What Does It Look Like? What Does it Do? DNA What Does It Look Like? What Does it Do?

DNA – A Piece of the Puzzle DNA must be able to give instructions for building and maintaining cells DNA must be able to be copied each time a cell divides so that each cell has identical chromosomes and genes.

Nucleotides: Building Blocks of DNA A nucleotide is made up of a sugar, a phosphate and a base. There are four bases Adenine Thymine Guanine Cytosine

Bases Match Up All four bases have different shapes Guanine only matches with Cytosine Adenine only matches with Thymine Chargaff’s rule says the amount of A=T and the amount of G=C

Rosalind Franklin Rosalind Franklin used x-ray diffraction to take pictures of DNA molecules. Her research suggested the spiral shape of DNA

Watson and Crick’s Model After seeing Franklin’s image, Watson and Crick determined that DNA must be shaped like a long, twisted ladder. This explained how DNA is copied and how it works in the cell

DNA – The Double Helix The twisted ladder is called a double helix The sides of the ladder are made of alternating sugar and phosphate molecules The rungs of the ladder are made of base pairs Adenine – Thymine Guanine - Cytosine

Making Copies of DNA Pairs of bases allows the cell to replicate or to make copies of DNA The pairs are complementary since they always pair A-T and C-G

How Copies are Made DNA molecules split down the middle The bases on each side are used as a pattern for the new strand Complementary bases are added (A-T and C-G) TWO DNA molecules are formed. Half of each molecule is the old DNA, half is new DNA

DNA and Chromosomes DNA is coiled tightly in the nucleus of cells DNA is bundled into structures called chromosomes. DNA is copied every time a cell divides The job of unwinding, copying and rewinding the DNA is done by proteins in the cell.