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Intro to DNA

Chapter 11

Before we understood

� People believed that the material responsible for heredity was Protein

What is DNA?

� DNA is a molecule made of two long strands of nucleotides

� These nucleotides are held together by hydrogen bonds

� These two strands twist around each other to form a shape called the double helix

Watson and Crick

� In 1953, two scientists determined that the shape of DNA is a double helix� James Watson� Francis Crick

� They were awarded a Nobel Prize in 1962 for their discovery

Rosalind Franklin � Rosalind Franklin took a

special kind of photograph (called an electron micrograph) of DNA

� This photo was very important in determining the shape of DNA

Rosalind Franklin � Watson and Crick stole

information from this photo, then took full credit for the discovery

� Rosalind Franklin died in 1958� Did not receive the 1962

Nobel Prize for her contribution because she was not alive

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Nucleotides� A nucleotide is made of three units:

a) a 5 carbon sugar � deoxyribose in DNA� ribose in RNA

b) a phosphate group c) a nitrogenous base

Nucleotides� The four nitrogenous bases in DNA are:

� guanine� adenine� thymine� cytosine

Base Pairs

� Hydrogen bonds only form between certain base pairs

� A-T

� G-C

Base Pairs

� Erwin Chargaff

� Discovered that there equal numbers of these complementary base pairs.

� This discovery helped Watson & Crick with their model of DNA.

Purines

� Purines have tworings in their nitrogenous base

� A� G

Pyrimidines

� Pyrimidines have one ring in their nitrogenous base

� T� C

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Importance of Base Pairing

� It’s important that A-T and C-G total 3-rings wide � Enables the DNA

strand is the same width

� Allows DNA to coil without becoming tangled