6.2.1 Watson and Crick: The Clues

Question 1

James Watson and Francis Crick

Answer 1

• were the first scientists to accurately describe DNA structure. They constructed a hypothetical model that incorporated the Erwin Chargaff’s findings and the findings of Rosalind Franklin and Maurice Wilkins.
• constructed a model of DNA using three sets of data:
  1. Chargaff’s rules. Erwin Chargaff found that within a given species, the amount of adenine and thymine are equal, and the amount of guanine and cytosine are equal.
  2. Repetitive amount ratios. Franklin and Wilkins’ data showed that some element repeated every 0.34 nanometers (nm) and another repeated every 3.4 nm. The molecule appeared to be 2.0 nm wide.
  3. Shape of molecule. Based on the diffraction pattern, the DNA molecule appeared to be helical in shape.

Question 2

Linus Pauling

Answer 2

• a contemporary of Watson and Crick, proposed that DNA consisted of three nucleotide strands. Watson
  and Crick’s double-stranded version proved to be correct

Question 3

note

Answer 3

• Based on the three sets of data, Watson and Crick proposed that DNA was a double-stranded helix, each strand consisting of a series of nucleotides.
• Franklin and Wilkins’ data showed that the two strands were exactly 2.0 nm apart (a). Illustration (b) shows that each strand consists of nucleotide subunits of a sugar, a phosphate, and a base. The subunits are 0.34 nm apart. The sugar and phosphate groups form the backbone of the strand, and the base is available for hydrogen bonding with a base on the other strand.
• To remain consistent with the width data, adenine (A) must bond only with thymine (T) and guanine (G) only with cytosine (C). The specific bonding between adenine and thymine and between guanine and cytosine also fits Chargaff’s results!

Question 4

Watson and Crick discovered that for the DNA helix to be 2.0 nm wide

Answer 4

• one pyrimidine and one purine need to be paired together

Question 5

True or false?
Crick and Watson were the first to use an X-ray diffraction crystallography technique to determine that DNA had a helical structure.

Answer 5

• false

Question 6

Watson and Crick determined that adenine paired with thymine and guanine paired with cytosine in DNA. How do the adenine bases bond to the thymine bases?

Answer 6

• hydrogen bonds

Question 7

Which is not a nucleotide base in DNA?

Answer 7

• uracil

Question 8

James Watson and Francis Crick were the first scientists to

Answer 8

• accurately describe the molecular structure of DNA

Question 9

The significance of the 0.34 and 3.4 nanometer measurements of DNA is that

Answer 9

• there are 10 bases per 3.4 nanometer-long turn of the helix