What Does DNA Look What Does DNA Look Like?Like?

The Pieces of the PuzzleThe Pieces of the Puzzle

DNA is sometimes DNA is sometimes called "the called "the blueprintblueprint of of life" because it contains life" because it contains the the codecode, or instructions , or instructions for building an organism for building an organism and ensuring that and ensuring that organism functions organism functions correctly. correctly.

Just like a builder uses Just like a builder uses a blueprint to build a a blueprint to build a house, DNA is used as house, DNA is used as the blueprint, or the blueprint, or plansplans, , for the entire organism.for the entire organism.

Scientists knew that the material that Scientists knew that the material that makes up makes up genes genes must be able to do 2 must be able to do 2 things:things:

1.1. Give instructions for building and Give instructions for building and maintaining maintaining cellscells

2.2. Be able to be copied each time a cell Be able to be copied each time a cell dividesdivides and contain identical genes and contain identical genes

It is the chemical It is the chemical component of component of chromosomes, which are chromosomes, which are located in the located in the nucleusnucleus of of every cell. every cell.

Stretches of DNA (or Stretches of DNA (or stretches of stretches of chromosomes) code for chromosomes) code for genesgenes. .

Gene - a segment of DNA Gene - a segment of DNA that codes for a that codes for a proteinprotein, , which in turn codes for a which in turn codes for a traittrait (skin tone, eye color, (skin tone, eye color, etc), a gene is a stretch of etc), a gene is a stretch of DNA.DNA.

DNA is made up of DNA is made up of subunitssubunits called called nucleotidesnucleotidesNucleotide = consists of a sugar, a Nucleotide = consists of a sugar, a phosphate, and a basephosphate, and a baseFourFour bases bases adenine, thymine, guanine, adenine, thymine, guanine, and cytosine (A, T, G, C)and cytosine (A, T, G, C)Each base has a different Each base has a different shapeshape

Chargaff’s RuleChargaff’s Rule

In 1950’s, In 1950’s, biochemist named biochemist named Erwin Chargaff Erwin Chargaff found that the found that the amount of adenine amount of adenine in DNA in DNA alwaysalways equals amount of equals amount of thyminethymine

Also found that Also found that amount of amount of guanineguanine always equals always equals amount of cytosineamount of cytosine

Chargaff’s Rule Chargaff’s Rule helped scientists helped scientists understand the understand the structurestructure of DNA of DNA

Franklin’s DiscoveryFranklin’s Discovery

Chemist Rosalind Chemist Rosalind Franklin was able Franklin was able to make to make imagesimages of DNA of DNA moleculesmolecules

X ray diffraction = X ray diffraction = X rays are aimed X rays are aimed at at DNADNA molecules molecules and the ray and the ray bounces offbounces offThe pattern made The pattern made by the bouncing by the bouncing rays is captured on rays is captured on film, suggesting film, suggesting that DNA has a that DNA has a spiral spiral shapeshape

Watson and Crick’s ModelWatson and Crick’s Model

James Watson James Watson and Francis Crick and Francis Crick used Franklin’s X used Franklin’s X ray images to ray images to conclude that conclude that DNA must look DNA must look like a long, like a long, twisted twisted ladderladder

They built a They built a modelmodel of DNA in of DNA in their laboratorytheir laboratory

DNA’s Double HelixDNA’s Double Helix

Double helix = Double helix = shaped like a twisted shaped like a twisted ladderladder

Two sides of the Two sides of the ladder are made up of ladder are made up of alternating alternating sugarsugar parts and phosphate parts and phosphate partsparts

Each rung of the Each rung of the ladder is made of aladder is made of a pairpair of bases of bases

The rungs of the The rungs of the ladder can occur in ladder can occur in any order (as long as any order (as long as the the basebase-pair rule is -pair rule is followed)—A always followed)—A always with with TT, G always with , G always with CC

For instance, a stretch of DNA could be For instance, a stretch of DNA could be AATGACCAT (which would code for a AATGACCAT (which would code for a different different genegene than a stretch that read: than a stretch that read: GGGCCATAG). GGGCCATAG).

All in all, there are billions of bases in All in all, there are billions of bases in cells, which code for all the things an cells, which code for all the things an organism needs to organism needs to functionfunction..

Making Copies of DNAMaking Copies of DNA

The pairing of bases allows the cell to The pairing of bases allows the cell to “replicate”, or make “replicate”, or make copiescopies of, DNA of, DNA

Pairs of bases are complementary—Pairs of bases are complementary—meaning they only pair with each meaning they only pair with each other, and both other, and both sidessides of the DNA of the DNA ladder pair togetherladder pair together

How Copies are How Copies are MadeMade

During replication During replication (copying process), (copying process), DNA molecule splits DNA molecule splits down the down the middlemiddle, , where the bases where the bases meetmeet

The bases on each The bases on each side of the molecule side of the molecule are used as a are used as a patternpattern for a new for a new strandstrand

As the bases are As the bases are exposed, exposed, complementarycomplementary nucleotides are nucleotides are added to each added to each side of the ladderside of the ladder

Two DNA Two DNA molecules are molecules are formedformed half of half of each DNA is each DNA is oldold and half is newand half is new

When Copies are MadeWhen Copies are Made

DNA is copied DNA is copied every time a cell every time a cell dividesdivides

The job of The job of unwinding, unwinding, copying, and re-copying, and re-winding the DNA is winding the DNA is done by done by proteinsproteins within the cellwithin the cell

DNA is usually DNA is usually found with several found with several kinds of proteinskinds of proteins

Other proteins help Other proteins help with the process of with the process of carrying out the carrying out the instructionsinstructions written written in the code of DNAin the code of DNA

Scientists’ ContributionsScientists’ Contributions

Erwin ChargaffErwin Chargaff: : discovered base-pair discovered base-pair rulerule

Rosalind FranklinRosalind Franklin: : used X-rays to find used X-rays to find DNA’s spiral shapeDNA’s spiral shape

Watson and CrickWatson and Crick: : built 3-D model of built 3-D model of DNADNA