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Edexcel International A Level Biology

Revision Notes

Home / International A Level / Biology / Edexcel / Revision Notes / 2. Membranes, Proteins, DNA & Gene Expression / DNA & Gene Expression / 2.10 DNA Replication


2.10 DNA Replication


DNA Replication

  • Before a (parent) cell divides, it needs to copy the DNA contained within it
  • Doubling the DNA ensures that the two new (daughter) cells produced will both receive full copies of the parental DNA
  • The DNA is copied via a process known as semi-conservative replication (semi = half)
  • The process is called this because in each new DNA molecule produced, one of the polynucleotide DNA strands (half of the new DNA molecule) is from the original DNA molecule being copied
  • The other polynucleotide DNA strand (the other half of the new DNA molecule) has to be newly created by the cell
  • Therefore, the new DNA molecule has conserved half of the original DNA and then used this to create a new strand

The importance of retaining one original DNA strand

  • It ensures there is genetic continuity between generations of cells
  • In other words, it ensures that the new cells produced during cell division inherit all their genes from their parent cells
  • This is important because cells in our body are replaced regularly and therefore we need the new cells to be able to do the same role as the old ones
    • Replication of DNA and cell division also occurs during growth

Semi-conservative replication of DNA

Semi conservative replication of DNA

Semi-conservative replication

  • DNA replication occurs in preparation for mitosis, the number of DNA molecules in the parent cell must be doubled before mitosis takes place
  • DNA replication occurs during the S phase of the cell cycle (which occurs during interphase, when a cell is not dividing)
  • The enzyme helicase unwinds the DNA double helix by breaking the hydrogen bonds between the base pairs on the two antiparallel polynucleotide DNA strands to form two single polynucleotide DNA strands
  • Each of these single polynucleotide DNA strands acts as a template for the formation of a new strand made from free nucleotides that are attracted to the exposed DNA bases by base pairing
  • The new nucleotides are then joined together by the enzyme DNA polymerase
  • The original strand and the new strand join together through hydrogen bonding between base pairs to form the new DNA molecule
  • This method of replicating DNA is known as semi-conservative replication because half of the original DNA molecule is kept (conserved) in each of the two new DNA molecules

DNA Polymerase

  • In the nucleus, there are free nucleotides which contain three phosphate groups
    • These nucleotides are known as nucleoside triphosphates or ‘activated nucleotides’
    • The extra phosphates activate the nucleotides, enabling them to take part in DNA replication
  • The bases of the free nucleoside triphosphates align with their complementary bases on each of the template DNA strands
  • The enzyme DNA polymerase synthesises new DNA strands from the two template strands
  • It does this by catalysing condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides within the new strands, creating the sugar-phosphate backbone of the new DNA strands
  • DNA polymerase cleaves (breaks off) the two extra phosphates and uses the energy released to create the phosphodiester bonds (between adjacent nucleotides)
  • Hydrogen bonds then form between the complementary base pairs of the template and new DNA strands

Process of Semi-Conservation Replication 1Process of Semi-Conservation Replication 2

Nucleotides are bonded together by DNA polymerase to create the new complementary DNA strands

Leading & lagging strands

  • DNA polymerase can only build the new strand in one direction (5’ to 3’ direction)
  • As DNA is ‘unzipped’ from the 3’ towards the 5’ end, DNA polymerase will attach to the 3’ end of the original strand and move towards the replication fork (the point at which the DNA molecule is splitting into two template strands)
  • This means the DNA polymerase enzyme can synthesise the leading strand continuously
  • This template strand that the DNA polymerase attaches to is known as the leading strand
  • The other template strand created during DNA replication is known as the lagging strand
  • On this strand, DNA polymerase moves away from the replication fork (from the 5’ end to the 3’ end)
  • This means the DNA polymerase enzyme can only synthesise the lagging DNA strand in short segments (called Okazaki fragments)
  • A second enzyme known as DNA ligase is needed to join these lagging strand segments together to form a continuous complementary DNA strand
  • DNA ligase does this by catalysing the formation of phosphodiester bonds between the segments to create a continuous sugar-phosphate backbone

Leading and lagging strands

The synthesis of the complimentary strand occurs differently on the leading and lagging strands of DNA

Evidence for Semi-Conservative Replication

  • Scientists were unsure if DNA replication was conservative or semi-conservative
  • Two scientists called Matthew Meselson and Franklin Stahl, showed that DNA replication was semi-conservative by experimenting with isotopes of nitrogen

