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Leader: Course: Instructor: Date: Unit 2 DNA Supplemental Instruction Iowa State University Morgan Bio 212 Shen 9/30/2009 DNA Structure and Replication 1. Central Dogma ____DNA_______ → ___RNA________ → ___Protein________ ↓ ↓ ____Transcription_ ___Translation________ 2. Which famous experiment characterized DNA as the material of heredity? Briefly describe. Hershey-Chase. Using a phage, they infected bacteria cells with radioactively tagged virus. They measured radioactivity in their final product (after removing phage) and found that only those cells infected with DNA radioactive DNA glowed. Infection was due to transfer of DNA, not protein. 3. DNA is a _double helix composed of two _anti-parallel___ sugar-phosphate backbones. 4. What are Chargraff’s rules? If a segment of DNA is composed of 30% C, what is the % of A? A=T and C=G 20% (C=30%=G, A+G=C+T, A=T) 5. A always pairs with _T__, forming _2__ H-bonds. C always pairs with _G__, forming _3_Hbonds. 6. What is the important relationship between structure and function relative to DNA replication? The complementary strands of the double helix each individually act as templates for the new strand. Therefore, DNA encodes the information needed to reproduce itself. 7. The accepted model for DNA replication is the _semi-conservative___ model. Who proved it? Meselson and Stahl 8. Which of the following images shows prokaryotic DNA replication. Which depicts eukaryotic replication? What is the key difference? eukaryotic prokaryotic Supplemental Instruction 1060 Hixson-Lied Student Success Center 294-6624 www.si.iastate.edu 9. In what direction does elongation ALWAYS occur? Why is this significant? What is an Okazaki fragment? 5’→ 3’ (always on 3’ end!) This means that the lagging strand has to be replicated in pieces (Okazaki fragments) that have to be joined together by DNA ligase) 10. Fill in the table of key enzymes: Enzyme Helicase Primase DNA Polymerase I DNA Polymerase II Ligase Nuclease Function Unwinds the original strands of DNA so they can act as templates Starts initiation with a small strand of RNA that is later replaced (primer) Removes primer and starts adding DNA Continues elongation, proofreading Joins Okazaki fragments Cuts out damaged DNA 11. DNA replication is not perfect and damage can occur. What enzymes are involved in fixing problems with DNA (error, damage)? DNA Pol III, nuclease, ligase Gene → Protein (I) 12. How is DNA actually expressed in a cell? As a protein product that has some specific function 13. What are the two stages of protein synthesis? Define them. Transcription: mRNA synthesis using DNA as the complementary template Translation: Polypeptide synthesis using mRNA as the template (ribosome) 14. How does protein synthesis differ in prokaryotes and eukaryotes? Location: P- coupled in cytoplasm E- Transcription in nucleus, translation in cytoplasm P- 1 RNA Pol, E- 3 RNA Pol; Opposite RNA synthesis direction P- directly use RNA, Emodifies RNA before use E- introns and exons, P- no introns 15. What is a codon? What does it mean that the genetic code is highly conserved? What is the start codon and what amino acid does it code for? 3 genes that code for one amino acid. The same code is used for all organisms, viruses, chloroplast, mitochondria AUG- methionine 16. The substitution of the nitrogenous base _Thymine___ for ___Uracil___ is a key difference in DNA and RNA. 17. What are three ways that RNA is modified in eukaryotes? 5 prime cap, 3 prime poly A tail, removal of introns by splicing 18. Why do humans have almost 20,000 more proteins than genes? Because humans have introns and exons, some genes can be spliced different ways, making the sequences altered in resulting in different protein products from a single gene.