Download Newitt AP Biology DNA

Newitt AP Biology
DNA
Determination that DNA is the genetic material ­ happened around mid 1900s
Link to Griffith's experiment ­ click on 12.1 Activity and follow steps
Frederick Griffith Experiment (1928) ­ ­Smooth Pneumococcus bacteria ­ virulent form,
killed mice
­Rough Pneumococcus bacteria ­ nonvirulent form
Link to animation of Hershey & Chase's experiment
Alfred Hershey & Martha Chase Experiment(1952) ­ Proteins contain CHON(S) Nucleic Acids contain CHONP
Protein coats of viruses labeled with radioactive sulfur
­Heat killed Smooth bacteria ­ mice survived
DNA labeled with radioactive phosphurus
­Heat killed Smooth + living Rough ­ Living Rough cells were "transformed" by something from the dead S cells The part of the virus that entered the bacteria and turned it into a virus producing factory, was DNA, not protein.
Conclusion ­ a "transforming factor" was transferred
from the dead S cells to the living r cells, that gave
R cells new traits ­ pathogenicity
Avery, McCarty, MacLeod (1944) ­ determined
that "transforming factor" was DNA (in bacteria)
Erwin Chargaff ­ measured the percent composition
of different bases in DNA in different species and determined that
A=T, C=G
Rosalind Franklin­ specialist
in X­Ray crystallography. Took famous "Photo 51" which confirmed that DNA must be a helix. 1
James Watson & Francis Crick ­ 1953
They put the data together from Chargaff and Franklin and built a model. Realized
strands run anti­parallel, A­T, C­G. 1962 won Nobel Prize (with Wilkins)
(Franklin died in 1958). Link to Watson and Crick's Nature Paper
Link to Watson's TED talk
Key features:
3 H bonds between C and G
3 H bonds between A and T
Phosphodiester bond links nucleotides
Strands run antiparalel
A and G both purines, double ring structures
C and T pyrimidines single ring structures
"PUGA"
"PYCUT"
DNA is packed together in predictable, precise way with proteins, forming chromatin ­DNA wraps around histone proteins forming 8 histone clumps called nucleosome "beads on a string"
­Heterochromatin ­ more compacted regions found around centromere and
telomeres
­Euchromatin ­ less compacted regions, more accessible to cell machinery for transcription of DNA
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DNA Replication
Meselson and Stahl 1958
What would the results over three rounds of replication have looked like with each of the models?
1st Replication
2nd Replication
3rd Replication
Conservative
Semiconservative
Dispersive
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The 6 billion "letters" or nucleotide pairs in the human genome
can be copied in a cell in just a few hours, with very few errors (1/10 billion nucleotides. Bacteria ­ start replicating at a site called the origin, opens up a replication "bubble." DNA is built in both
directions until done.
Eukaryotes ­ 100s­1000s of origins, forming bubbles
that enlarge and meet to speed up process.
DNA nucleotides are added as a nucleoside triphosphate (dATP, dTTP,
dGTP, dCTP ­ d for deoxyribose.) DNA polymerase adds the nucleotides in a coupled reaction using energy from removal of 2 phosphate groups
Enzymes involved in DNA Replication:
Helicase ­ untwist the double helix at the replication fork
Binding proteins ­ keep the DNA strands separate from each other
Topoisomerase ­ because part of DNA untwists, part in front of it twists tighter ­ this enzyme relieves that strain by breaking, twisting and rejoining the DNA strands
Primase ­ synthesizes short RNA strands that act as a [primer because DNA pol can only add to an existing chain
DNA Polymerase ­adds nucleotides to a 3' end. Only adds nucleotides in a 5'­3' direction.
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If DNA can only be built in a 5'­3' direction, since the strands run antiparallel, both
strands need to be built in opposite directions:
3'
Leading strand ­ can be built DNA Replication Animation
5'
continuously from 5'­3' direction
helicase
5'
3'
3'
Replication Fork
Summary of Leading Strand Replication
Lagging strand ­ built discontinuously from 5'­3'
3'
because going opposite 5'
direction from the helicase movement. Forms "Okazaki Origin of Replication fragments" which later joined.
Summary of Lagging Strand Replication
DNA Pol I
Ligase
Steps:
1. RNA primase adds a short sequence of RNA in the 5'­3' direction at the origin
2. DNA polymerase III starts adding nucleotides to the open 3' end of the RNA
3. A sliding clamp is associated with the DNA Pol III and slides it along the strand
Steps:
1. Primase adds RNA nucleotides to form a primer
a little distance away from origin.
2. DNA Pol III adds DNA nucleotides to the primer, forms first Okazak. fragment
3. Once fragment reaches next primer, DNA pol III detaches
4. Fragment 2 is primed, then the next Okazaki fragment is made
5. DNA Pol I replaces the first RNA primer with DNA 6. DNA ligase forms a bond between the two pieces of DNA
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Proofreading and Repairing DNA ­ DNA polymerases constantly proofread DNA strands to make sure nucleotides correctly paired, if not replace with correct ones.
­Sometimes mismatches happen as a result of damage to DNA, such as by tobacco smoke, radiation, etc. Diagram to left shows a type of Excision repair on two thymines that have formed a dimer. DNA Pol cuts out incorrect piece and replaces nucleotides, ligase joins the pieces together. Xeroderma pigmentosum ­ disorder where people lack the ability to repair UV light damage to skins' DNA. High incidence of skin cancer. ­DNA Shortening ­ ends of the lagging strand end up not
getting fully replicated, because once the RNA primer is removed from the end, there is no existing 3'­OH to add nucleotides to, so the DNA isn't filled in. When the strands separate for the next replication, that piece doesn't get replicated. This results in DNA strands getting slightly shorter with each replication. ­To protect important genes from getting left off, the ends of chromosomes have sequences called telomeres which are noncoding pieces of DNA. As DNA replicates, bits of telomere are lost each time, and give a kind of lifespan to cells. Once telomere gone, doesn't divide anymore.
­Germ cells, that produce gametes have telomerase
to keep rebuilding telomeres so they can continue to divide many times. ­Cancer ­ telomerase activity has been found which allows cells to keep dividing and form tumors. Target of cancer therapy. 6
Attachments
17­17­Translation.mov