6.2 DNA Structure 

DNA (deoxyribonucleic acid) is a slightly acidic molecule, rich in nitrogen and phosphorus. 

It carries hereditary information, controls cellular processes, and is able to replicate itself.

In the mid-20th century, scientists discovered the structure of DNA. 

Rosalind Franklin used a technique called X-ray diffraction analysis, where DNA was bombarded with X-rays, which produced a pattern on a photographic film. It appeared that DNA had a corkscrew, or helical shape. Watson and Crick were then presented with this information, without permission or credit, and built their famous model of the double-helix structure of DNA. It looks like it has two “backbones”. 

They were awarded the Nobel Prize for Physiology or Medicine in 1962.

1865: Gregor Mendel cross pollinated pea plants, as he wanted to know how to draw genotypes and phenotype charts. 

1868: Frederick Meisher discovered nuclein

1928: Frederick Griffith discovered transformation by studying bacteria and attempting to transform them.

1944: Oswald Avery, Colin McLeod, and Maclyn McCarty discover nda as the transforming substance. 1952: Alfred D. Hershey and Martha Chase conclude that DNA carries genetic material.

1952: Rosalind Franklin and Maurice Wilkins use X-ray crystallography to study DNA shape. Franklin suggests the double helix model. 

1952: James Watson and Francis Crick confirm the double helix model of DNA. \n

DNA is made of three main components:

  1. A deoxyribose sugar
  2. A phosphate group
  3. A nitrogenous base
  • The source of variation in DNA is due to the organization of its nitrogenous bases.

DNA is a polymer (a molecule of many repeating units) and each unit is called a nucleotide. \n

A nucleotide is a molecule that consists of a 5-carbon sugar with a nitrogenous base attached to their 1’ carbon of the sugar (using a glycosyl bond), and one phosphate group that is attached to the 5’ carbon of the sugar (using an ester bond, called a phosphodiester bond).

The five carbon atoms are numbered clockwise, starting with the carbon just to the right of the oxygen atom. This carbon is called the 1’ (or “one prime”) carbon.

Along the backbone, the 3’ and 5’ carbons of the sugar are used, and the 1’ is used to attach the base. The DNA molecule consists of two antiparallel strands of nucleotides. The bases on one strand are paired with the bases on the other, but one strand is “upside down” compared to the other one. One strand runs from 3’ to 5’ (referring to the sugar) and the other goes from 5’ to \n

As discussed before, there are 4 types of bases:

  1. Adenine (A)
  2. Thymine (T)
  3. Guanine (G)
  4. Cytosine (C)

A purine (A & G)  is always bonded with a pyrimidine (T & C). This is called a complementary base pairing. 

The rules are: 

  • A always bonds with T, using 2 hydrogen bonds
  • G always bonds with C, using 3 hydrogen bonds.

Therefore, if you know the sequence of one strand of DNA, you will automatically know the sequence of the other, based on these pairing “rules”. Because of this, by convention, only the 5’ to 3’ strand is written, as the other one doesn’t need to. \n

Visually, the purines (A or G) have two chemical rings in their structure, and the pyrimidines (C or T) have one chemical ring in the base structure.