DNA Replicaiton

Question 1

DNA synthesis is semi conservative

Answer 1

Each strand acts as a template

Question 2

DNA is synthesized in the 5’->3’ direcition

Answer 2

A 3’OH primer and a template are required

• phsophdiester bond former
• energy from hydrolysis

Question 3

DNA replicaiton is semi-discontinuous

Answer 3

• Leading strand is continuous
• Bottom strand has to keep restarting because new DNA is becoming available
• Okazaki fragments on lagging starnd
  
  • Primase provides free 3’OH for lagging strand replication

Question 4

4 Steps of DNA replication

Answer 4

1. Separation of the two complimentary strands at the origin of replication
2. Formation of the replicaiton fork (primers and Okazaki fragments)
3. Chain Elongation
4. Removal of the primers

Question 5

Properties of E. coli DNA Pol I

Answer 5

One protein, 3 catyltic activities

1. 5’-3’ polymerase
2. 5-3 exonuclease
3. 3-5 exonuclease

Proofreading activity removes incorrectly based paired nucleotides

Nuclease degrade phosphodiester bonds

Question 6

Error correction by the 3’-5’ exonuclease

Answer 6

1. Polymerase mispairs dC with dT
2. Polymerase repositions the mispaired 3’ terminus into the 3’-5’ exonuclease site
3. Exonuclease hydrolyzes the mispaired dC
4. The 3’ terminus repositions back to the polymerase site
5. Polmerase incorpates the correct nucleotide, dA

Question 7

DNA Pol 1 vs. DNA Pol III

Answer 7

DNA Pol III is very process, synthesizes > 500,000 nucleotides before dissociating from its template

DNA Pol I is not a very process polymerase, synthesies only 3-200 nucleotides before falling off

DNA Pol III lacks 5’-3’ exonuclease activity

Question 8

Summary of machinery

Answer 8

• SSB - binds to single strand to keep the strands from coming back together due to hydrogen bond force
• Primase - synthesizes RNA primer
• DNA Pol III - does bulk of synthesis
• DNA Pol I - filling up gaps and for lagging strand, replaces RNA primer with DNA and resynthesizes that stretch
• Ligase - fills up nicks
• Gyrase - relieves torsional stress

Question 9

Maintenacne of Chromosode Ends

Answer 9

Incomplete daughter strand synthesis at the telomere can result in shortening of chromosome length.

Telomerase and unique properties of telomeres help in maintenance of chromosome integrety

Question 10

Telomerase Mechanism

Answer 10

1. 3’ end of pareantal DNA starnd is extended by RNA-templated DNA synthesis
2. Lagging strand is comleted by DNA polymerase, which carriers a DNA primae as one of its subunits

* Never makes the strand completely flush but keeps it intact