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 pairsAdenine – ThymineGuanine - 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.