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DNADNA• DNA DNA is often

called the blueprint of life..

• In simple terms, DNA contains the instructions for making proteins within the cell.

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Watson & Crick’s Model

Watson and Crick were the first scientists to model DNA in 1953.

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Why do we study DNADNA?

We study DNA for many reasons, e.g.,

• its central importance to all life on Earth,

• medical benefits such as cures for diseases,

• better food crops.

What is DNA?• DNA (deoxyribonucleic acid)

carries the genetic code for all organisms

• In Eukaryotes DNA is found in the nucleus of the cell.

• All organisms contain DNA or RNA • DNA is a Nucleic Acid made up of

nucelotides.

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DNA StructureDNA Structure• Nucleotides include:Nucleotides include:

1. phosphate group1. phosphate group2. pentose sugar (5-carbon)2. pentose sugar (5-carbon)3. nitrogenous bases:3. nitrogenous bases:

adenine (A)adenine (A)thymine (T) DNA onlythymine (T) DNA onlycytosine (C)cytosine (C)guanine (G)guanine (G)

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NucleotideNucleotide

OO=P-O O

PhosphatePhosphate GroupGroup

NNitrogenous baseNitrogenous base (A, G, C, or T)(A, G, C, or T)

CH2

O

C1C4

C3 C2

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SugarSugar(deoxyribose)(deoxyribose)

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Chromosomes and DNAChromosomes and DNA

• Our genes are on our chromosomes.

• Chromosomes are made up of a chemical called DNA.

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The Shape of the The Shape of the MoleculeMolecule

•DNA is a very long polymer.

•The basic shape is like a twisted ladder or zipper.

•This is 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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DNA - double helixDNA - double helix

P

P

P

O

O

O

1

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4

5

5

3

3

5

P

P

PO

O

O

1

2 3

4

5

5

3

5

3

G C

T A

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One Strand of DNAOne Strand of DNA• The backbone

of the molecule 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 together with its phosphate and

base make a nucleotide.

Nitrogenous base

Deoxyribose

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One Strand of DNAOne Strand of DNA

• One strand of DNA is a polymer of nucleotides.

• One strand of DNA has many millions of nucleotides.

nucleotide

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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 pyrimidinesare pyrimidines

• Thymine and cytosine each have one Thymine and cytosine each have one ring of carbon and nitrogen atoms.ring of carbon and nitrogen atoms.

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 purinesare purines

• AdenineAdenine and and guanineguanine each have each have two rings of carbon and two rings of carbon and nitrogen atoms.nitrogen atoms.

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• Remember,

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

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

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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• 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

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Hydrogen Bonds, 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

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 and Thymine always join together

A T

• Cytosine and Guanine always join together

C 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.

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