Download Ch. 16 The Molecular Basis of Life

Ch. 16
The Molecular
Basis of Life
Nov 15 ­ 5:19 PM
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Essential Question:
On a molecular basis, how does information get passed on from one cell to another?
Nov 15 ­ 5:20 PM
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DNA
Nov 15 ­ 5:22 PM
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History behind discovery of DNA
Morgan ­ early 1900's ­genes located on chromosomes
­chromosomes made of DNA and Protein
­DNA or proteins could be genetic Nov 15 ­ 5:23 PM
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Griffith (1928) ­ studied Streptococcus pneumoniae ­ virulent/nonvirulent strain
conclusion: living bacteria had been transformed into pathogenic bacteria by an unknown, heritable substance
Nov 15 ­ 5:33 PM
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up to 1940's­ believed to be proteins
­specific in function
­essential
­little known about nucleic acids
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Avery, McCarty and MacLeod ­ 1944
­purified molecules of heat killed pathogenic bacteria
­tried to transform live nonpathogenic bacteria with each type
­only DNA worked
­scientists still skeptical because believed in proteins
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Hershey/Chase ­ 1952
­used bacteriophages (viruses that infect bacteria), T2 = phage that infects Escherichia coli
­knew T2 contained DNA and protein
­knew viruses infected bacteria to make more viruses
­experiment was to see if it was the protein or the DNA that could reprogram a host cell to make viruses
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Nov 15 ­ 8:30 PM
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Dec 3­10:31 AM
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Hershey/Chase proved it is DNA that is the genetic material, not proteins for viruses
Erwin Chargaff ­ 1947
­knew that DNA was polymer of nucleotides
­made of sugar (deoxyribose), a nitrogenous base and a phosphate group
­DNA differs from one organism to another
­saw ratios of A, C, G, A, T
C = G and A= T (Chargaff's rules)
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Also known:
­ DNA doubled before mitosis
­equally distributed in daughter cells
­each species: diploid set of chromosomes has double amount of DNA as haploid set
Once DNA was believed to be genetic material the race was on to find the structure of it
Nov 15 ­ 9:08 PM
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1950's knew arrangement of covalent bonds in nucleotide
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Players in the race to find the structure:
Linus Pauling of Cal Tech
Nov 15 ­ 9:24 PM
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Maurice Wilkins and Rosalind Franklin at King's College in London
worked on x­ray crystallography
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James Watson and Francis Crick at Cambridge
http://nobelprize.org/nobel_prizes/medicine/laureates/1962/watson­bio.html
http://nobelprize.org/nobel_prizes/medicine/laureates/1962/crick­bio.html
used model building techniques
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Possible pairings of purines and pyrimidines
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base pairing in DNA
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DNA found to be double helix structure
­nitrogen bases on the inside
­alternating sugar and phosphates on the outside
­ adenine and guanine are purines (double ring structure)
­cytosine and thymine are pyrimidines (single ring structures)
­hydrogen bonds hold the strands together
­3 bonds between G and C
­2 bonds between A and T
­explained Chargaff's rules
Nov 15 ­ 9:28 PM
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Dec 4­7:03 AM
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April 1953 ­ Watson and Crick published in Nature their structure of DNA
Nobel Peace Prize was shared by Watson, Crick, and Wilkins
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How does the structure of DNA affect its function?
"It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material" (Watson and Crick in Nature)
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How does DNA replicate?
Watson and Crick's basic idea for replication
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Three models for DNA replication
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Meselson and Stahl ­ 1950's
­tested the 3 hypotheses
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Meselson and Stahl proved that DNA replication is semiconservative
work on studying DNA replication is done in bacteria because they have fewer base pairs and they reproduce quickly (every 20 minutes for E. coli)
Nov 15 ­ 9:50 PM
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Procedure for replication
1. begins at origins of replication
­helicase recognizes certain DNA sequences and attach to those sites
­ has many sites to speed up replication
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2. helicase causes DNA strands to untwist and separate to form a replication bubble
