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AP Biology
DNAStructure andReplication
AP Biologyproteins
DNA
Nucleic Acids§ Function:
ugenetic material§ stores information
wgeneswblueprint for building proteins
n DNA ® RNA ® proteins§ transfers information
wblueprint for new cellswblueprint for next generation
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Nucleic Acids§ Examples:
u RNA (ribonucleic acid)§ single helix
u DNA (deoxyribonucleic acid)§ double helix
§ Structure:u monomers = nucleotides
RNADNA
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RNA & DNA§ RNA
u single nucleotide chain§ DNA
u double nucleotide chain§ N bases bond in pairs
across chainsu spiraled in a double helix
§ double helix 1st proposed as structure of DNA in 1953 by James Watson & Francis Crick
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DNA§ Function
u series of bases encodes information§ like the letters that make up words in a book
u stored information is passed from parent to offspring§ need to copy accurately!
u stored information = genes§ genetic information
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Nucleotides§ 3 parts
u nitrogen base (C-N ring)u pentose sugar (5C)
§ ribose in RNA§ deoxyribose in DNA
u phosphate (PO4) group
Are nucleic acidscharged molecules?
Nitrogen baseI’m the A,T,C,G or Upart!
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Nucleotides§ 2 types of nucleotides
u Differ in nitrogen bases u purines
§ double ring N base § adenine (A)§ guanine (G)
u pyrimidines§ single ring N base § cytosine (C)§ thymine (T)§ uracil (U)
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DNA Structure - Nucleic polymer§ Backbone
u sugar to PO4 bondu phosphodiester bond (a type
of covalent bond)§ new base added to sugar of
previous base§ polymer grows in one direction
u N bases hang off the sugar-phosphate backbone
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DNA Structure – Paired nucleotides§ Nucleotides bond between
DNA strandsu H bondsThe base pairing ules:u purine :: pyrimidineu A :: T
§ 2 H bondsu G :: C
§ 3 H bonds
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DNA molecule§ Double helix
u H bonds between bases join the 2 strands§ A :: T§ C :: G
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Building the polymer
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Copying DNA§ Replication
u 2 strands of DNA helix are complementary§ have one, can build other§ have one, can rebuild the
whole
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DNA replication“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”
James WatsonFrancis Crick
1953
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Rosalind Franklin (1920-1958)Rosalind’s X-Ray diffraction pictures helped Watson and Crick develop their theory
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Interesting note…§ Ratio of A-T::G-C
affects stability of DNA moleculeu 2 H bonds vs. 3 H bondsu biotech procedures
§ more G-C = need higher T° to separate strands
u high T° organisms§ many G-C
u parasites§ many A-T (don’t know why)
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Another interesting note…§ ATP
Adenosine triphosphate
++
u modified nucleotide§ adenine (AMP) + Pi + Pi
AP Biology
HELIXHELIX
DNA REPLICATION
Preparing for Mitosis
DNA REPLICATIONDNA IS COPIED DURING
the S-phase of INTERPHASE
PREPARATION FOR CELL DIVISION (when each daughter cell gets a complete set of DNA)
SEPARATIONHelicase is the enzyme that
splits the 2 strands of DNA
(Scissors)
THE POINT OF SEPARATION IS CALLED THE REPLICATION FORK
SEPARATION
Helicase will separate the strands of dna in multiple places along the molecule so that separation occurs quickly
Multiple Replication bubbles form along the entire dna molecule
PREPARATIONWITHIN EACH
REPLICATION BUBBLE, primase, The second enzyme, primes the dna for replication.
EXTENDING THE STRANDS
DNA POLYMERASEMOVEs ALONG THE OPEN CHAINS AND adds COMPLEMENTARY NUCLEOTIDES to build a new strand of DNA
The 2 ends of a strand of DNA are different and antiparallel- one end is 5 prime (5’), other is 3 prime (3’), complementary to opposite ends
Dna polymerase can ONLY add nucleotides to the 3’ end, never to the 5’ end.
BUILDING DNASTRANDS
The leading strand is the strand that is made by adding nucleotides to the 3’ end as the replication fork opens up
The lagging strand is the strand that has to somehow build a dna strand in the 5’ direction
EXTENSION
How does DNA build a lagging strand that is
extending in the 5’ direction?
PROBLEM!!!
ANSWER!!:
Okazaki fragments are partial segments of DNA that form as the replication fork exposes more of the DNA
FINALIZING THE NEW STRANDS
The final enzyme, Ligase, “lags” behind and reforms the phosphate & Sugarbackbone, “gluing” the okazaki fragments together
TWO STRANDSNOW THERE ARE TWO double
STRANDS of DNA• EACH STRAND contains HALF
OF THE ORIGINAL STRAND
EACH Daughter STRAND IS IDENTICAL TO THE ORIGINAL, parent, STRAND
Unzipped by:Helicase
Unzipped by:Helicase
New base pairs added by:DNA PolymerasePrimed by:
Primase
Unzipped by:Helicase
New base pairs added by:DNA PolymerasePrimed by:
Primase
Finalized by:Ligase
MOVIEShttp://www.johnkyrk.com/DNAreplication.htmlhttp://www.pbs.org/wgbh/aso/tryit/dna/#http://www.ncc.gmu.edu/dna/repanim.htmhttp://sites.fas.harvard.edu/~biotext/animations/replication1.swfhttp://www.hhmi.org/biointeractive/dna-replication-advanced-detail