1. DNA & It’s replication
Unit 1 – Human Cells

2. Structure of DNA Deoxyribonucleic Acid
Stores genetic information in it’s sequence of bases
Consists of two strands, made of nucleotides
3 parts:
- Deoxyribose sugar
- Phosphate
- Base
There are four types of base:
Adenine (A), guanine (G), cytosine (C), thymine (T)
Bases pair up : A-T, G-C

3. Structure of DNA
A strong chemical bond forms between phosphate groups and the deoxyribose sugars
Phosphate groups on the 5th carbon (5’) end
are added to the 3 carbon (3’) end of the deoxyribose
This forms the sugar- phosphate backbone
Nucleotides can only be added at the 3’ end
Therefore DNA strands run in opposite directions (anti- parallel)
Nucleotides are held together in the middle by hydrogen bonds

4. Structure of DNA
The 2 strands of DNA assemble themselves into a spiral ladder
- known as the double helix
DNA strands need to be tightly coiled in order to fit inside the nuclei of cells

5. DNA replication DNA is able to direct it’s own replication
- e.g. for cell division
Process starts with hydrogen bonds breaking
DNA strands then “unzip”
Bases exposed at a replication fork
Replication starts with a primer
- forms at the 3’ end
- complimentary nucleotides then form a continuous strand
- this is the leading strand
DNA polymerase helps join the nucleotides together
On the other side, the strand is copied in fragments (discontinuous)
- this is the lagging strand
- fragments are joined together by DNA ligase

6. DNA replication Many replication forks appear simultaneously
Requirements for replication:
- DNA
- primers
- nucleotides
- enzymes (ligase/polymerase)
- ATP (for energy)
DNA replication ensures an exact copy of genetic information is passed from cell to cell
- and from generation to generation