Meselson and Stahl's Experiment

  • Bacteria were grown in a broth containing the heavy (15N) nitrogen isotope
    • DNA contains nitrogen in its bases
    • As the bacteria replicated, they used nitrogen from the broth to make new DNA nucleotides
    • After some time, the culture of bacteria had DNA containing only heavy (15N) nitrogen
  • A sample of DNA from the 15N culture of bacteria was extracted and spun in a centrifuge
    • This showed that the DNA containing the heavy nitrogen settled near the bottom of the centrifuge tube
  • The bacteria containing only 15N DNA were then taken out of the 15N broth and added to a broth containing only the lighter 14N nitrogen. The bacteria were left for enough time for one round of DNA replication to occur before their DNA was extracted and spun in a centrifuge
    • If conservative DNA replication had occurred, the original template DNA molecules would only contain the heavier nitrogen and would settle at the bottom of the tube, whilst the new DNA molecules would only contain the lighter nitrogen and would settle at the top of the tube
    • If semi-conservative replication had occurred, all the DNA molecules would now contain both the heavy 15N and light 14N nitrogen and would therefore settle in the middle of the tube (one strand of each DNA molecule would be from the original DNA containing the heavier nitrogen and the other (new) strand would be made using only the lighter nitrogen)
  • Meselson and Stahl confirmed that the bacterial DNA had undergone semi-conservative replication.
    • The DNA from this second round of centrifugation settled in the middle of the tube, showing that each DNA molecule contained a mixture of the heavier and lighter nitrogen isotopes
    • If more rounds of replication were allowed to take place, the ratio of 15N:14N would go from 1:1 after the first round of replication, to 3:1 after the second and 7:1 after the third

Meselson and Stahl Experiment (1)Meselson and Stahl Experiment (2)

Meselson and Stahl's experiment that showed bacterial DNA replicated via semi-conservative DNA replication



