1. DNA Deoxyribonucleic Acid Structure and Replication
Higher Biology
DNA
Deoxyribonucleic Acid
Structure and Replication

2. DNA and Genes A determines an organisms characteristics.
Genes are found on
Chromosomes are found in the of all cells.
Chromosomes are threadlike structures made from
gene
chromosomes
nuclei
DNA
DNA
nuclei
chromosomes
gene

3. DNA Structure
In 1953 the structure of the DNA molecule was explained for the first time by two scientists in Cambridge- James Watson and Francis Crick.
They showed that DNA:
Consists of two strands
Each strand is made of repeating units (like monomers) called nucleotides
The two strands are antiparallel (run in opposite directions)
The two strands wrap around each other to form a double helix.

4. DNA Structure
DNA structure 1
DNA structure 2

5. Nucleotides Nucleotide structure
Insert and label the “nucleotide diagram”, from your pack, with the following:
Nitrogenous or organic base
Deoxyribose sugar
Phosphate
These structures are held together by strong chemical bonds.

6. Nucleotides In DNA there are 4 different organic bases. They are:
Adenine
Thymine
Cytosine
Guanine

7. Nucleotides
In DNA the bases ALWAYS bond (or pair) together according to the following rule
Adenine Thymine
Cytosine Guanine
The bonds that hold the complementary bases together are weak hydrogen bonds.
Insert and complete the “Structure of DNA” diagram from your diagram pack.

8. Quick Questions What shape is the DNA molecule?
What type of bonding holds the two DNA strands together?
Name the three components of a nucleotide.
Name the organic base which pairs with cytosine.
Name the organic base which pairs with thymine.

9. Quick Questions What do the following symbols represent? P S A T G C
phosphate
sugar
adenine
thymine
guanine
cytosine

10. DNA Replication
During cell division DNA must be replicated (or copied) so that each daughter cell carries an identical set of instructions to those in the parent cell.
DNA is a unique molecule because it can copy itself exactly.
The replication is described as semi-conservative because each of the 2 resulting DNA molecules is composed of one original (or conserved) and one new strand.

11. DNA Replication
Replication of DNA is a fast but complex process that requires:
A DNA template
Several enzymes including DNA polymerase (the enzyme which joins new nucleotides to the growing DNA strand)
A supply of the 4 types of DNA nucleotides
A supply of ATP to provide energy
Watch the DNA replication animation.

12. DNA Replication Briefly the stages in DNA replication are:
The DNA molecule unwinds.
The DNA molecule unzips (as the weak hydrogen bonds, between complementary bases, break).
Free nucleotides align themselves with their complementary nucleotide on the open chain.

13. DNA Replication
New weak hydrogen bonds form between complementary bases (remember A-T, G-C).
Adjacent new nucleotides are linked through the sugar and phosphate molecules by strong chemical bonds to form the “backbone” of the new strand.
The new molecule winds up into a double helix.

14. DNA Replication

15. DNA Replication
Insert and complete the “DNA replication” diagram from your pack.
Use 2 different colours to draw a diagram to illustrate what is meant by semi- conservative replication.

16. Practice Questions From the Torrance textbook: TYK page 47 Q4
AYK page 49 Q1-6.