Download Biology 102 Lecture 11: DNA

11/1/2015
DNA and Information Storage
Biology 102
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DNA stores a vast amount of information in a
cell
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By far the most compact, efficient method
known
Lecture 11: DNA
Cell Physiology
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Virtually all cell function is a result of proteins
and their interactions
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Information for every protein is stored in DNA
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Used to be thought that DNA was too simple to
be important
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Two experiments disproved this…
DNA
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Change was termed transformation
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1944 – proved that the transforming agent is
DNA
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Results from bacteria’s ability to take up DNA
from environment and replicate it
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We now exploit this to generate DNA for
experiments
DNA
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Virus = protein coat with RNA or DNA inside
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DNA contains phosphorus
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Grow viruses in radioactive
phosphorus = viruses with
radioactive DNA
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Protein contains sulfur
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Grow viruses in radioactive
sulfur = viruses with
radioactive protein coat
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DNA
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Proved that only DNA is transferred from virus
to infected cell
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Despite this, new viruses were made in infected
cells that contained both DNA and protein coats
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Protein is not required for information transfer
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DNA must be
Structure of DNA
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Considered one of the most significant
discoveries in the history of science
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Credited to James Watson and Francis Crick in
1953
Structure of DNA
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Their discovery was only possible because of
X-ray reflection images by Rosalind Franklin
(and some unauthorized access)
Structure of DNA
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These images led to the discovery of the doubledoublehelical structure of DNA
Nucleotides
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Recall: DNA strands are long chains of
nucleotide monomers
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All DNA nucleotides contain…
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Phosphate
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Deoxyribose (sugar)
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Nitrogenous base (1 of 5)
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Nucleotide Bases
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DNA = A G C T
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RNA = A G C U
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Gout
Adenine nucleotide
Structure of Nucleic Acids
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The phosphate of one nucleotide binds to the
sugar of another nucleotide
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Structure of DNA
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RNA is singlesingle-stranded
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DNA made of two antianti-
Sugar--phosphate backbone
Sugar
Bases project out at right angle
parallel strands
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Complimentary basebasepairing with hydrogen bonds
holds the strands together
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Structure of DNA
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Twisted into a double helix
Structure of DNA
Only purine:pyramidine pairs fit inside the
double helix
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Structure of DNA
Can A Bind to A? NO!!
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Hydrogen bonds have to line up too
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Number of hydrogen bonds? None
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Let’s see how some combinations might bind to
each other
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Width of base pair? Too wide
Can C Bind to A? NO!!
Can A Bind to T? YES!!
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Number of hydrogen bonds? None
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Number of hydrogen bonds? 2
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Width of base pair? Correct
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Width of base pair? Correct
Can G Bind to G? NO!!
Can G Bind to C? YES!!
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Number of hydrogen bonds? None
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Number of hydrogen bonds? 3
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Width of base pair? Too wide
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Width of base pair? Correct
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Can T Bind to T? NO!!
Structure of DNA
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Number of hydrogen bonds? 2?
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Shapes have to complimentary
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Width of base pair? Too narrow
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Hydrogen bonds have to line up
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Means only 2 combinations are possible
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A binds to T
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C binds to G
Structure of DNA
Reconstruct The Missing Strand
(Or DNA Matchmaking)
Reconstruct The Missing Strand
(Or DNA Matchmaking)
DNA Organization
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DNA strand is very long and delicate
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PRECIOUS – must be protected
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Associated with organizing proteins called
histones
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Histones + DNA = chromatin
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Subdivided into smaller portions called
chromosomes
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DNA Organization
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Most human cells have 46 chromosomes
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If all were of equal length, each would contain 65
million base pairs
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Actual length varies considerably
DNA Organization
Human Genome Project: 19901990-2003
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Each chromosome carries information to
synthesize several proteins
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Massive international project to map the human
genome
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Portion of DNA that codes for a single protein is
called a gene
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25,000 genes
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Physical location of a gene on a chromosome is
called a locus
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3 billion base pairs
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2-3% code for proteins (mostly the same)
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The rest = ? (DNA “fingerprint”)
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