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Molecular Basis for Inheritance
Campbell Chapter 16
Discovery of DNA
O To understand genetics discover the
chemical nature of a gene
O Identify the molecule that carries
genetic information understand how
genetics works
O Frederick Griffith (1928) wanted to
know why people got pneumonia but
discovered the chemical nature of a
gene
Griffith’s ExperimentsO Isolated two different strains of bacteria
O Disease-causing bacteria pneumonia (S strain)
O Harmless bacteria (R-strain)
O Inject mice with S strain developed pneumonia and died
O Inject mice with harmless R-strain stayed healthy
O What killed the mice?
Griffith’s Next Experiments
O Heated and killed the S-strain cells and injected mice.
Mice survived the cause of pneumonia was not a toxin
from the bacteria
O Mixed the heat-killed, S-strain bacteria with live, harmless
R-strain bacteria and injected mice. Injected mice
developed pneumonia and died
O How could that happen if the S-strain cells were dead?
Harmless
bacteria (R strain)
Bacterial TransformationO Somehow the heat-killed bacteria passed their
disease-causing ability to the harmless bacteria
O Transformation – process in which one strain of
bacteria takes as piece of DNA and integrates it in
its genome
O Ability to cause disease was inherited by the
offspring of the transformed bacteria
concluded that the
transforming factor
had to be a gene
The Molecular Cause of Transformation
O Which molecule in the heat-killed bacteria was most important for transformation?
O Canadian biologist Oswald Avery and his team destroyed all parts of the bacterium except the DNA transformation still occurred
O repeated the experiment but destroyed DNA transformation did not occur
O by observing bacterial transformation, Avery and other scientist discovered that the nucleic acid DNA stores and and transmits genetic info from one generation to generation
Bacteriophages
O Bacteriophage - type of virus that
infects bacteria
O When a bacteriophage enters a
bacterium, it attaches to the surface of
the bacterial cell and injects its DNA
into it
O The bacteriophage genes produce
many new bacteriophages, which
gradually destroys and splits open the
bacterium
O Hundreds of new viruses burst out
The Hershey-Chase ExperimentO American scientists Alfred Hershey and Martha Chase studied
a bacteriophage to determine which part of the bacteriophage—the protein coat or the DNA core—entered the bacterial cell
O Grew viruses containing radioactive isotopes (markers) enabling the scientists to tell if the protein coat or the DNA core carried the genetic information of the virus
O Nearly all the radioactivity in the bacteria was from the marker found in DNA
O DNA stores and transmits genetic information from one generation of bacteria to the generation
Solving the Structure of DNA
O In the 1950s, British scientist Rosalind Franklin
used a technique called X-ray diffraction to get
information about the structure of the DNA
molecule.
The Work of Watson and Crick
O At the same time, James Watson, an American biologist, and Francis Crick, a British physicist, were also trying to understand the structure of DNA.
O Early in 1953, Watson was shown a copy of Franklin’s X-ray pattern.
O Watson and Crick’s breakthrough model of DNA was a double helix, in which two strands were wound around each other.
Summary
O Used to think that proteins not DNA were the molecular basis of inheritance
O Griffith (1927) Bacteria have the ability to transform harmless cells in to virulent ones by transferring some genetic factor from one bacteria cell to another
O Avery, MacLeod and McCarty (1944) provided direct experimental evidence that DNA, not protein, was the genetic material
O Hershey & Chase (1952) backed up that DNA was genetic material
Summary
O Franklin (1950-1953) working in Wilkins lab, took
X-ray crystallography picture of DNA, showed
helical shape
O Died before Nobel Prize
O Watson and Crick (1953) proposed double helix
structure of DNA
O Received Nobel in 1962 with Wilkins
O Meselson & Stahl (1958) proved that DNA
replicates in a semiconservative fashion as
predicted by Crick
DNA Timeline
How Long is DNA?
Complete 1-8
The Components of DNA
O Nucleotides - building blocks (monomers) of
nucleic acid
O DNA (deoxyribonucleic acid) - nucleic acid
made up of nucleotides joined into long
strands or chains by covalent bonds
(polymer)
Nucleic Acids and Nucleotides
O Three parts of a nucleotide :
O 5-carbon sugar called deoxyribose
O phosphate group
O nitrogenous base
Nitrogenous Bases and Covalent Bonds
O The nucleotides in a strand of DNA are joined by
covalent bonds formed between their sugar and
phosphate groups.