  • 1. Molecules, Transport & Health
    • Biological Molecules
      • 1.1 The Importance of Water
        • 1.2 Saccharides
          • 1.3 Core Practical 1: Estimating the Concentration of Sugars & Starch
            • 1.4 Condensation & Hydrolysis
              • 1.5 Triglycerides & Ester Bonds
              • The Circulatory System
                • 1.6 The Need for a Circulatory System
                  • 1.7 Blood Vessels: Structure & Function
                    • 1.8 The Cardiac Cycle
                      • 1.9 The Role of Haemoglobin
                        • 1.10 Atherosclerosis
                          • 1.11 Blood Clotting
                          • Diet & Health
                            • 1.12 Reducing Risk Factors of CVD
                              • 1.13 Dietary Antioxidants & CVD
                                • 1.14 Core Practical 2: Investigate the Vitamin C Content of Food & Drink
                                  • 1.15 Interpreting Data on Risk Factors
                                    • 1.16 Designing Studies into the Effects of Risk Factors
                                      • 1.17 Perception of Risk vs Actual Risk
                                        • 1.18 Data on Cholesterol & Lipoproteins
                                          • 1.19 Data on Effect of Diet
                                            • 1.20 Treatments for CVD: Benefits & Risks
                                          • 2. Membranes, Proteins, DNA & Gene Expression
                                            • Gas Exchange, Cell Membranes & Transport
                                              • 2.1 Properties of Gas Exchange Surfaces
                                                • 2.2 Cell Membranes
                                                  • 2.3 Core Practical 3: Investigating Membrane Structure & Permeability
                                                    • 2.4 Osmosis
                                                      • 2.5 Diffusion, Facilitated Diffusion & Active Transport
                                                      • Proteins
                                                        • 2.6 Amino Acids, Proteins & Protein Structure
                                                          • 2.7 Enzymes: Roles & Modes of Action
                                                            • 2.8 Core Practical 4: Investigating the Rate of Enzyme Reactions
                                                              • 2.9 Nucleotides, DNA & RNA, Base Pairing
                                                              • DNA & Gene Expression
                                                                • 2.10 DNA Replication
                                                                  • 2.11 The Nature of the Genetic Code
                                                                    • 2.12 How Bases Code for a Polypeptide Chain
                                                                      • 2.13 Transcription & Translation
                                                                      • Inheritance
                                                                        • 2.14 Mutations
                                                                          • 2.15 Patterns of Inheritance & Sex Linkage
                                                                            • 2.16 Cystic Fibrosis
                                                                              • 2.17 Genetic Screening
                                                                                • 2.18 Ethical & Social Issues of Genetic Screening
                                                                              • 3. Cell Structure, Reproduction & Development
                                                                                • Cell Structure & Organisation
                                                                                  • 3.1 Cell Theory
                                                                                    • 3.2 Levels of Organisation of Cells
                                                                                      • 3.3 Eukaryotic Cells
                                                                                        • 3.4 The Rough Endoplasmic Reticulum & Golgi
                                                                                          • 3.5 Prokaryotic Cells
                                                                                            • 3.6 Electron Microscopy of Animal Cells
                                                                                              • 3.7 Microscopy: Magnification & Resolution
                                                                                                • 3.8 Core Practical 5: Light Microscopy
                                                                                                • Reproduction & Inheritance
                                                                                                  • 3.9 Gene Locus
                                                                                                    • 3.10 Meiosis & Variation
                                                                                                      • 3.11 Mammalian Gametes
                                                                                                        • 3.12 Fertilisation: Mammals
                                                                                                          • 3.13 Fertilisation: Flowering Plants
                                                                                                            • 3.14 The Cell Cycle & Mitosis
                                                                                                              • 3.15 Core Practical 6: Observing the Stages of Mitosis
                                                                                                                • 3.16 Calculation of Mitotic Index
                                                                                                                  • 3.17 Stem Cells & Cell Potency
                                                                                                                    • 3.18 Cell Specialisation
                                                                                                                      • 3.19 Post-Transcriptional Changes to mRNA
                                                                                                                        • 3.20 Gene Interaction & Epigenetics
                                                                                                                          • 3.21 Polygenic Inheritance & Continuous Variation
                                                                                                                        • 4. Plant Structure & Function, Biodiversity & Conservation
                                                                                                                          • Plant Structure & Function
                                                                                                                            • 4.1 Plant Cell Structure
                                                                                                                              • 4.2 Electron Microscopy of Plant Cells
                                                                                                                                • 4.3 Starch & Cellulose: Structure & Function
                                                                                                                                  • 4.4 Properties of Cellulose
                                                                                                                                    • 4.5 The Vascular Structure of Plants
                                                                                                                                      • 4.6 Core Practical 7: Identifying Tissue Types Within Stems
                                                                                                                                      • Plants & Conservation
                                                                                                                                        • 4.7 Plant-Based Products for Sustainability
                                                                                                                                          • 4.8 Water & Inorganic Ions in Plants
                                                                                                                                            • 4.9 Core Practical 8: Determining the Tensile Strength of Plant Fibres
                                                                                                                                            • Plants & Bacterial Growth
                                                                                                                                              • 4.10 Bacterial Growth Conditions
                                                                                                                                                • 4.11 Plant Products with Antimicrobial Properties
                                                                                                                                                  • 4.12 Core Practical 9: Antimicrobial Properties of Plants
                                                                                                                                                    • 4.13 Development of Drugs & Drug Testing
                                                                                                                                                    • Classification & Biodiversity
                                                                                                                                                      • 4.14 The Three Domains of Life
                                                                                                                                                        • 4.15 The Variety of Life
                                                                                                                                                          • 4.16 Biodiversity & Endemism
                                                                                                                                                            • 4.17 Species Richness & Heterozygosity Index
                                                                                                                                                              • 4.18 Index of Biodiversity
                                                                                                                                                                • 4.19 Ecological Niches & Adaptations
                                                                                                                                                                  • 4.20 Hardy-Weinberg Equation
                                                                                                                                                                    • 4.21 Roles of Seed Banks & Zoos in Conservation
                                                                                                                                                                  • 5. Energy Flow, Ecosystems & the Environment
                                                                                                                                                                    • Photosynthesis
                                                                                                                                                                      • 5.1 Photosynthesis: Overview
                                                                                                                                                                        • 5.2 The Role of ATP in Photosynthesis
                                                                                                                                                                          • 5.3 Photosynthesis: Light-Dependent Stage
                                                                                                                                                                            • 5.4 Photosynthesis: Light-Independent Stage
                                                                                                                                                                              • 5.5 Chloroplasts
                                                                                                                                                                                • 5.6 Absorption & Action Spectra
                                                                                                                                                                                  • 5.7 Separation of Photosynthetic Pigments with Chromatography
                                                                                                                                                                                    • 5.8 Core Practical 10: Rate of Photosynthesis
                                                                                                                                                                                    • Ecosystems
                                                                                                                                                                                      • 5.9 Ecological Productivity
                                                                                                                                                                                        • 5.10 Energy & Biomass Transfers
                                                                                                                                                                                          • 5.11 Ecology: Key Terms
