Molecular GeneticsChapter 4

Self-StudyRead sections 4.1 and 4.2 on your own. You

will be responsible for knowing a few of the experiments, which led to the discovery of location of Hereditary Material and DNA Structure.

Suggested Questions: Pg. 216 #1-6 and 10.

Example – summarize each experiment and significance

Key Players Experiment and DiscoveredGriffith Used mice with different strains of pneumococcus (virulent and non-virulent).

Observed that when heat treated virulent, mixed with non-virulent was injected into healthy mouse it died. Discovered process of transformation (meaning something in the diseased bacteria even when heat treated acts on the healthy bacteria)

Hammerling Observed regeneration of alga “cap” but not “foot” when foot cut off. Nucleus contained in the foot, meaning that hereditary info is stored in the nucleus

Chargaff Discovered that in DNA the number of A=T and G=C

Hershey & Chase

Used radioactive labelled viruses(bacteriophages) to infect bacteria cells seeing if protein or DNA passed along hereditary information. Concluded it was DNA that was injected into the bacteria cell and thus the genetic material necessary to make new viruses.

Franklin Used Xray crystallography that suggested a double helix (also noted the accurate diameter of the DNA molecule)

Watson & Crick

Deduced the structure of DNA using the information from Chargaff, Franklin and Wilkins

4.3 DNA Replication and Repair

How does DNA replicate?• Meselson and Stahl

determined a Semiconservative Model

• Means that a replicated DNA molecule is composed of one parent strand and one newly synthesized strand

• Experiment used radioactively labeled DNA from E.coli

DNA Replication

III

Leading and Lagging Strands

5’

3’

5’

3’

Direction ofunwinding

Leading Strand

5’

3’RNA Primer

RNA Primer

5’

3’

Primer

5’

3’Lagging Strand

Reads template 3’5’Builds new strand 5’3’

Enzymes Acting During New DNA Strand Synthesis:

DNA Helicase – unzippingDNA Gyrase – reduce tension of unwindingSSB’s – help keep strands apartDNA Polymerase III – adds nucleotides to create new

strand (complimentary base pairing)

DNA Primase – lays down RNA primer (“temporary” 6-10bp)

Acting After Elongation of New DNA Strand:DNA Polymerase I – replaces RNA primer and

proofreading DNA Ligase – joins gaps between okazaki fragments

How It Happens

DNA helicase

http://www.youtube.com/watch?v=teV62zrm2P0

Reads template 3’5’

Mechanism for Incorporating New Nucleotides

DNA Polymerase III adds free Dioxyribonucleoside triphosphate (dNTP)

Always added to the 3’-OH end (since new strand builds in 5” 3’ direction)

Bond breaks between first two phosphates to drive process of elongating the strand (2p recycled to make more dNTP)

3 Phosphates

Deoxyribose

Mechanism for Incorporating New Nucleotides

Key Points to Know:Each parent strand acts as a template

(semiconservative model)

Reads DNA template 3’5’ and Builds 5’3’

Mechanism for joining dNTP’s

Know which is your Leading strand (towards fork) and Lagging strand (away from fork)

Know all acting enzymes and proofreading process

Try Some QuestionsPg. 223 Q# 1,2,5

Pg. 230 Q# 4,6,7

Handout – will help quiz and review steps

“Awareness Presentation” (due Dec19th)

More realistic animation of DNA replication

http://www.youtube.com/watch?v=4jtmOZaIvS0&feature=related