Molecular Biology

(BIO1004/2001)

Kathleen LobbanKathleen LobbanKathleenlobban@yahoo.coKathleenlobban@yahoo.co

mm

MODULE CONTENTMODULE CONTENT Structure of DNAStructure of DNA DNA ReplicationDNA Replication Transcription and Protein synthesis Transcription and Protein synthesis Molecular Biology TechniquesMolecular Biology Techniques Recombinant DNA technologyRecombinant DNA technology Gene Expression and Regulation in Gene Expression and Regulation in

Prokaryotes and EukaryotesProkaryotes and Eukaryotes

ASSESSMENT PROCEDURESASSESSMENT PROCEDURES

Test 1 (Test 1 (Week of October 12, 2009Week of October 12, 2009 ) ) 15% 15% Test 2 (Test 2 (Week of November 16, 2009Week of November 16, 2009 ) ) 15%15% PracticalsPracticals 10%10% ProjectProject ( (Week of October 26, 2009Week of October 26, 2009 ) ) 10%10% Final Exam (Final Exam (Week of October 12, 2009Week of October 12, 2009 ) ) 50%50%

UNIT 1UNIT 1

Structure of Structure of DNADNA

DNADNA Deoxyribonucleic acidDeoxyribonucleic acid Belongs to the class of Belongs to the class of

macromolecules called nucleic macromolecules called nucleic acids.acids.

The other nucleic acid is RNAThe other nucleic acid is RNA (ribonucleic acid)(ribonucleic acid)

Composition of DNAComposition of DNA DNA is a DNA is a polymer.polymer. The monomer units of DNA are The monomer units of DNA are

nucleotidesnucleotides Therefore the polymer is known as a Therefore the polymer is known as a

"polynucleotide.""polynucleotide." Each nucleotide consists of Each nucleotide consists of

a 5-carbon a 5-carbon sugarsugar (deoxyribose) (deoxyribose) a nitrogen-containing a nitrogen-containing basebase a a phosphatephosphate group. group.

Composition of DNAComposition of DNA

Nitrogenous BasesNitrogenous Bases

The NucleotidesThe NucleotidesBaseBase NucleosideNucleoside Nucleotide (dNTP)Nucleotide (dNTP)AdenineAdenine adenosineadenosineadenosine triphosphateadenosine triphosphate

ATP / ATP / dATPdATPGuanineGuanine guanosineguanosine guanosine triphosphateguanosine triphosphate

GTP / GTP / dGTPdGTPCytosineCytosine cytidinecytidine cytidine triphosphatecytidine triphosphate

CTP / CTP / dCTPdCTPUracilUracil uridineuridine uridine triphosphateuridine triphosphate

UTP /UTP /dUTPdUTPThymineThymine thymidinethymidinethymidine triphosphatethymidine triphosphate

TTP / TTP / dTTPdTTP

Other Functions of Other Functions of NucleotidesNucleotides

They carry chemical in their bonds which They carry chemical in their bonds which can be easily released for use by the cell eg can be easily released for use by the cell eg ATP and GTPATP and GTP

They combine with other group to form They combine with other group to form coenzymes: eg coenzyme Acoenzymes: eg coenzyme A

They are used as specific signaling molecules They are used as specific signaling molecules within cells, eg. Cyclic AMP (cAMP) serves to within cells, eg. Cyclic AMP (cAMP) serves to signal switching on of the lac operon in signal switching on of the lac operon in prokaryotes.prokaryotes.

Composition of DNAComposition of DNABase PairsBase Pairs AdenineAdenine forms 2 hydrogen bonds with forms 2 hydrogen bonds with

TThymine on the opposite strand hymine on the opposite strand Adenine (A) will only bond with Thymine Adenine (A) will only bond with Thymine

(T)(T)

GuanineGuanine forms 3 hydrogen bonds forms 3 hydrogen bonds with with CytosineCytosine on the opposite strand. on the opposite strand. Guanine (G) will only bond with Cytosine Guanine (G) will only bond with Cytosine

(C). (C).

