amount of adenine equals the amount of thymine in a DNA molecule?

Accepted Answer

Adenine pairs with thymine via hydrogen bonds

Question 7

In the Meselson-Stahl experiment, what result after one round of replication in $$^{14}	ext{N}$$ medium would support the semi-conservative model of DNA replication?

Accepted Answer

One band at an intermediate density between heavy and light DNA

Question 8

Which chemical group is found at the 5′ end of a DNA strand?

Accepted Answer

A phosphate group ($$-PO_4$)

Question 9

If a DNA molecule is 3.4 nm long, how many base pairs does it contain? (Assume $$0.34\ 	ext{nm}$$ per base pair.)

Accepted Answer

$$10$$ base pairs

Question 10

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

Accepted Answer

It is hydrophilic and found on the outside of the helix

Question 11

Which base pairing is stabilized by three hydrogen bonds in DNA?

Accepted Answer

Guanine–Cytosine

Question 12

A scientist isolates a DNA molecule and finds that $$22\% of $$the bases are guanine. What percentage of the bases are thymine?

Accepted Answer

$$28\%$$

Question 13

Which of the following best explains why DNA replication is considered semi-conservative?

Accepted Answer

Each new DNA molecule contains one old strand and one new strand

Question 14

Which scientist’s data was crucial for Watson and Crick’s model but was not recognized with a Nobel Prize?

Accepted Answer

Rosalind Franklin

Question 15

What would happen to the DNA double helix if purines always paired with purines and pyrimidines with pyrimidines?

Accepted Answer

The helix would have regions that are too wide or too narrow

Question 16

Which of the following is a direct result of the antiparallel arrangement of DNA strands?

Accepted Answer

Replication can proceed in both directions from a single origin

Question 17

In the Meselson-Stahl experiment, what would you expect to see after two generations if DNA replication was dispersive?

Accepted Answer

One band at an intermediate position

Question 18

Which of the following best describes the chemical structure of a nucleotide in DNA?

Accepted Answer

A phosphate group, a deoxyribose sugar, and a nitrogenous base

Question 19

Which of the following is NOT a feature of the Watson-Crick model of DNA?

Accepted Answer

The two strands are parallel

LEC 18: The Structure and Replication of DNA: Discovery, Evidence, and Models

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