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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

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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