DNA moleculeDNA molecule First described by First described by

James D. Watson and James D. Watson and Francis Crick in 1953. Francis Crick in 1953.

In a DNA molecule, In a DNA molecule, the two strands are the two strands are not parallel, but not parallel, but intertwined with each intertwined with each other. other. 

    The two strands form The two strands form

a "a "double helixdouble helix" " structurestructure

DNA moleculeDNA molecule The DNA backbone is The DNA backbone is

an alternating sugar-an alternating sugar-phosphate sequence. phosphate sequence.

The deoxyribose The deoxyribose sugars are joined at sugars are joined at both the 3'- and 5'-both the 3'- and 5'-carbon to phosphate carbon to phosphate groups by groups by phosphodiesterphosphodiester bonds. bonds.

DNA moleculeDNA molecule Chain has a Chain has a

direction (polarity)direction (polarity)

55΄́ end - phosphate end - phosphate

33΄́ end - hydroxyl end - hydroxyl

DNA moleculeDNA moleculeThe two polynucleotide chains run in The two polynucleotide chains run in

opposite directions - opposite directions - antiparallelantiparallel

The DNA Double HelixThe DNA Double Helix Two DNA strands form a right-handedTwo DNA strands form a right-handed

helical spiral helical spiral The sugar-phosphate backbones of the The sugar-phosphate backbones of the

two DNA strands wind around the helix two DNA strands wind around the helix axis like the railing of a spiral staircase axis like the railing of a spiral staircase

The bases of the individual nucleotides The bases of the individual nucleotides are on the inside of the helix, stacked are on the inside of the helix, stacked on top of each other like the steps of a on top of each other like the steps of a spiral staircasespiral staircase

DNA moleculeDNA molecule The sugar-phosphate backbone of DNA The sugar-phosphate backbone of DNA

is polar, and therefore hydrophilic. is polar, and therefore hydrophilic. The bases, are relatively non-polar and The bases, are relatively non-polar and

therefore hydrophobic. therefore hydrophobic. These These hydrostatic forceshydrostatic forces have a very have a very

stabilizing effect on the overall stabilizing effect on the overall structure of the DNA double helixstructure of the DNA double helix

Therefore there is a strong pressure Therefore there is a strong pressure holding the two strands of DNA holding the two strands of DNA together.together.

The DNA Double HelixThe DNA Double Helix The helix makes a turn every 3.4 nm The helix makes a turn every 3.4 nm The distance between two The distance between two

neighboring base pairs is 0.34 nm.  neighboring base pairs is 0.34 nm.  Hence, there are about 10 pairs per Hence, there are about 10 pairs per

turn.  turn.  The intertwined strands make two The intertwined strands make two

grooves of different widthsgrooves of different widths The The major groovemajor groove

facilitate binding with facilitate binding with specific proteins. specific proteins. The minor grooveThe minor groove

Watson and Crick’s Model Watson and Crick’s Model of DNA of DNA

In 1928 In 1928 Frederick GriffithFrederick Griffith was working on a was working on a project that enabled others to point out that DNA project that enabled others to point out that DNA was the molecule of inheritance.  was the molecule of inheritance. 

The experiment involved mice and bacteria that The experiment involved mice and bacteria that cause pneumonia - a virulent and a non-virulent cause pneumonia - a virulent and a non-virulent kind.  kind. 

He injected the virulent into a mouse and the He injected the virulent into a mouse and the mouse died. mouse died. 

Next he injected the non-virulent into a Next he injected the non-virulent into a mouse and the mouse lived.  mouse and the mouse lived. 

After this, he heated up the virulent strain to kill it After this, he heated up the virulent strain to kill it and then injected it into a mouse.  The mouse and then injected it into a mouse.  The mouse lived.  lived. 

