6.1.6
DNA Replication
DNA Replication
DNA Replication
When a cell divides, it forms two daughter cells. This means that before cell division, the cell must duplicate its DNA so that each daughter cell can inherit the full set of DNA.


DNA replication
DNA replication
- DNA is made up of two polynucleotide strands that form a double helix.
- During DNA replication, each of the two strands are used as a template from which new strands are copied.


Semi-conservative model
Semi-conservative model
- After replication, the new DNA is made up of one original polynucleotide strand and a new, complementary strand.
- This explains why DNA replication is described as semi-conservative because one original strand is conserved.


The process of replication (1)
The process of replication (1)
- DNA helicase binds to DNA and breaks the hydrogen bonds between the two strands.
- The DNA helix unwinds and the two strands separate.


The process of replication (2)
The process of replication (2)
- Free-floating nucleotides form hydrogen bonds with the complementary bases.
- Each strand is used as a template to produce complementary strands.
- Two hydrogen bonds form between A & T nucleotides. Three hydrogen bonds form between C & G nucleotides.
- DNA polymerase forms phosphodiester bonds between the nucleotides.
- Two new DNA strands are synthesised.


The process of replication (3)
The process of replication (3)
- DNA polymerase can only extend in the 5' to 3' direction. This poses a slight problem as one strand of DNA is in the 5' to 3' direction and the other is oriented in the 3' to 5' direction.
- The leading strand, which is complementary to the 3' to 5' parental DNA strand, is synthesized continuously. This is because the polymerase can add nucleotides in the 5'-3' direction.
- The other strand, complementary to the 5' to 3' parental DNA, is extended in small fragments.


The process of replication (4)
The process of replication (4)
- These fragments are known as Okazaki fragments. Each Okazaki fragments is synthesized in the 5'-3' direction.
- The fragments are joined together by an enzyme called DNA ligase. They are joined together to form an overall strand that runs in the 3'-5' direction.
Strands of the Double Helix
Strands of the Double Helix
Nucleotides join together to form a polynucleotide strand. Two polynucleotide strands coil together to form a double helix.


DNA strands
DNA strands
- The carbon atoms of the sugar molecule are numbered as 1′, 2′, 3′, 4′, and 5′ (1′ is read as “one prime”).
- The phosphate residue attaches to the hydroxyl group of the 5′ carbon of one sugar and the hydroxyl group of the 3′ carbon of the sugar of the next nucleotide.
- The bond is called a 5′–3′ phosphodiester linkage.


Antiparallel
Antiparallel
- This is called an antiparallel orientation because the helix's two strands run in opposite directions.
- The 5′ carbon end of one strand will face the 3′ carbon end of its matching strand.


DNA polymerase
DNA polymerase
- DNA polymerase catalyses the condensation reaction between the nucleotides in a DNA strand.
- DNA polymerase forms phosphodiester bonds between adjacent nucleotides.
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
Jump to other topics
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
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