1. DNA and the Genome
Key Area 1a
The Structure of DNA

2. Learning Intentions By the end of this topic you should be able to:
Name the molecules in a DNA nucleotide and identify them in a diagram
Name the type of bond on the backbone of the DNA molecule
Give the names of the 4 DNA bases
Describe the base pairing rule for DNA bases
Describe the role of hydrogen bonds in the DNA structure
State the name of the coiled structure adopted by DNA

3. Learning Intentions
Identify the positions of 3’ and 5’ carbons on a DNA nucleotide
Identify the positions of 3’ and 5’ ends on a DNA strand
Describe how 2 strands of DNA align themselves to each other

4. You should already know that:
DNA is inherited.
DNA is the genetic material of living things.
DNA is located within the nucleus of all cells apart from red blood cells.
DNA is a long chemical sequence and this sequence contains the information needed for that living thing to develop, survive and pass its genetic information on to the next generation.
The DNA chemical sequence differs between individuals. The pattern of this sequence is called the genotype.

5. DNA = Deoxyribonucleic acid
What is DNA?
DNA = Deoxyribonucleic acid
DNA is the storage molecule found in ALL living cells whose sequence of bases forms the genetic code. This determines the organisms genotype and the structure of its proteins.
Chromosomes are made of DNA and are INHERITED
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6. Structure of DNA
DNA molecules are made of two strands wound together like a spiral ladder, this is called the DOUBLE HELIX.
The uprights are formed by ‘sugar-phosphate backbones’ and bases pair forming the rungs

7. Structure of DNA
A molecule of DNA has two strands of repeating units called nucleotides.
5’ end
3’ end
deoxyribose sugar
phosphate
nitroegeousbase
5’ pronounced “5 prime”

8. Carbon 1
Carbon 2
Carbon 3
Carbon 4
Carbon 5
Note the numbering of the carbons
C1 has the base attached, C5 the phosphate

9. Structure of DNA
DNA has four different types of nucleotides each with a different base (attached to C1).
Thymine
Adenine
Cytosine
Guanine

10. Base Pairs
Weak hydrogen bonds join each base to its complementary partner.
Adenine (A) only pairs with Thymine (T)
Guanine (G) only pairs with Cytosine (C)

11. A = adenine T =thymine
G = guanine C = cytosine
Strong chemical bond
Weak hydrogen bonds A G T C
Note that there are 2 hydrogen bonds between the bases A-T and 3 hydrogen bonds between G-C. This means more energy is required to break the G-C bonds in DNA than the A-T

12. Strong chemical covalent bonds (PHOSPHODIESTER BONDS) form between the phosphate group of one nucleotide (5C) and the sugar (3C) of another to form a permanent strand…(sugar-phosphate backbone)
DNA Strand
DNA Nucleotide
5 Carbon Sugar
(deoxyribose)
Phosphate Group
Base
strong chemical bond

13. The enzyme which links nucleotides in a chain is known as DNA Polymerase and it can only add nucleotides in the 5’ to 3’ direction (or onto the 3’ end of the growing strand) OH P O
CH2
H or OH H N 1 2 3 4 5 6 - 5’ 3’
Phosphate
Organic Base
Deoxyribose Sugar
Strong chemical bond

14. The result of the joining of the nucleotides is the SUGAR PHOSPHATE BACKBONE with the bases attached
A NUCLEOTIDE CHAIN

15. DNA is double stranded with the two strands running in opposite directions. This is known as a double stranded antiparallel arrangement. 5’ 3’