Download DNA Handout KEY - Iowa State University

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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
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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.