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DNADNA• DNA DNA is the

blueprint of life..• DNA contains

the instructions for making proteins

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Watson & Crick’s Model

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Why do we study DNADNA?

• all life on Earth depends on DNA to pass on genetic traits and run cells

• medical benefits such as cures for diseases,

• better food crops.

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Chromosomes and DNAChromosomes and DNA

•genes are on chromosomes.

•Chromosomes are made of DNA.

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The Shape of the The Shape of the MoleculeMolecule

•DNA is a very long polymer.

•Basic shape is a twisted ladder

•called a double helix.

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The Double Helix The Double Helix MoleculeMolecule

•The DNA double helix has two strands twisted together.

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One Strand of DNAOne Strand of DNA• The backbone

of DNA is alternating phosphates and deoxyribose sugar

• The teeth are nitrogenous bases.

phosphate

deoxyribose

bases

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NucleotidesNucleotides

C C

C

OPhosphate

O

CC

O -P OO

O

O -P OO

O

O -P OO

O One deoxyribose plus phosphate and base make

a nucleotide.

Nitrogenous base

Deoxyribose

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Four nitrogenous basesFour nitrogenous bases

• Cytosine C• Thymine T

• Adenine A

• Guanine G

DNA has four different bases:

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Two Kinds of Bases in DNA

• PyrimidinesPyrimidines are single ring are single ring basesbases..

• PurinesPurines are are double ring double ring bases.bases.

C

C

C

C

N

N

OO

N

CC

CC

N

N

N

N

N

C

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Thymine and Cytosine Thymine and Cytosine are pyrimidines and are pyrimidines and have one ring onlyhave one ring only

C

C

C

C

N

N

OO

N

cytosinecytosine

C

C

C

C

N

N

OO

OO

thyminethymine

C

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Adenine and Guanine Adenine and Guanine are purines and have are purines and have

two ringstwo rings

C

C

C

C

N

N

N

AdenineAdenine N

N

C

C

C

C

C

N

NOO

N

GuanineGuanine N

N

C

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Two Stranded DNATwo Stranded DNA• The 2 DNA fit

together like a zipper.

• Why do the bases stick together?

• Hydrogen bonds!

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C

C

C

C

N

N

OO

N

C

C

C

C

N NOO

N

N

N C

Hydrogen BondsHydrogen Bonds• Bases are attracted

to each other by hydrogen bonds.

• Hydrogen bonds are weak attractive forces between a hydrogen on one strand and a nitrogen or oxygen on the other strand of DNA.

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Hydrogen Bonds, Hydrogen Bonds, cont.• Cytosine pairs

with guanine using 3 H bonds

• Adenine pairs with thymine using

• 2 H bonds

C

C

CC

N

N

N

N

N

C

C

C

C

C

N

N

OO

OO

C

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Chargraff’s Rule:Chargraff’s Rule:

• Adenine with ThymineA T

• Cytosine with GuanineC G

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DNA by the NumbersDNA 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.

The earth is 150 billion mor 93 million miles from the sun.