                                                                                                                                                                                            • 5.12 Factors Affecting Populations
                                                                                                                                                                                              • 5.13 Niche
                                                                                                                                                                                                • 5.14 Core Practical 11: Quadrat & Transect Study
                                                                                                                                                                                                  • 5.15 Ecological Succession
                                                                                                                                                                                                  • Environmental Biology
                                                                                                                                                                                                    • 5.16 Evidence for Climate Change
                                                                                                                                                                                                      • 5.17 Anthropogenic Climate Change
                                                                                                                                                                                                        • 5.18 Carbon Cycle & Environmental Management
                                                                                                                                                                                                          • 5.19 Models for Predicting Climate Change
                                                                                                                                                                                                            • 5.20 How Climate Change Affects Species
                                                                                                                                                                                                              • 5.21 The Effect of Temperature on Enzyme Reactions
                                                                                                                                                                                                                • 5.22 Core Practical 12: Effect of Temperature on Development
                                                                                                                                                                                                                  • 5.23 Mutation, Natural Selection & Evolution
                                                                                                                                                                                                                    • 5.24 Isolation & Speciation
                                                                                                                                                                                                                      • 5.25 Contentious Issues in Environmental Science
                                                                                                                                                                                                                        • 5.26 Examples of Sustainable Conservation
                                                                                                                                                                                                                      • 6. Microbiology, Immunity & Forensics
                                                                                                                                                                                                                        • Microbiology
                                                                                                                                                                                                                          • 6.1 Culturing Microorganisms
                                                                                                                                                                                                                            • 6.2 Measuring the Growth of Microorganisms
                                                                                                                                                                                                                              • 6.3 The Bacterial Growth Curve
                                                                                                                                                                                                                                • 6.4 Core Practical 13: Rate of Growth of Microorganisms
                                                                                                                                                                                                                                  • 6.5 Comparison of Bacterial & Viral Structure
                                                                                                                                                                                                                                  • Immunity
                                                                                                                                                                                                                                    • 6.6 Tuberculosis & HIV
                                                                                                                                                                                                                                      • 6.7 Pathogens: Routes of Entry
                                                                                                                                                                                                                                        • 6.8 Non-Specific Immune Responses
                                                                                                                                                                                                                                          • 6.9 Specific Immune Responses
                                                                                                                                                                                                                                            • 6.11 Developing Immunity
                                                                                                                                                                                                                                              • 6.12 Pathogens vs Hosts: An Evolutionary Race
                                                                                                                                                                                                                                              • Antibiotics
                                                                                                                                                                                                                                                • 6.13 Antibiotics
                                                                                                                                                                                                                                                  • 6.14 Core Practical 14: The Effects of Different Antibiotics
                                                                                                                                                                                                                                                    • 6.15 Current Usage of Antibiotics
                                                                                                                                                                                                                                                    • Decomposition & Decay
                                                                                                                                                                                                                                                      • 6.16 The Role of Microorganisms
                                                                                                                                                                                                                                                      • Forensics
                                                                                                                                                                                                                                                        • 6.17 Polymerase Chain Reaction
                                                                                                                                                                                                                                                          • 6.18 Gel Electrophoresis in Forensics
                                                                                                                                                                                                                                                            • 6.19 DNA Profiling
                                                                                                                                                                                                                                                              • 6.20 Types of Data Provided by Forensic Analysis
                                                                                                                                                                                                                                                            • 7. Respiration, Muscles & the Internal Environment
                                                                                                                                                                                                                                                              • Respiration
                                                                                                                                                                                                                                                                • 7.1 Overview of Respiration
                                                                                                                                                                                                                                                                  • 7.2 Glycolysis
                                                                                                                                                                                                                                                                    • 7.3 Link Reaction & Krebs Cycle
                                                                                                                                                                                                                                                                      • 7.4 The Electron Transport Chain
                                                                                                                                                                                                                                                                        • 7.5 Anaerobic Respiration
                                                                                                                                                                                                                                                                          • 7.6 Respiratory Quotient
                                                                                                                                                                                                                                                                            • 7.7 Core Practical 15: Investigation of Respiration in Yeast
                                                                                                                                                                                                                                                                              • 7.8 Core Practical 16: Respirometer to Calculate RQ
                                                                                                                                                                                                                                                                              • Muscular Movement
                                                                                                                                                                                                                                                                                • 7.9 The Musculoskeletal System
                                                                                                                                                                                                                                                                                  • 7.10 The Muscle Fibre