Last he injected non-virulent pneumonia and Last he injected non-virulent pneumonia and virulent pneumonia, that had been heated and virulent pneumonia, that had been heated and killed, into a mouse.  This mouse died.    killed, into a mouse.  This mouse died.    

Watson and Crick’s Model Watson and Crick’s Model of DNAof DNA

Why?  Griffith thought that the Why?  Griffith thought that the killed virulent bacteria had killed virulent bacteria had passed on a characteristic to the passed on a characteristic to the non-virulent one to make it non-virulent one to make it virulent - virulent - TRANSFORMATIONTRANSFORMATION.  .  He thought that this He thought that this characteristic was in the characteristic was in the “inheritance molecule”. “inheritance molecule”. 

Watson and Crick’s Model Watson and Crick’s Model of DNAof DNA

In 1942 Oswald AveryIn 1942 Oswald Avery continued with Griffith’s experiment to see continued with Griffith’s experiment to see

what the inheritance molecule was.  what the inheritance molecule was.  He destroyed the lipids, ribonucleic acids, He destroyed the lipids, ribonucleic acids,

carbohydrates, and proteins of the virulent carbohydrates, and proteins of the virulent pneumonia.  Transformation still occurred pneumonia.  Transformation still occurred after this.  after this. 

Next he destroyed the deoxyribonucleic acid.  Next he destroyed the deoxyribonucleic acid.  Transformation did not occur.  Avery had Transformation did not occur.  Avery had

found the inheritance molecule, found the inheritance molecule, DNA!DNA!   

Watson and Crick’s Model Watson and Crick’s Model of DNAof DNA

1940’s Erwin Chargaff1940’s Erwin Chargaff     noticed a pattern in the amounts of the four noticed a pattern in the amounts of the four

bases: adenine, guanine, cytosine, and bases: adenine, guanine, cytosine, and thymine.  thymine. 

In samples of DNA from different organisms In samples of DNA from different organisms : : the amount of adenine = to the amount of the amount of adenine = to the amount of

thyminethymine the amount of guanine = to the amount of the amount of guanine = to the amount of

cytosine.  cytosine.  This discovery later became This discovery later became Chargaff’s RuleChargaff’s Rule. .

  

Watson and Crick’s Model Watson and Crick’s Model of DNAof DNA

Rosalind Franklin and Maurice WilkinsRosalind Franklin and Maurice Wilkins Decided to try to make a crystal of the DNA Decided to try to make a crystal of the DNA

molecule.  They obtained an x-ray pattern.  molecule.  They obtained an x-ray pattern.  The pattern appeared to contain rungs, like The pattern appeared to contain rungs, like

those on a ladder between to strands that those on a ladder between to strands that are side by side.  It also showed by an “X” are side by side.  It also showed by an “X” shape that DNA had a helix shape. shape that DNA had a helix shape.

www.who2.com/.../watson-v-franklin-round-27.html www.who2.com/.../watson-v-franklin-round-27.html

Watson and Crick’s Model Watson and Crick’s Model of DNAof DNA

In 1953 In 1953 James Watson and Francis Crick James Watson and Francis Crick saw Franklin and Wilkin's picture of the X-ray and saw Franklin and Wilkin's picture of the X-ray and made an accurate model - a made an accurate model - a double helixdouble helix with with little little

rungs connecting the two strandsrungs connecting the two strands But how to bond the bases together ?But how to bond the bases together ? How to solve the problem of the sizes of the How to solve the problem of the sizes of the

bases?  bases?  Adenine and Guanine were purines having two Adenine and Guanine were purines having two

carbon-nitrogen rings in their structures.  carbon-nitrogen rings in their structures.  Thymine and Cytosine were pyrimidines having one Thymine and Cytosine were pyrimidines having one

carbon-nitrogen ring in its structure. carbon-nitrogen ring in its structure. If purines and the pyrimidines were together, then If purines and the pyrimidines were together, then

DNA would look wobly and crooked.  DNA would look wobly and crooked.  If they paired Thymine with Adenine and Guanine If they paired Thymine with Adenine and Guanine

with Cytosine, DNA would look uniform (Chargaff's with Cytosine, DNA would look uniform (Chargaff's rule)  rule) 

Each side is a complete compliment of the other. Each side is a complete compliment of the other.        