3. When strands untwist ­ become tighter farther down helix. Topoisomerase relieves some of the strain.
Dec 4­7:08 AM
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4. single­strand binding protein binds to the unpaired strands and keeps them stable until new strands can be synthesized
5. DNA polymerase adds new nucleotides by adding a nucleoside triphosphate. When added it loses two phosphate groups (pyrophosphate) which then gets hydrolyzed to two phosphate molecules
­DNA polymerase adds the nucleotides to the 3' end of the strand (sugar end) only, never to 5' end
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Some of the proteins involved in the initiation of DNA replication
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Dec 4­7:10 AM
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6. Initial nucleotide chain added is called a primer (can be DNA or RNA) = short piece (5­10 nucleotides long) of RNA with a 3' end
7. primase joins RNA nucleotides together one at a time making a primer complementary to the DNA template
8. DNA polymerase III adds a DNA nucleotide to the 3' end of the RNA primer and then adds more DNA nucleotides to the growing strand
­only one primer is needed for DNA to make strand
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5' and 3' ends
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­this happens with the leading strand = the strand of the DNA gets read easily in the 3' to 5' direction
9. For the 5' to 3' strand (lagging strand) ­DNA polymerase works away from replicating fork
­gets synthesized in short fragments (called Okazaki fragments­ 100­200 nucleotides long in eukaryotes)
­same scenario as before with primer, primase, DNA polymerase III, but each Okazaki fragment must be primed separately
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Synthesis of the leading strand during DNA replication
Feb 8­4:33 PM
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Priming
Dec 4­7:11 AM
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10. For Okazaki fragments, DNA polymerase I replaces the RNA nucleotides of the primers with DNA versions ­ add to 3' end of adjacent Okazaki fragment
11. DNA ligase joins the sugar­phosphate backbones of the two Okazaki fragments together.
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Replication
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Summary of Bacterial DNA replication
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Dec 4­7:16 AM
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can have errors in the replication of DNA ­ 1 in 100,000 base­pairs
­DNA polymerase proofreads the nucleotide with the template
­if an error is found, DNA polymerase removes the nucleotide and then continues synthesis
­if error is missed can get mismatch repair ­ enzymes fix the incorrectly paired nucleotides
­cell continually monitors and repairs genetic material
­over 130 DNA repair enzymes in humans found so far
Nov 15 ­ 11:30 PM
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in nucleotide excision repair:
Nuclease = enzyme that cuts out the damaged DNA segment
­ DNA polymerase and ligase are responsible for filling the empty spot back in.
ex. for skin cells ultraviolet light can cause thymine dimers = two adjacent thymines that covalently link together and cause buckling of DNA ­ interferes with replication
­ if not repaired can cause skin cancer
­xeroderma pigmentosum ­ hypersensitivity to light ­ get skin cancer very easily
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Nucleotide excision
What are the functions of nuclease, DNA polymerase and DNA ligase here?
Dec 4­7:17 AM
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Replicating process problem
at end of replication, have no way of completing 3' end of lagging strand because an RNA primer is there
­therefore after replication, the strands would get shorter and shorter
­only problem in eukaryotes, Why?
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Shortening of the ends of linear DNA molecules
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Eukaryotes have telomeres ­do not contain genes
­contain multiple repetitions of a short nucleotide chain
­TTAGGG
­number of repetitions can be 100 to 1,000
­telomeres postpone the shortening of DNA and the loss of genes on the ends of the DNA molecules in normal cells
­also prevent cell death
­shortening of teleomeres may have something to do with aging of certain tissues
Nov 15 ­ 11:58 PM
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For germ cells (give rise to gametes)
­telomerase catalyzes the lengthening of the telomeres in germ cells
­restores original length
­uses a short RNA molecule that acts as a template for new telomere synthesis
­have found telomerase in cancer cells that are not germ cells
­if we can find a way to stop the telomerase in tumor cells, could possibly target chemotherapy towards this
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Telomeres on mouse
Chromosomes
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So what is a chromosome?
Prokaryote ­ circular strand of DNA with small amount of protein
Eukaryote ­ linear strand of DNA with large amount of protein
(histones)
Feb 18­3:18 PM
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nucleoid region = dense region of DNA in bacteria, not membrane bound
in eukaryote = chromatin = strand of a complex of DNA and proteins Feb 18­3:30 PM
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heterochromatin = clumped chromatin in interphase
Euchromatin = "true chromatin", less compact, more dispersed
• genes can be expressed from this structure
­histones help change chromatin organization
help chromosomes condense
Feb 18­3:33 PM
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So......
On a molecular basis, how does information get passed on from one cell to another?
Feb 18­3:43 PM
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