Lesson 1: What is DNA?

Basic Structure of the DNA Module

James D. Watson and Francis H.C. Crick, working at Cambridge University in the 1950’s elucidated the basic structure.

Structure contains nucleotides and will be explained further in text

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The basic building block of DNA is a molecule called a nucleotide. Each nucleotide is made up of three parts:

  1. a molecule of sugar (deoxyribose)
  2. a phosphate module
  3. a molecule called a base that contains several atoms of nitrogen
    1. There are two classes of bases in DNA:
      1. large molecules composed of two rings containing nitrogen and carbon atoms called purines: adenine (A) and guanine (G) are examples
      2. smaller molecules composed of one ring containing nitrogen and carbon atoms called pyrimidines: cytosine (C) and thymine (T) are examples

Chromosomes Consists of Modules of DNA

Each of our forty-six chromosomes consists of very long molecules of DNA. Each DNA molecule is like a tremendously long ladder, with tens of hundreds of thousands of rungs. To form the DNA molecule, the ladder is twisted into a helix. If we straighten out the ladder, we can visualize the molecule. The two uprights (the side pieces) are composed of alternating sugar and phosphate molecules. The rungs of the straightened-out-ladder are pairs of bases. Bases are always attached to deoxyribose and never to phosphate. Phosphates always occur between two deoxyribose molecules, linking them together. Each upright and the bases attached to it are called the backbone. A backbone is composed of repeating subunits called nucleotides. Each nucleotide is composed of one sugar (deoxyribose), one phosphate, and one base chemically bonded together.

NHGRI Fact Sheet- Deoxyribonucleic Acid (DNA) (26990477451).jpg
By National Human Genome Research Institute (NHGRI) from Bethesda, MD, USA – NHGRI Fact Sheet: Deoxyribonucleic Acid (DNA), CC BY 2.0, Link

Another way to look at a portion of the backbone where we can better evaluate and work within writing is:

+ - + - + - + - + - + - +-
A   G   C   T   G   C   A


  • + = deoxyribose sugar molecule
  • – = phosphate molecule
  • A = molecule of the base adenine
  • G = molecule of the base guanine
  • C = molecule of the base cytosine
  • T = molecule of the base thymine
  • +- A = an individual nucleotide

As Watson and Crick discovered, one molecule of DNA has two backbones. They also discovered that an adenine (A) of one backbone always paired with a thymine (T) in the other, and a guanine (G) in one backbone is always paired with a cytosine (C) in the other. Each pair of bases has one purine (large) paired with one pyrimidine (small). Thus, the distance between two sugar-phosphate backbones never varies – another finding of Watson and Crick. The bases of one backbone are held to their complementary counterparts in the other backbone by weak bonds called hydrogen bonds. There are two hydrogen bonds between each adenine-thymine pair and three hydrogen bonds between each guanine-cytosine pair. Because of conformational differences, adenine is unable to pair with cytosine, and guanine is unable to pair with thymine.

We can now visualize both backbones of our DNA molecule as a ladder lying on its sides with the rungs made up of adenine hydrogen bonded to thymine and guanine hydrogen bonded to cytosine.

  +  - +  - + - + - + -  + - + -
  A    G    C   T   G    C   A
  **  ***  ***  ** ***  ***  **
  T    C    G   A   C    G   T 
- +  - +  - + - + - + -  + - +
  • + = deoxyribose sugar molecule
  • – = phosphate molecule
  • A = molecule of the base adenine
  • G = molecule of the base guanine
  • C = molecule of the base cytosine
  • T = molecule of the base thymine
  • * = hydrogen bonds between base pairs
  • AT, GC, CG, TA, GC, CG, and AT are base pairs
  • +- and a base are individual nucleotides

Several additional facts are needed to complete our understanding of DNA:

  1. The pattern described above runs continuously from one end of the molecule to the other – there are no exceptions, no gaps, no bulges.
  2. If I know one member of a base pair is adenine (A), its partner in the other backbone will always be thymine (T), G will always be paired with C, etc. Thus, if I can find the base sequence of one backbone, I know what the base sequence on the other backbone must be.
  3. Even though I know what base must occur across from an adenine for that rung (base pair), I do NOT know what bases are adjacent to it on the rung above or below.
That is,  if one rung is
the adjacent base pairs could be any one of the following base pairs:
 C       G      T
***  or ***  or **
 G       C      A

Since a chromosome is a series of genes, linked end to end, how does this DNA structure fit in with the concept of genes? Go to  Section 1: Genes