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The Structure of DNA
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Slide 14

Adenine and Guanine are purines

Adenine and Guanine are purines

Adenine and guanine each have two rings of carbon and nitrogen atoms.

Slide 15

Two Stranded DNA

Two Stranded DNA

Remember, DNA has two strands that fit together something like a zipper.

The teeth are the nitrogenous bases but why do they stick together?

Slide 16

Hydrogen Bonds

Hydrogen Bonds

The bases attract each other because of hydrogen bonds.

Hydrogen bonds are weak but there are millions and millions of them in a single molecule of DNA.

The bonds between cytosine and guanine are shown here with dotted lines

Slide 17

Hydrogen Bonds, cont.

Hydrogen Bonds, cont.

When making hydrogen bonds, cytosine always pairs up with guanine

Adenine always pairs up with thymine

Adenine is bonded to thymine here

Slide 18

Chargraff’s Rule:

Chargraff’s Rule:

Adenine and Thymine always join together

A T

Cytosine and Guanine always join together

C G

Slide 19

DNA by the Numbers

DNA by the Numbers

Each cell has about 2 m of DNA.

The average human has 75 trillion cells.

The average human has enough DNA to go from the earth to the sun more than 400 times.

DNA has a diameter of only 0.000000002 m.

Slide 20

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