2.3

Discovering the
structure of DNA

At the end of this subtopic you will be able to outline the key events in
the discovery and understanding of the structure of DNA.

The discovery of

DNA

Friedrich Miescher discovered
DNA in 1869.
Miescher isolated DNA from the
nuclei of white blood cells taken
from pus on bandages.
Miescher named the compound
he isolated ‘nuclein’.

There are 4 nitrogenous bases
found in nucleotides in DNA:

1. Adenine
2. Thymine
3. Cytosine
4. Guanine

The phosphate and sugar
(deoxyribose) is the same in each.

The nitrogenous base changes in
each nucleotide.

Types of nucleotides in DNA

Nucleotides in DNA

Purines have 2 rings and pyrimidines have 1.
Why do you think they are called “nitrogenous bases”?
Complementary bases (AT and CG) - a purine matches up with a
pyrimidine

Purines
Pyrimidines

Complementary bases

Chargaff’s rule:
Cytosine and Guanine will always
pair with each other (held
together by 3 hydrogen bonds).

Adenine and Thymine will always
pair with each other (held
together by 2 hydrogen bonds).

Hydrogen bonds are
intermolecular forces that can
easily be broken.

Anti-parallel structure

The two chains of DNA run in
opposite directions.

Chain 1: runs from a 5’ to 3’
direction

Chain 2: runs from a 3’ to 5’
direction

3’ and 5’ refer to the numbers on
the carbons in the sugar.

DNA is stored as Chromosomes

DNA is stored as a linear
chromosome in the nucleus of
eukaryotic cells.

DNA is wrapped around
proteins known as histones to
form a complex called a
nucleosome.

Nucleosomes compact to form
a chromosome.