DNA is made of two strands that twist together in a spiral shape called a double helix. Each strand contains a series of bases - adenine (A), cytosine (C), guanine (G), and thymine (T). The bases bond together in a complementary pairing with A always bonding to T and C always bonding to G. The sequence of these bases provides a code that determines the proteins made in cells and thus controls development and characteristics.

1. DNA
UNIT 4 Biological Molecules

2. Starter – Food tests
• A student carried out tests on two foods.
– Starch test:
• Food A went brown
• Food B went black
– Benedict’s reagent:
• Food A went orange-red
• Food B went blue
• What do the results tell you about Food A and
Food B?

3. Learning Objectives
• Describe the structure of DNA

4. What is DNA?
• DNA stands for deoxyribonucleic acid.
• It is the chemical that makes up our genes &
chromosomes.
• It is inherited (passed down from your
ancestors) and is responsible for giving us
many of our characteristics.

5. The structure of DNA
• DNA is made of 2 long strands.
• Each strand has a series of bases
along it.
• The bases are held together by
bonds forming crosslinks
• The two strands twist together
to make a spiral called a double
helix. (see Figure 4.14 on page
47)

6. DNA base pairing
• There are 4 bases:
– Adenine A
– Cytosine C
– Guanine G
– Thymine T
For iGCSE you only need to
know the letters!!!

7. DNA Base Pairing
• If you look at the diagram on page 47, you will
see that:
– A always pairs up with T
– C always pairs up with G
• This is called complimentary base pairing.
T A
G C

8. Base pairing
• Base-pairing means you will always find equal
amounts of A and T in a DNA sample – as well
as equal amounts of G and C.
– For example:
• If a DNA sample contains 30% A
• There will be 30% T
• How much C will there be in this sample?
• How much G?

9. What does the sequence of bases do?
• The sequence provides a code that is used to
determine the kinds of proteins that are made
in our cells.
• This will determine how our cells, tissues and
organs develop.
• It will determine all kinds of this such as
personal characteristics (hair colour, eye
colour, blood group).

11. End of chapter questions
• Coursebook page 48