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Higher Human Biology

Explore the structure of DNA, its replication process, and the significance of DNA Replication. Learn about leading and lagging strands, requirements for DNA Replication, and the Polymerase Chain Reaction. Discover how DNA amplification is utilized in genome sequencing and various applications in forensics and medicine.

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Higher Human Biology

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  1. 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 consists of 2 strands of repeating units called nucleotides twisted into a double helix 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

  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 at 3’ end and phosphate at 5’ ends of each strand Complementary base pairs are Adenine with Thymine, Guanine with Cytosine the base sequence of DNA forms the genetic code Chromosomes consist of tightly coiled DNA packaged around bundles of proteins

  5. 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) using the enzyme DNA polymerase Stages in DNA Replication 1. 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.

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

  8. 2d) 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 sequence of nucleotides) binds to the 3’ end of the template DNA allowing polymerase to add DNA nucleotides 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

  9. 2f) DNA Replication – Formation of the Lagging Strand

  10. 2d) 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

  11. 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 is very important to ensure that new cells carry out their correct role

  12. 2h) Polymerase Chain Reaction Amplification of DNA Sequences Genome Sequencing involves using a method called Polymerase Chain Reaction (PCR) to amplify DNA (copy it in the lab) DNA is heated to between 92 and 98oC to separate the DNA strands It is then cooled to between 50 and 65oC to allow primers to bind 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 Repeated cycles of heating and cooling amplify this region of DNA This technique can help solve crimes, settle paternity suits and diagnose genetic disorders

  13. 2h) Amplification & Detection of DNA Sequences Polymerase Chain Reaction (page 71 Torrance)

  14. 2h) Uses of PCR Medical Uses of PCR 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 can be used to confirm genetic relationships between individuals

  15. 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. Testing Your knowledge 2 Page 76 Q 1 and 3 3. Quick Quiz

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