Watson and Crick Model Watson and Crick Model of DNAof DNA

http://www.johnkyrk.com/DNAanatohttp://www.johnkyrk.com/DNAanatomy.htmlmy.html

Remembering RosalindRemembering Rosalind By using the picture of the By using the picture of the

crystallized DNA, crystallized DNA, Watson and Crick were able to put Watson and Crick were able to put

together the model of DNA. together the model of DNA.      Watson and Crick used Watson and Crick used

information from Avery, Chargaff, information from Avery, Chargaff, Griffith, and others.  Griffith, and others. 

They simply pieced together the They simply pieced together the puzzle.  puzzle. 

The Nobel Prize was awarded to The Nobel Prize was awarded to Watson, Crick, and Maurice Watson, Crick, and Maurice Wilkins.  Wilkins. 

Rosalind Franklin did not receive Rosalind Franklin did not receive the prize because she had died of the prize because she had died of cancer by this time.  Maurice cancer by this time.  Maurice Wilkins was able to share the prize Wilkins was able to share the prize with Watson and Crick, though, with Watson and Crick, though, because of his work with Franklin.  because of his work with Franklin.  Her accomplishment should never Her accomplishment should never be forgotten.  be forgotten. 

Forms of DNAForms of DNA DNA exists in many possible DNA exists in many possible

conformations.conformations. only only A-DNAA-DNA, , B-DNAB-DNA, and , and Z-DNAZ-DNA have have

been observed in cells.been observed in cells. conformation depends on:conformation depends on:

the DNA sequence the DNA sequence the amount and direction of supercoilingthe amount and direction of supercoiling chemical modifications of the bases chemical modifications of the bases solution conditions (eg concentration of solution conditions (eg concentration of

metal ions, salt concentration and level metal ions, salt concentration and level of hydrationof hydration).).

B -Form of DNA B -Form of DNA Most common under the conditions Most common under the conditions

found in cells. found in cells. Has a major and minor groove Has a major and minor groove Has 10 base pairs per turnHas 10 base pairs per turn One turn spans 3.4 nmOne turn spans 3.4 nm

A -Form of DNA A -Form of DNA With higher salt concentrations or With higher salt concentrations or

with alcohol added, the DNA with alcohol added, the DNA structure may change to structure may change to A formA form

A wider right-handed spiralA wider right-handed spiral A shallow, wider minor groove A shallow, wider minor groove A narrower, deeper major grooveA narrower, deeper major groove Has 11 bases per turnHas 11 bases per turn One turns spans 2.3 nmOne turns spans 2.3 nm

Z -Form of DNA Z -Form of DNA The DNA molecule with alternating G-The DNA molecule with alternating G-

C sequences in alcohol or high salt C sequences in alcohol or high salt solution tends to have such structuresolution tends to have such structure

Bases seem to zigzagBases seem to zigzag The strands turn about the helical The strands turn about the helical

axis in a left-handed spiral. axis in a left-handed spiral. Has a narrow deep grooveHas a narrow deep groove Has 12 bases per turn.Has 12 bases per turn. One turn spans 4.6 nm One turn spans 4.6 nm

A – DNA B-DNA Z-DNA

ChromosomesChromosomes Chromosomes – highly coiled condensed Chromosomes – highly coiled condensed

packages of DNApackages of DNA

Present in both eukaryotes and Present in both eukaryotes and prokaryotesprokaryotes Eukaryotes – linearEukaryotes – linear Prokaryotes – most closed circularProkaryotes – most closed circular

The human genome has 3,000,000,000 base The human genome has 3,000,000,000 base pairs packed into 23 chromosomes!pairs packed into 23 chromosomes!

