1 2 chromosomes and dna chromosomes are made up of a chemical called dna. they contain genes
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Chromosomes and DNAChromosomes and DNA
• Chromosomes are made up of a chemical called DNA.
• They contain genes.
Genes
• Instructions to build proteins.– Examples: gene for hitchhiker’s
thumb, widow’s peak, tongue-rolling…
• Humans have over 30,000 genes encoded in their genome.
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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.
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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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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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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.
http://www.youtube.com/watch?v=4LlBAVhhd-8
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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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Discovery of DNA Discovery of DNA StructureStructure
• Erwin Chargaff showed the amounts of the four bases on DNA (A,T,C,G)
• In a body or somatic cell: A = 30.3% T = 30.3% G = 19.5% C = 19.9%
Adapted from cmassengale
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Chargaff’s Rule:Chargaff’s Rule:
• Adenine and Thymine always join together
A T
• Cytosine and Guanine always join together
C G
T
A
G
C
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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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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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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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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