DNA Structure & DNA Structure & ReplicationReplication

Ch. 10.1Ch. 10.1GriffithGriffith’’s Transformation s Transformation

ExperimentExperiment1928 – Frederick Griffith is studying how

certain strains of bacteria cause pneumonia and inadvertently makes a discovery about how genetic information is passed from organism to organism

His Experiment:Grow two slightly different strains (types) of

bacteriaOne strain proven harmless and other

deadlyLaboratory mice are injected with these

strains

GriffithGriffith’’s Resultss Results

What caused GriffithWhat caused Griffith’’s s results?results?

• The heat-killed strain passed on its disease-causing ability to the live harmless strain.

• In Griffith’s words, one strain of bacteria was TRANSFORMED into another.

• Was the “Transforming” Factor: Protein or DNA?

Avery, MacLeod, McCarty

Isolate macromolecules present in S-strain (virulent) bacteria to see which is capable of transforming R-strain (non-virulent) bacteria.

Only DNA can transform!

The Hershey-Chase The Hershey-Chase ExperimentExperiment

• Alfred Hershey & Martha Chase studied viruses, which are non-living particles smaller than a cell that can infect living organisms.

• Bacteriophages: specific group of viruses that infect bacteria.

• OBJECTIVE: To determine which part of the virus (protein or DNA) enters a bacteria it is infecting.

What did Hershey & What did Hershey & Chase do?Chase do?

• If Hershey and Chase could determine which part of the virus entered an infected cell, they would learn whether genes were made of protein or DNA.

• To accomplish this, they grew viruses in cultures containing radioactive isotopes of phosphorus-32 (32P) and sulfur-35 (35S).

• Some viruses had P-32 in their DNA, and others had S-25 in their protein coat.

• If S-35 is found in the bacteria, it would mean that viruses release their protein and if P-32 is found in the bacteria it would mean that viruses release their DNA.

Recall: Method of Bacteriophage Recall: Method of Bacteriophage InfectionInfection

o When a bacteriophage enters a bacterium, the virus attaches to the surface of the cell and injects its genetic information into it.

o The viral genes replicate to produce many new bacteriophages, which eventually destroy the bacterium.

o When the cell splits open, from viral overload, hundreds of new viruses burst out and can infect surrounding cells

Hershey-Chase ResultsHershey-Chase Results

So… The genetic material in bacteriophages was the DNA,DNA, (not the protein)!!!

Ch. 10.2-10.3: DNA Ch. 10.2-10.3: DNA StructureStructure

• Made of monomers called nucleotidesnucleotides

• Nucleotide structure:

A nucleotide can have one A nucleotide can have one of four bases:of four bases:

Types of bases: Adenine Guanine Cytosine Thymine

A & G are bigger and are called purinesC & T are smaller and are called pyrimidines

ChargaffChargaff’’s Rule & s Rule & Rosalind FranklinRosalind Franklin

Chargaff (A=T; C=G)Discovered that in almost any DNA sample, the % G% G nearly equals the % C% C and the % A% A nearly equals the % T % T

Franklin: X-ray picture helps figure out structureused x-ray diffraction to get information about the structure of DNA. •She aimed an X-ray beam at concentrated DNA samples and recorded the scattering pattern of the X-rays on film.

Watson & CrickWatson & Crick• Using clues from Franklin’s X-ray pattern, shown

to them by Maurice Wilkins, James Watson and Francis Crick built a 3-D model that explained how DNA carried information and could be copied.

• Watson, Crick & Wilkins were awarded the 1962 Nobel Prize in Physiology or Medicine for their work.

Base-PairingBase-Pairing• Watson & Crick discovered that bonds can only form

between certain base pairs, Adenine & Thymine and Cytosine & Guanine.

• The base-pairing rule means that purinespurines only pair with pyrimidinespyrimidines, making the rungs equally spaced like a ladder.

• The nitrogenous bases are held together by hydrogen bonds. – A & T are held together by TWO hydrogen bonds– C & G are held together by THREE hydrogen bonds

DNA is a DNA is a ““double-helixdouble-helix”” or or twisted ladder:twisted ladder:

oThe “backbone” or sides of the DNA molecule are made up of alternating sugars and phosphates and the “rungs” are made up of interlocking nitrogen bases.

oThe sugars and the phosphates are held together by covalent bonds and the nitrogen bases are held together by hydrogen bonds.