Organization of Eukaryotic Organization of Eukaryotic GenomesGenomes

DNA is packed into chromosomes with the DNA is packed into chromosomes with the help of proteins - help of proteins - histoneshistones. .

The histones associate with DNA forming The histones associate with DNA forming structures called structures called nucleosomesnucleosomes. .

Each nucleosome complex consists of a Each nucleosome complex consists of a beadlike structure with 146 base pairs of beadlike structure with 146 base pairs of DNA wrapped around a disc-shaped core of DNA wrapped around a disc-shaped core of eight histone molecules. eight histone molecules.

NucleosomeNucleosome

ChromosomesChromosomes Nucleosomes are 11nm in diameter.Nucleosomes are 11nm in diameter. The packed nucleosome state occurs when a The packed nucleosome state occurs when a

ninth histone called H1, associates with the ninth histone called H1, associates with the linker DNA packing adjacent nucleosomes linker DNA packing adjacent nucleosomes together forming a 30nm diameter thread. together forming a 30nm diameter thread.

In the extended chromosome , these 30nm In the extended chromosome , these 30nm diameter threads form large coiled loops diameter threads form large coiled loops held together by a set of non-histone held together by a set of non-histone scaffolding proteins. scaffolding proteins.

Eukaryotic GenomesEukaryotic Genomes organized into multiple organized into multiple

chromosomes - varying in size and chromosomes - varying in size and numbers depending on the species:numbers depending on the species: Human cells haploid 23Human cells haploid 23 Fruit flies haploid 4Fruit flies haploid 4 Yeast haploid 16Yeast haploid 16 Cats haploid 19Cats haploid 19 Dogs haploid 39 Dogs haploid 39

Bacterial Genome Bacterial Genome OrganizationOrganization

Most have covalently closed, circular Most have covalently closed, circular chromosomes and plasmidschromosomes and plasmids

Not all bacteria have a single circular Not all bacteria have a single circular chromosome: chromosome: some bacteria have some bacteria have multiple circular multiple circular

chromosomeschromosomes many bacteria have many bacteria have linear chromosomeslinear chromosomes

and and linear plasmidslinear plasmids..

Bacterial GenomeBacterial Genome The genetic material of a bacterium lies in the The genetic material of a bacterium lies in the

cytoplasm and is not surrounded by a nuclear cytoplasm and is not surrounded by a nuclear envelope. envelope.

In most species it is contained in a single In most species it is contained in a single circular DNA molecule. circular DNA molecule.

If stretched out to its full length, this molecule If stretched out to its full length, this molecule would be 1000 times longer than the cell itself. would be 1000 times longer than the cell itself.

Unlike eukaryotic chromosomes, the bacterial Unlike eukaryotic chromosomes, the bacterial DNA has little proteins associated with it. DNA has little proteins associated with it.

PlasmidsPlasmids In addition to the genomic DNA, most bacteria In addition to the genomic DNA, most bacteria

have a small amount of genetic information have a small amount of genetic information present as one or more plasmids – present as one or more plasmids – extrachromosomal circular DNA. extrachromosomal circular DNA.

Plasmids can replicate independently of the Plasmids can replicate independently of the genomic DNA or become integrated into it. genomic DNA or become integrated into it.

Bacterial plasmids frequently have genes that Bacterial plasmids frequently have genes that code for:code for: catabolic enzymescatabolic enzymes genetic exchangegenetic exchange resistance to antibiotic. resistance to antibiotic.

