KEY AREA 2: Structure & Replication of DNA Higher Human Biology Unit 1 Human Cells KEY AREA 2: Structure & Replication of DNA

Human Cells Learning Intentions KEY AREA 2 – Structure & Replication of DNA Structure of DNA DNA Replication

2a) Structure of DNA 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 DNA consists of 2 strands of repeating units called nucleotides twisted into a double helix

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

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

2d) DNA Replication Before a cell divides the DNA must be replicated (copied) Stages in DNA Replication 1. DNA double helix unwinds 2. Weak hydrogen bonds between base pairs break (2 strands unzip) 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. 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) must be present to create an existing chain for the enzyme DNA polymerase to work on 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 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

2e) DNA Replication – Formation of the Leading Strand

2f) DNA Replication – Formation of the Lagging Strand

2g) DNA Replication DNA Replication (1:04) 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

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. Quick Quiz