As you can see from the model, the structure is

basically a long, …twisty, …uh laddery type thing.

1953: The structure of the DNA molecule is first described.

DNA - Introduction

DNA• How do genes work? What are they made of,

and how do they determine the characteristics of organisms?

• In the middle 1900’s questions like these were on the minds of biologists everywhere.

Gregor Mendel “Father of Genetics” (1864)

Genetics – the scientific study of heredity.

The Components and Structure of DNA

• When scientists found out that the molecule DNA was responsible for our genetics they still were not satisfied.

• How could DNA or any molecule for that matter have the ability to carry, translate, copy, and be responsible for our heredity?

DNA(Deoxyribose Nucleic Acid)• DNA is a long molecule made up of units called

nucleotides.• Nucleotide is made up of three basic structures:

- A 5-carbon sugar called deoxyribose- A phosphate group- And a combination of four nitrogenous bases.

The Backbone of DNA• The “backbone” of DNA

is made of up of sugar and phosphate.

• This backbone allows for the nitrogeneous bases (adenine, cytosine, guanine, and thymine) to combine and construct our genetic code.

Nitrogenous Bases• DNA’s nitrogenous bases are the letters of the

words which make up our genetic code.• Adenine always connects with Thymine.• Cytosine always connects with Guanine.

Together these bases make up the steps of a ladder.

Nitrogenous Bases

- Two nitrogenous bases belong to a group called the purines. (Adenine and Guanine) A and G

- The other two nitrogenous bases belong to a group called the pyrimidines. (Cytosine and Thymine) C and T

- Each base pair contains one purine and one pyrimidine.

Adenine and Thymine

Straights with StraightsA - T

Guanine and Cytosine

Curves with CurvesG – C

DNA• Each DNA molecule is made up of a series of

monomers called nucleotides. Each nucleotide has three parts:

• A deoxyribose sugar• A phosphate group• A nitrogenous base.

DNA’s History• Now that you know what DNA is and how it is

constructed, how do you think it was found?

Chargaff’s Rule

• One of the puzzling facts about DNA was a curious relationship between its nucleotides.

• Erwin Chargaff (an American biochemist) had discovered that the percentages of guanine and cytosine bases are almost always equal in any sample of DNA.

X-Ray Evidence• In the early 1950’s a British scientist named

Rosalind Franklin began to study DNA.• Just like anyone else that is curious Rosalind

wanted to see what she was studying, so she took pictures of DNA with an X-ray.

The Double Helix• At the same time that Franklin was continuing her

research, Francis Crick and James Watson were trying to build a three-dimensional model of the molecule.

• Watson and Crick used Rosalind’s picture to figure out DNA’s puzzle and the solved what is known as the double helix.

• DNA is a double helix in which two strands are wound around each other. Each strand is made up of a chain of nucleotides. The two strands are held together by hydrogen bonds between adenine and thymine and between guanine and cytosine.

100 Greatest Discoveries - DNA

Read Section 12-1 / pp. 287-294(Relationship between genes & DNA / Structure)

Questions to turn in…Page 294 - 2, 3 & 4Page 315 - 12 & 13