6.1.5 Chargaff and Franklin and Wilkins: The DNA Story Begins

Question 1

note

Answer 1

• Once experimental results established that DNA was the genetic material, scientists intensified their efforts to characterize DNA’s molecular structure. The structure of DNA must allow the four basic functions of the genetic material, which include a means of storing large quantities of information. The first illustration below depicts the four subunits of DNA.
  • In 1950, Erwin Chargaff reported interesting findings about the relative amounts of nucleotidesfound in different species. The total amount of DNA was species-specific. Within a given species the amount of adenine relative to thymine was
  equal, as was the amount of cytosine relative to guanine.
  • Rosalind Franklinand Maurice Wilkins used x-ray diffractionto study DNA structure. The technique involves striking DNA crystals with x rays and photographing the resulting diffraction pattern.
• At first glance, DNA seems to be a simple molecule incapable of storing large amounts of information. The illustrations on the left show the four subunits of DNA. The subunits are called nucleotides based on molecular structure. Note that the nucleotides have identical sugar and phosphate groups. Two of the subunits have purine bases (adenine and guanine), and the other two have pyrimidine bases (thymine and cytosine). How can a molecule consisting of only four subunits direct the synthesis of thousands of proteins?
• The x-ray diffraction image provides information about
  DNA structure.
• The crisscross pattern of spots provides evidence that DNA is helical in shape. Three important sets of measurements were 2.0 nm, 0.34 nm, and 3.4 nm.
• Following Franklin and Wilkins’ findings, other scientists
  used these measurements to further their understanding of DNA structure and function.

Question 2

In analyzing the number of different bases in a DNA sample, Chargaff found that

Answer 2

• A + G = C + T

Question 3

Hershey and Chase’s work established that

Answer 3

• DNA was the genetic material

Question 4

Rosalind Franklin and Maurice Wilkins used X-ray diffraction technique to study DNA structure. The technique

Answer 4

• can be used to analyze molecular structure at near-atomic resolution

Question 5

If a double-stranded segment of DNA has been found to have 23 purines and 15 thymines, how many adenines are there?

Answer 5

• 15

Question 6

If a sample of DNA is 34% guanine (G), what percentage of the sample will be thymine (T)?

Answer 6

• 16%

Question 7

Why was it important that the structure of DNA be determined?

Answer 7

• It was not understood how DNA could act as genetic material