Viral GenomesViral Genomes virus genomes may contain their virus genomes may contain their

genetic information encoded in genetic information encoded in either DNA or RNAeither DNA or RNA

Viral GenomesViral Genomes Many of the DNA viruses of Many of the DNA viruses of

eukaryotes closely resemble their eukaryotes closely resemble their host cells in terms of the biology of host cells in terms of the biology of their genomes: their genomes: Some DNA virus genomes are Some DNA virus genomes are

complexed with cellular histones to complexed with cellular histones to form a chromatin-like structure inside form a chromatin-like structure inside the virus particle. the virus particle.

Viral GenomesViral Genomes Genome may be :Genome may be :

Circular or linearCircular or linear segmented – (two or more separate molecules of segmented – (two or more separate molecules of

nucleic acid) nucleic acid) Single-stranded Single-stranded Double-stranded Double-stranded Double-stranded with regions of single-strandedness Double-stranded with regions of single-strandedness

DNA virusesDNA viruses RNA virusesRNA viruses

Positive sensePositive sense Negative senseNegative sense AmbisenseAmbisense

Both DNA and RNA (at different stages in the Both DNA and RNA (at different stages in the life cycle)life cycle)

Reverse transcribing viruses Reverse transcribing viruses

Viral GenomesViral Genomes I: I: dsDNA virusesdsDNA viruses

(e.g. Herpesviruses) (e.g. Herpesviruses) II: II: ssDNA virusesssDNA viruses (+)sense DNA (+)sense DNA

(e.g. Parvoviruses) (e.g. Parvoviruses) III: III: dsRNA virusesdsRNA viruses

(e.g. Reoviruses - Rotavirus) (e.g. Reoviruses - Rotavirus) IV: IV: (+)ssRNA viruses(+)ssRNA viruses

(+)sense RNA (e.g. Picornaviruses -Enterovirus) (+)sense RNA (e.g. Picornaviruses -Enterovirus) V: V: (-)ssRNA viruses(-)ssRNA viruses

(-)sense RNA (e.g. Orthomyxoviruses – Influenza A, B, C) (-)sense RNA (e.g. Orthomyxoviruses – Influenza A, B, C) VI: VI: ssRNA-RT virusesssRNA-RT viruses

(+)sense RNA with DNA intermediate in life-cycle (e.g. (+)sense RNA with DNA intermediate in life-cycle (e.g. Retroviruses – HIV, lukemia and tumor viruses) Retroviruses – HIV, lukemia and tumor viruses)

VII: VII: dsDNA-RT virusesdsDNA-RT viruses (e.g. Hepadnaviruses – hepatitis B) (e.g. Hepadnaviruses – hepatitis B)

GenesGenes Unit of inheritanceUnit of inheritance A sequence of nucleotides which A sequence of nucleotides which

provide a cell with the instructions provide a cell with the instructions for the synthesis of a specific for the synthesis of a specific polypeptide or a type of RNApolypeptide or a type of RNA

Genes determine traitsGenes determine traits Most are 1,000 to 4,000 nucleotides Most are 1,000 to 4,000 nucleotides

long (may be shorter or significantly long (may be shorter or significantly longer)longer)

GenesGenes

GenesGenes Most eukaryotic genes contain non Most eukaryotic genes contain non

coding regions - intronscoding regions - introns

Genes, Viruses and CancerGenes, Viruses and Cancer Cancer is a disease in which cells escape Cancer is a disease in which cells escape

the restraints on normal cell growth.the restraints on normal cell growth. Cancer is an inheritable disease (at least Cancer is an inheritable disease (at least

from cell to daughter cells). from cell to daughter cells). Once a cell has become cancerous, all of Once a cell has become cancerous, all of

its descendant cells are cancerous. its descendant cells are cancerous. Gross chromosomal abnormalities are Gross chromosomal abnormalities are

often visible in cancerous cells. often visible in cancerous cells. Most carcinogens (cancer-generating Most carcinogens (cancer-generating

factors) are also mutagens (mutation-factors) are also mutagens (mutation-generating factors). generating factors).

Oncogenes are genes resembling Oncogenes are genes resembling normal genes but in which something normal genes but in which something has gone wrong, resulting in a cancer. has gone wrong, resulting in a cancer.

