1. KEY AREA 2: Structure & Replication of DNA
Higher Human Biology
Unit 1
Human Cells
KEY AREA 2: Structure & Replication of DNA

2. Human Cells Learning Intentions
KEY AREA 2 – Structure & Replication of DNA
Structure of DNA
DNA Replication

3. 2a) Structure of DNA
DNA Nucleotides are made of deoxyribose sugar, phosphate, base (ATGC)
Deoxyribose in a nucleotide has a base attached to its carbon 1, and a phosphate attached to its carbon 5
DNA consists of 2 strands of repeating units called nucleotides twisted into a double helix

4. 2b) Structure of DNA
DNA has a sugar-phosphate backbone due to the strong chemical bonds forming between the phosphate of one nucleotide and the carbon 3 of the deoxyribose on another nucleotide
The 2 DNA strands are held together by hydrogen bonds and have an antiparallel structure, with deoxyribose sugar and phosphate at 3’ and 5’ ends of each strand
Complementary base pairs are Adenine with Thymine, Guanine with Cytosine
Chromosomes consist of tightly coiled DNA packaged around bundles of proteins

5. This is a simplified diagram to show the process of DNA Replication
2c) DNA Replication
This is a simplified diagram to show the process of DNA Replication

6. 2d) DNA Replication
Before a cell divides the DNA must be replicated (copied)
Stages in DNA Replication
1. DNA double helix unwinds
2. Weak hydrogen bonds between base pairs break (2 strands unzip)
3. DNA template strands become stabilised and expose their bases at a Y-shaped replication fork
4. One strand of the replication fork is a template for the Leading Strand and the other is the template for the Lagging Strand.
Leading Strand
5. Replication of the DNA template strand that has the 3’ end is continuous and forms the leading strand of the replicated DNA
6. The DNA primer (made of a short sequence of nucleotides) must be present to create an existing chain for the enzyme DNA polymerase to work on
7. Free DNA bases align with complementary base pairs (A-T, G-C) on the template strand and become bound to the 3’ end of the primer
*NOTE* A DNA chain of nucleotides can only grow by adding nucleotides to
its 3’ end
8. DNA polymerase brings about the formation of the sugar-phosphate bonds
between the primer and the nucleotides, and individual nucleotides at the 3’ end
9. This results in the leading strand being replicated continuously
Lagging Strand
10. Replication of the DNA template strand that has the 5’ end is discontinuous (as it has to be replicated in fragments starting with the 3’ end of a primer)
11. Each fragment has to be primed to allow DNA polymerase to bind nucleotides together.
12. Once replication of a fragment is complete, ligase enzyme joins the fragments together to create the lagging strand of DNA

7. 2e) DNA Replication – Formation of the Leading Strand

8. 2f) DNA Replication – Formation of the Lagging Strand

9. 2g) DNA Replication DNA Replication (1:04)
Requirements for DNA Replication
1. DNA (to act as a template)
2. Primers (to create an existing chain for DNA polymerase to work on)
3. DNA nucleotides (Adenine, Thymine, Guanine, Cytosine)
4. Enzymes (DNA polymerase, ligase)
5. ATP (for energy)
DNA Replication is very important to ensure that new cells carry out their correct role

10. Human Cells Questions KEY AREA 2 – Structure & Replication of DNA
Testing Your Knowledge 1 Page 23 Q’s 1-3
2. Testing Your Knowledge 2 Page 31 Q’s 1-3
3. Quick Quiz