Page 157 - Big Ideas Year 10 sciPAD 2025

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SCIPAD - BIG IDEAS - BOOK 2
THE BASIS OF HEREDITY
C G
A
T A
G C
T
Base
Sugar
(deoxyribose)
Phosphate
Nucleotide
Base pair
C
A Closer Look at DNA
The structure of deoxyribonucleic acid (DNA) is the same in all living things. It looks
like a twisted ladder, known as a double helix. DNA is made up of simple repeating
units called nucleotides. A nucleotide is comprised of a sugar, a phosphate and a
base. The sides of DNA are made up of the sugar and the phosphate. These sides
are linked together by chemicals called nucleotide bases.
There are four different nucleotide bases in DNA – adenine (A), thymine (T), cytosine (C) and
guanine (G). Their different chemical make-up means that adenine always bonds with thymine,
and cytosine always bonds with guanine. This specific pairing of bases is called the “complementary
base-pairing rule”.
1. Describe the structure of a nucleotide. 


2. Describe the “complementary base-pairing rule”. 



3. Use the complementary base-pairing rule to complete the DNA strands shown below.
T A C
C T G
G A T T
C G C
C C G
C T A
G C T
A T T C
T C G
T A T
Remember ‘straight’
letters go together (i.e. A and T) and
curved letters go together (i.e. C and G)
G
4. Label the diagram of a nucleotide shown below.
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