Download Chapter 7 I. DNA Structure DNA

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Chapter 7
I. DNA Structure
DNA
• DNA = Deoxyribonucleic acid
• Provides instructions to the cell.
• Trait = An inherited characteristic
(physical, behavioral, chemical)
– Examples: height, weight, strength,
metabolism, personality….
• DNA is the genetic material which
codes for traits.
• Heredity = The passing of traits
from one generation to the next.
• DNA controls heredity.
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Discovery of DNA
• For many years scientists
believed the genetic material
to be proteins.
• 1928 – Frederick Griffith
– Studied a type of bacteria
which caused pneumonia in
mammals.
– Transformation  Harmless
bacteria changed into deadly
bacteria.
– What caused the
transformation?
Discovery of DNA
• 1944 – Avery and McCarty
– Repeated Griffith’s experiment with some changes.
– Treated the heat-killed bacteria with enzymes which
destroyed lipids, proteins and carbohydrates 
transformation still occurred.
– Treated the heat killed bacteria with enzymes which
destroyed RNA  transformation still occurred.
– Treated the heat killed bacteria with enzymes which
destroyed DNA  Transformation did not occur!!
– Conclusion  DNA is the transforming material!
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Discovery of DNA
• 1952 – Alfred Hershey
and Martha Chase
• Bacteriophages –
Viruses which infect
bacteria.
• 35S radioactively labels
the protein coat of the
virus.
• 32P radioactively labels
the DNA of the virus.
• Only 32P found inside of
the bacteria.
• Proves conclusively
that DNA is the
genetic material.
Discovery of the Structure of DNA
• 1950’s Rosalind
Franklin
– Extracted DNA and
took pictures of it
using a beam of xrays.
– The x-rays scattered
producing a pattern
which provided clues
to the 3-D structure
of DNA.
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Discovery of the Structure of DNA
• Francis Crick – A British Physicist
• James Watson – An American Biochemist
• Both worked to together in Cambridge
England trying to discover the 3-D structure of
DNA.
• Watson visited Franklin’s colleague Maurice
Wilkins at King’s College in London and
viewed Franklin’s photographs.
• Watson was familiar with interpreting these
photos and recognized the helical shape of
the DNA.
Discovery of the Structure of DNA
• Watson and Crick built models of DNA and
matched it to the patterns in the x-ray
photograph.
James Watson
(born 1928)
Francis Crick
(1916 - 2004)
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The Structure of DNA
• The monomer of DNA is a
nucleotide.
• A nucleotide consists of
– Phosphate group
– Deoxyribose (a sugar)
– Nitogenous base
• There are two sides to a
nucleotide.
– The phosphate group is
attached to the 5' carbon.
– A hydroxyl group (-OH) is
attached to the 3' carbon.
The Structure of DNA
• The 5’ side (phosphate) of
one nucleotide joins
together with the 3’
(Hydroxyl) side of another
nucleotide
• Nucleotides join together by
dehydration synthesis.
• A polymer of nucleotides
make up a molecule of
DNA
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The Structure of DNA
• There are two strands of
DNA in a DNA molecule.
• The sides of the molecule
consist of alternating sugar
and phosphate.
– Called a sugar phosphate
backbone.
• The center of the molecule
contains the nitrogenous
bases.
The Structure of DNA
• Base Pairing - The
nitrogenous bases hydrogen
bond to each other.
• There are two types of
nitrogenous bases.
– Purines
• Adenine(A) and Guanine (G)
– Pyrimidines
• Thymine (T) and Cytosine ( C )
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The Structure of DNA
• Base Pairing Rules - A
Purine bonds to a
Pyrimidine
– Adenine +Thymine (Two
hydrogen bonds)
– Guanine +Cyotosine (Three
hydrogen bonds)
The Structure of DNA
• Anti-parallel - The sugar
phosphate backbones of
DNA run in opposite
directions
• One side runs 5’ to 3’
• The other side runs 3’ to 5’
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The structure of DNA
• A DNA molecule is
shaped like a double
helix.
• The nitrogenous bases
bind together on the
inside of the the double
helix following the base
pairing rules.
Watson and Crick’s Discovery
• Watson and Crick
discovered that DNA is
shaped like a double
helix
• Watson and Crick
discovered that the
nitrogenous bases are on
the inside of the double
helix.
• Watson and Crick
discovered that
– Adenine binds to Thymine
– Guanine binds to
Cytosine.
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Genes and Chromosomes
• Gene - A sequence of nitrogenous bases which
code for a trait.
• A DNA molecule contains many genes.
• Each gene has a unique sequence of bases.
Genes and Chromosomes
• Each DNA molecule
makes up a
chromosome
• Chromosome = A
strand of DNA
• Each cell in our body
contains 23 pairs of
chromosomes = 46 in
total.
• Each chromosome
contains many genes
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Chromosome
Genes and Chromosomes
• DNA is wrapped around
proteins called histones.
• Histones enable DNA to fit
inside the nucleus.
DNA • The combination of DNA and a
histone is called a
nucleosome.
Histone• DNA and the histones coil up
to form a chromosome.
Nucleosome
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