Lesson 1 — DNA Structure


The Structure of DNA

Read pages 628 - 629

As you have already learned, Rosalind Franklin successfully took an X-ray diffraction picture of the DNA molecule in 1953. Imagine looking at a DNA strand vertically from the top, and think about the shape that would result in a pattern shown below. As you may have realized, the x-pattern on the diffraction image suggested that DNA molecules had a helical shape.




Rosalind Franklin. From the personal collection of Jenifer Glynn.  National Library of Medicine.



Rosalind Franklin's X-ray Diffraction Image of DNA.  Franklin, R. E.; Gosling, R. G. (1953-09-10). "The structure of sodium thymonucleate fibres. I. The influence of water content". Acta Crystallographica. 6 (8): 673–677. doi:10.1107/S0365110X53001939 Used under the Fair Dealings Provision of the Canadian Copyright Act.

Structure of DNA. National Human Genome Research Institute. Public Domain

When Watson and Crick obtained a copy of this diffraction image, they were able to deduce that DNA formed a double helix similar to a twisted ladder. The bases formed pairs on the inside similar to the rungs of the ladder and the sugar-phosphate backbones acted as the rails of the ladder. By measuring the distance between the patterns, they were able to calculate the dimensions of the double helix and determine the number of bases involved in a single turn of the helix.

DNA Nucleotides
The information in DNA molecules is stored as a code of repeating units called nucleotides. These nucleotides are connected into a long strand, and each DNA molecule consists of two nucleotide strands.

A nucleotide consists of a phosphate group, a deoxyribose sugar, and a nitrogen base. It is the basic building block for DNA and RNA. (You will investigate RNA more closely later in the module.) The sequence of these bases contains the information for growth and maintenance of an organism. Humans have approximately 3.2 billion bases packaged into 23 chromosomes.

Nucleotide. National Human Genome Research Institute.  Public Domain.

Complementary Base Pairs

Complementary Base Pairs. National Human Genome Research Institute. Public Domain. 
Four bases occur in a DNA molecule: adenine (A), cytosine (C), guanine (G), and thymine (T). As mentioned before, the DNA is a double-stranded molecule. The bases on one strand pair with the complementary bases on the other strand. These base pairs are held together by hydrogen bonds, and they make up the rungs on the DNA ladder.

The bases always pair with each other in the same way:

  • adenine with thymine

  • cytosine with guanine
The A-T and C-G base pairs are called complementary base pairs.

Example
A tissue sample DNA contains 32% adenine. Determine the percent composition of guanine in this sample.

Solution
According to Chargaff's Rule and what you have learned about the complementary base pairs, the amount of adenine should equal the amount of thymine.

  • 32% adenine = 32% thymine
This leaves 36% of the DNA remaining for cytosine and guanine.

  • 100% - (32% adenine + 32% thymine) = 36% for cytosine and guanine
Because cytosine and guanine are in equal presence, divide the remaining percentage of DNA into two.

    «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mfrac»«mrow»«mn»36«/mn»«mo»%«/mo»«/mrow»«mn»2«/mn»«/mfrac»«mo»=«/mo»«mo»§#160;«/mo»«mn»18«/mn»«mo»%«/mo»«mo»§#160;«/mo»«/math»
Thus, 18% guanine is in the tissue sample DNA.