Molecular Structure of DNA

DNA Structure

Chromosomes & DNAPROKARYOTES:• In prokaryotic cells (bacteria), DNA is

located in the cytoplasm.• Most prokaryotic cells have a single DNA

molecule that holds all of the genetic information

EUKARYOTES:• 1000x the DNA as prokaryotes• DNA is held in chromosomes located in

the nucleus• Chromosome # varies widely from one

species to the next

DO NOW:

• What category of macromolecules does DNA fit into?

• What is the monomer of a DNA molecule?

• What are the three parts of this molecule?

• How do the nucleotides fit together in a molecule of DNA?

• How is the 3D structure of DNA described?

33 end end 55 end end

33 end end55 end end

33

55

22

44

1133

55

22

4411

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DNA Replication Tutorials

• http://www.hhmi.org/biointeractive/dna/DNAi_replication_vo2.html Excellent narrated animation; pause, highlight, take notes; use Styrofoam model

• Good additional animation of lagging strand http://sites.fas.harvard.edu/~biotext/animations/replication1.html

DNA Replication• Before a cell can divide, it’s DNA must be replicated or

copied in the S-phase of the cell cycle. • In most prokaryotes, replication begins at a single point

and continues in two directions.• In eukaryotes, replication occurs in hundreds of places

simultaneously and proceeds until complete.

• Sites of replication are called replication forks.

– DNA replication begins at the origins of replication– DNA unwinds at the origin to produce a “bubble”– Replication proceeds in both directions from the origin– Replication ends when products from the bubbles merge

with each other

– DNA replication occurs in the 5’ 3’ direction– Replication is continuous on the 3’ 5’ template– Replication is discontinuous on the 5’ 3’ template,

forming short segments

10.5 DNA replication proceeds in two directions at many sites simultaneously

Copyright © 2009 Pearson Education, Inc.Copyright © 2009 Pearson Education, Inc.

Animation: Animation: Leading StrandLeading Strand

10.5 DNA replication proceeds in two directions at many sites simultaneously

– Proteins involved in DNA replication– DNA helicase unwinds the double helix– DNA polymerase adds nucleotides to a growing chain– DNA ligase joins small fragments into a continuous chain

Copyright © 2009 Pearson Education, Inc.Copyright © 2009 Pearson Education, Inc.

Animation: Animation: Lagging StrandLagging Strand

Animation: Animation: DNA Replication ReviewDNA Replication Review

Animation: Animation: Origins of ReplicationOrigins of Replication

Origin of replicationOrigin of replication Parental strandParental strand

Daughter strandDaughter strand

BubbleBubble

Two daughter DNA moleculesTwo daughter DNA molecules

Parental DNAParental DNA

3355

DNA polymeraseDNA polymerasemoleculemolecule

DNA ligaseDNA ligase

3355

Overall direction of replicationOverall direction of replication

Daughter strandDaughter strandsynthesizedsynthesizedcontinuouslycontinuously

3355

3355

Daughter Daughter strandstrandsynthesizedsynthesizedin piecesin pieces

Visual Summary of DNA replication

AnimationAnimation

Replication Bubbles

ParentalParentalmoleculemoleculeof DNAof DNA

NucleotidesNucleotides

Both parentalBoth parentalstrands servestrands serveas templatesas templates

Two identicalTwo identicaldaughterdaughter

molecules of DNAmolecules of DNA Each Each ““newnew”” strand of DNA consists of one strand of DNA consists of one original original

templatetemplate strand and one strand and one newly madenewly made strand. strand. This allows for This allows for proofreadingproofreading, using the template , using the template

strand as the strand as the ““mastermaster””..

DNA REPLICATION is semi-conservative

Chromosome Structure• Eukaryotic chromosomes contain DNA and

protein tightly packed together to from chromatin.

• Chromatin consists of DNA tightly coiled around proteins called histones.

• DNA & histone molecules form nucleosomes, which pack together to form thick fibers of chromosomes.

Do Now:

• Why would DNA need to be photocopied? What is the name of this process? Where does it occur?

• Replication is described as semi-conservative. Explain.

 •  Distinguish between the leading and

lagging strand during DNA replication.