DNA
(Deoxyribonucleic Acid)
What is DNA?
DNA is an encoded molecule that determines
traits by giving instructions to make proteins.
Discovery of DNA Structure
• 1953 – James Watson
and Francis Crick
(Nobel Prize)
• Rosalind Franklin
Structure of DNA
• Very long molecule
Structure of DNA
• Made of nucleotides
– Each nucleotide:
• Sugar (deoxyribose)
• Phosphate group
(negatively charged)
• Nitrogen base
adenine (A)
guanine (G)
cytosine (C)
thymine (T)
Structure of DNA
• Double helix – 2 strands connected like a
ladder and twisted
Structure of DNA
• Base pairing rules
– In DNA …
• Adenine (A) always pairs with thymine (T)
• Cytosine (C) always pairs with guanine (G)
Calculating % of Bases
• If you know % of one base, you can calculate
the rest.
– Example: % of adenine = 34%
– Because A & T pair up, # adenine = # thymine
– So, thymine = 34%
– So, adenine and thymine make up 68% (34 + 34)
– So, cytosine and guanine together make up 32%
(100 – 68)
– Because C & G pair up, just divide 32 by 2.
– So, cytosine = 16% and guanine = 16%.
Importance of Base Sequences
• Sequence = order of bases
• Sequence of bases determines the proteins
made by the cell
• We can also use sequence to determine:
– How closely 2 organisms are related
– If 2 people are related (paternity)
– If crime scene DNA matches suspect’s DNA