Genes, Viruses and CancerGenes, Viruses and Cancer Viruses seem able to cause cancer in Viruses seem able to cause cancer in

three ways: three ways: Presence of the viral DNA may Presence of the viral DNA may

disrupt normal host DNA functions. disrupt normal host DNA functions. Viral proteins needed for virus Viral proteins needed for virus

replication may also affect normal replication may also affect normal host gene regulation. host gene regulation.

Since most cancer-causing viruses Since most cancer-causing viruses are retroviruses, the virus may are retroviruses, the virus may serve as a vector for oncogene serve as a vector for oncogene insertion. insertion.

Images’ sourceImages’ source http://www2.le.ac.uk/departments/emfpu/profiling/explained/images/PrimaryDNA.gifhttp://www2.le.ac.uk/departments/emfpu/profiling/explained/images/PrimaryDNA.gif http://3.bp.blogspot.com/_DZH2cmCoois/Rp_K_5IavqI/AAAAAAAAClI/9GdTDazv-oQ/s400/figure+19-http://3.bp.blogspot.com/_DZH2cmCoois/Rp_K_5IavqI/AAAAAAAAClI/9GdTDazv-oQ/s400/figure+19-

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table : table : http://www.sci.sdsu.edu/~smaloy/MicrobialGenetics/topics/chroms-genes-prots/chromosomes.htmlhttp://www.sci.sdsu.edu/~smaloy/MicrobialGenetics/topics/chroms-genes-prots/chromosomes.html

History of DNA History of DNA http://library.thinkquest.org/20830/Textbook/HistoryofDNAResearch.htm#Watson%20and%20Crickhttp://library.thinkquest.org/20830/Textbook/HistoryofDNAResearch.htm#Watson%20and%20Crick

http://images.google.com.jm/imgres?imgurl=http://genome.jgi-psf.org/Chr16/http://images.google.com.jm/imgres?imgurl=http://genome.jgi-psf.org/Chr16/dna.jpg&imgrefurl=http://genome.jgi-psf.org/Chr16/dna.jpg&imgrefurl=http://genome.jgi-psf.org/Chr16/Chr16.home.html&usg=__zOSiNtQbR9DdY2ZBsog2FGR1Ag8=&h=284&w=280&sz=51&hl=en&starChr16.home.html&usg=__zOSiNtQbR9DdY2ZBsog2FGR1Ag8=&h=284&w=280&sz=51&hl=en&start=6&um=1&tbnid=zYTbfjqc9ubbgM:&tbnh=114&tbnw=112&prev=/images%3Fq%3Drosalindt=6&um=1&tbnid=zYTbfjqc9ubbgM:&tbnh=114&tbnw=112&prev=/images%3Fq%3Drosalind%2Bfranklin%26hl%3Den%26sa%3DG%26um%3D1%2Bfranklin%26hl%3Den%26sa%3DG%26um%3D1

http://www.daviddeerfield.com/NIH/A-B_DNA.gifhttp://www.daviddeerfield.com/NIH/A-B_DNA.gif http://commons.wikimedia.org/wiki/File:A-B-Z-DNA_Side_View_Transparent.pnghttp://commons.wikimedia.org/wiki/File:A-B-Z-DNA_Side_View_Transparent.png http://bioweb.wku.edu/courses/biol566/Images/ChromatinF09-35.JPGhttp://bioweb.wku.edu/courses/biol566/Images/ChromatinF09-35.JPG http://www.microbiologybytes.com/introduction/genomes.htmlhttp://www.microbiologybytes.com/introduction/genomes.html http://www.accessexcellence.org/RC/VL/GG/images/genes.gifhttp://www.accessexcellence.org/RC/VL/GG/images/genes.gif http://www.daviddarling.info/images/exon_and_intron.gif http://www.daviddarling.info/images/exon_and_intron.gif

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