                                                                                                                                                                                                                                                                                    • 7.11 The Process of Skeletal Muscle Contraction
                                                                                                                                                                                                                                                                                      • 7.12 The Role of Muscle in the Cardiac Cycle
                                                                                                                                                                                                                                                                                        • 7.13 Calculation of Cardiac Output
                                                                                                                                                                                                                                                                                          • 7.14 The Role of Adrenaline
                                                                                                                                                                                                                                                                                            • 7.15 Core Practical 17: The Effects of Exercise
                                                                                                                                                                                                                                                                                            • The Internal Environment
                                                                                                                                                                                                                                                                                              • 7.16 Negative & Positive Feedback
                                                                                                                                                                                                                                                                                                • 7.17 Homeostasis
                                                                                                                                                                                                                                                                                                  • 7.18 The Kidney: Structure
                                                                                                                                                                                                                                                                                                    • 7.19 Urea as a Waste Product
                                                                                                                                                                                                                                                                                                      • 7.20 Selective Reabsorption in the Kidney
                                                                                                                                                                                                                                                                                                        • 7.21 Hormones in Osmoregulation
                                                                                                                                                                                                                                                                                                          • 7.22 Control of Gene Expression
                                                                                                                                                                                                                                                                                                        • 8. Coordination, Response & Gene Technology
                                                                                                                                                                                                                                                                                                          • The Nervous System
                                                                                                                                                                                                                                                                                                            • 8.1 Neurones: Types and Functions
                                                                                                                                                                                                                                                                                                              • 8.2 How a Response is Generated by Effectors
                                                                                                                                                                                                                                                                                                                • 8.3 The Reflex Arc
                                                                                                                                                                                                                                                                                                                  • 8.4 The Nerve Impulse
                                                                                                                                                                                                                                                                                                                    • 8.5 Myelination & Saltatory Conduction
                                                                                                                                                                                                                                                                                                                      • 8.6 Synapses & Neurotransmitters
                                                                                                                                                                                                                                                                                                                        • 8.7 The Effects of Drugs on Nervous Transmission
                                                                                                                                                                                                                                                                                                                          • 8.8 Detection of Stimuli
                                                                                                                                                                                                                                                                                                                            • 8.9 Habituation
                                                                                                                                                                                                                                                                                                                              • 8.10 Central & Peripheral Nervous System
                                                                                                                                                                                                                                                                                                                              • Plant Hormones
                                                                                                                                                                                                                                                                                                                                • 8.11 Effects of Plant Hormones
                                                                                                                                                                                                                                                                                                                                  • 8.12 Core Practical 18: Amylase in Germinating Cereal Grains
                                                                                                                                                                                                                                                                                                                                    • 8.13 Nervous & Hormonal Coordination
                                                                                                                                                                                                                                                                                                                                    • The Brain
                                                                                                                                                                                                                                                                                                                                      • 8.14 Human Brain Structures & Functions
                                                                                                                                                                                                                                                                                                                                        • 8.15 Techniques to Investigate the Brain
                                                                                                                                                                                                                                                                                                                                          • 8.16 Brain Disease
                                                                                                                                                                                                                                                                                                                                          • Gene Technology
                                                                                                                                                                                                                                                                                                                                            • 8.17 Drug Production from Genetically Modified Organisms
                                                                                                                                                                                                                                                                                                                                              • 8.18 Recombinant DNA
                                                                                                                                                                                                                                                                                                                                                • 8.19 Transfer of Recombinant DNA into Other Cells
                                                                                                                                                                                                                                                                                                                                                  • 8.20 Identification of Active Genes
                                                                                                                                                                                                                                                                                                                                                    • 8.21 Bioinformatics
                                                                                                                                                                                                                                                                                                                                                      • 8.22 Risks & Benefits of Using GMOs


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                                                                                                                                                                                                                                                                                                                                                    Author: Marlene

                                                                                                                                                                                                                                                                                                                                                    Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.


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