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Higher Biology

This text provides a simplified overview of the DNA replication process, including the formation of leading and lagging strands. It also explores the technique of Polymerase Chain Reaction (PCR) and its various applications in medical and forensic fields.

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Higher Biology

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  1. Higher Biology Unit 1 DNA and the Genome KEY AREA 2: Replication of DNA

  2. DNA and the Genome Learning Intentions KEY AREA 2 – Replication of DNA • Replication of DNA • PCR (polymerase chain reaction)

  3. 2a) DNA Replication This is a simplified diagram to show the process of DNA Replication

  4. 2b) DNA Replication Before a cell divides the DNA must be replicated (copied) using the enzyme DNA polymerase Stages in DNA Replication DNA double helix unwinds 2. Weak hydrogen bonds between base pairs break (to form 2 template strands) 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.

  5. 2c) DNA Replication 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 strand of nucleotides) binds to the 3’ end of the template DNA allowing polymerase to add DNA nucleotides 7. Free DNA nucleotides align with complementary base pairs (A-T, G-C) on the template strand and become bound to the deoxyribose 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

  6. 2d) DNA Replication – Formation of the Leading Strand

  7. 2e) DNA Replication 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

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

  9. DNA Replication (1:04) 2g) DNA Replication 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 allows cells to pass on an exact copy of a species’ genetic information during growth or reproduction This is very important to ensure that new cells carry out their correct role

  10. 2h) Many Replication Forks Long chromosomes form several replication forks dotted along the DNA Replication of DNA occurs simultaneously at each replication fork This speeds up the copying of the DNA molecule

  11. 2i) Polymerase Chain Reaction Amplification of DNA Sequences Genome Sequencing involves using a method called Polymerase Chain Reaction (PCR) to amplify DNA (make many copies in the lab) DNA is heated to between 90 and 98oC to separate the DNA strands It is then cooled to between 50 and 65oC to allow primers to bind/anneal PCR uses primers which are complementary to a specific target sequence It is then heated to between 70 and 80oC for heat-tolerant DNA polymerase to replicate the region of DNA (also known as Taq polymerase) Repeated cycles of heating and cooling amplify this region of DNA The amplified DNA can then be used for other experiments The DNA polymerase used must be heat-tolerant so that it is not denatured by the high temperature used in the first step

  12. 2j) Amplification & Detection of DNA Sequences Polymerase Chain Reaction (page 22 Torrance)

  13. 2k) Uses of DNA From PCR Medical Uses of PCR DNA from PCR can be used to:- 1. Diagnose a disease Estimate the risk of disease onset Forensic Uses of PCR Forensic scientists use PCR to amplify DNA samples from a crime scene Paternity Disputes PCR followed by Gel Electrophoresis of the amplified DNA can be used to confirm genetic relationships between individuals

  14. DNA and the Genome Questions KEY AREA 2 – Replication of DNA • Testing Your Knowledge 2 P 24-25 Q 1-3 2. Problem Solving P 5 Q5-7 * you must be able to calculate the number of DNA molecules made after a given number of PCR cycles 3. Quick Quiz

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