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Chapter 8

Section 8.2: The Structure of DNA

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Objectives

• SWBAT describe the interaction o the four nucleotides that make up DNA.

• SWBAT describe the three-dimensional structure of DNA.

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Vocabulary – Section 8.2

• Nucleotide • Double helix• What are the four nitrogen-containing bases

of DNA?– How are they paired up inside of the DNA double

helix?• Review: Covalent bond• Review: Hydrogen bond

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James Watson

• TED Talk by James Watson about his contribution to the discovery of DNA’s structure.

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The Structure of DNA

• DNA is a very long polymer.– It is made up of monomers

called nucleotides.– Each nucleotide has three parts:

1. A phosphate group (one phosphorus with four oxygens)

2. A ring-shaped sugar called deoxyribose.

3. A nitrogen-containing base (a single or double ring built around nitrogen and carbon atoms (these form the genes).

phosphate group

deoxyribose (sugar)

nitrogen-containingbase

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The Structure of DNA

• One molecule of human DNA (one chromosome) contains billions of nucleotides.– There are, however, only

four types of nucleotides.– The 4 nucleotides differ

only in their nitrogen-containing bases.

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The Four Nitrogen Containing Bases

• The bases are Adenine (A) and Thymine (T), and Guanine (G) and Cytosine (C). – Adenine and guanine (A and G) have a larger

double-ring structure.– Cytosine and Thymine (C and T) have a smaller

single-ring structure.

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The 4 Nitrogen-Containing Bases of DNA

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Chargaff’s Rule

• Until Edwin Chargaff, many scientists hypothesized that DNA was made up of the four nucleotides in equal amounts. – They believed that DNA was exactly the same in

all organisms and that DNA was not the genetic material (and thus could not be responsible for all of the variation seen among all organisms).

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Chargaff’s Rule

• In 1950, Chargaff analyzed the DNA of several different organisms. – Chargaff found that the same four bases are

found in all organisms but the proportions of the bases was unique to each organism.

– He found that the amount of adenine equals the amount of thymine; and that the amount of cytosine equals the amount of guanine.

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Watson, Crick, and the Double Helix

• As in all science, James Watson (a geneticist from the US) and Francis Crick (a British physicist) based their work on the previous work of many other scientists (this is how science works). – Others included:

• Erwin Chargaff• Rosalind Franklin (whose x-rays were key data in the building of

the Watson/Crick DNA model).• Alfred Hershey and Martha Chase (working with bacteriophages).• Linus Pauling (noticed that some proteins were a helix)• And Maurice Wilkins – shared the Nobel Prize with Watson and

Crick.

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X-Ray Evidence

• Franklin and Wilkins used x-ray crystallography to study DNA. – When hit with x-rays, DNA diffracts the x-rays in a

pattern that can be captured on film. – Franklin’s x-ray photographs showed an X

surrounded by a circle.– This gave Watson and Crick the idea that DNA was

a double helix.

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X-Ray Evidence

This is Franklin’s x-ray photograph with which Watson and Crick worked. They created wood and metal models to figure out the DNA’s structure.

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Double Helix Model

• Watson and Crick placed the sugar-phosphate backbones on the outside and the bases inside (the rungs).

• The reason DNA is a helix is because A and G are about twice the width of C and T. This causes the twisting.

• A and G are double ring Purines while C and T are single ring Pyrimidines.

• When they put A and T together and C and G together, the helix structure came together in the double helix model.

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Double Helix Model

Note the role of hydrogen and covalent bonds to hold the molecule together.

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DNA Structure

• Alternating sugars and phosphates form the sides of a double helix.

• The nucleotides of a single strand are held together by covalent bonds.

• The DNA double helix is held together by hydrogen bonds between the bases in the middle (the bases are the rungs that hold the two single strands together).

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Base Pairing Rules

• Thymine (T) always pairs with Adenine (A)

• Cytosine (C) always pairs with Guanine (G).

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Base Pairing Rules

• The pairings are centered on the sizes of the bases, note the double versus single rings, and the ability of the bases to form hydrogen bonds with each other.

• Of note, A and T form two hydrogen bonds while C and G form three. – Which do you think will be stronger and more

difficult (relatively speaking) to break?

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Question

• If a sequence of bases on one DNA strand is CTGCTA then what would the sequence be on the corresponding, other, strand?