Nitrogenous Bases and Covalent Bonds
O DNA has four kinds of nitrogenous bases:
O Adenine (A)
O Guanine (G)
O Cytosine (C)
O Thymine (T)
Nitrogenous Bases
O 2 Types of Nitrogenous Bases
O Purines
O Adenine and Guanine
O 2 Carbon rings
O Pyrimidines
O Cytosine and Thymine
O Single Carbon Ring
Nitrogenous Bases and Covalent Bonds
O The nitrogenous bases stick
out sideways from the
nucleotide chain.
O The nucleotides can be joined
together in any order, meaning
that any sequence of bases is
possible
Complete 9-10
Chargaff ’s Rules
O Erwin Chargaff discovered that the percentages of
adenine [A] and thymine [T] bases are almost
equal in any sample of DNA. The same thing is
true for the other two nucleotides, guanine [G] and
cytosine [C].
O The observation that:
O [A] = [T]
O [G] = [C]
O became known as “Chargaff’s rules.”
Chargaff's Rule Practice#16 on POGIL
Hydrogen Bonding
O Hydrogen bonds form
between bases, holding
the two DNA strands
together.
O Which are stronger,
covalent or hydrogen
bonds?
O Why is this important?
Base P
airing
Structure of DNA
O DNA is a double helix: Two strands run in opposite directions
O One strand runs 5’ to 3’, other runs 3’ to 5’
O Carbons in Deoxyribose are numbered 1-5
O Histones: proteins that coil DNA into chromatin
O Packed and unpacked in nucleus as needed
Numbering Carbons
DNA ReplicationBefore a cell divides, its DNA must
first be copied. How might the
double-helix structure of DNA
make that possible?
Flashback…
O During S phase of the cell cycle, DNA
is replicated
O A new set of DNA must be made for
each new cell that is formed
O DNA is also passed from parent to
offspring
O Need a way to copy DNA so this can
happen
Complete 11-15
Semiconservative Replication
O Watson and Crick hypothesized that there must be template involved in the copying of DNAO “It has not escaped our notice that the
specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Watson & Crick
O Based on complementary base pairs
O If you know the sequence of one strand you can figure out the sequence of the other
O How can we do this?
Copying the Code
O Each strand has all the information needed to reconstruct the other half by base pairing
O Strands are said to be complementary
O As each new strand forms, new bases are added following the rules of base pairing
Template Mechanism
O Watson and Crick’s theory proven in 1950’s
O During DNA Replication:
O Two strands split
O Each strand acts as a template for the creation of a new strand
O One copy becomes two
O Each piece of DNA has one old strand and one new daughter strand
Summary
DNA Replication
O Eukaryotic chromosomes are SUPER LONG
O Replication may begin at dozens or even hundreds
of places on the DNA molecule, proceeding in both
directions until each chromosome is completely
copied
Complete 17-20
Role of Enzymes
O More than a dozen enzymes involved
O Nucleotides are brought in and paired
with complimentary nucleotides on
the parent strand
O Process is super fast and accurate,
only 1 error every billion nucleotides
Replication of the Double Helix
O Replication begins at specific sites, Origins of
Replication
O Copying moves outward (in both directions)
creating bubbles
O Having multiple bubbles lets the replication
happen faster
O Each end of the bubble is a replication fork
O Eventually all the bubbles fuse
Beginning Replication
O One of our cells can replicate its entire DNA in a
few hours
O Helicase, an enzyme unzips the DNA at the
replication fork
O Single strand binding proteins hold strands apart
like scaffolding
O Primase, an enzyme, makes an RNA primer to
guide DNA Polymerase
DNA Polymerase
O DNA Polymerase: an enzyme which catalyzes the
antiparallel elongation of the new DNA strands
O Builds new strand 5’ to 3’ direction
O In humans 50 nucleotides per second
O DNA Polymerase does not initiate synthesis
O Only adds nucleotides to 3’ end
O Carries out mismatch repair, proofreading
O DNA nuclease removes damaged regions
Problem with ForksO Leading strand: towards the replication fork
O One continuous string, unbroken
O Lagging Strand: away from the fork
O Series of segments: Okazaki fragments
O Joined by DNA ligase
Excision Repair
O DNA Polymerase can “proofread” DNA to
check for errors.
O Nuclease will cut out a short piece containing
the mutation
O DNA Polymerase will then replace that piece
with new DNA nucleotides
O Ligase seals the DNA
Telomeres
O Each time DNA replicates some nucleotides at the end of chromosomes are lost
O Lost genes?
O Nonsense sequence to protect against this
O TTAGGG repeats thousands of times
O Ends called TelomeresCreated and maintained by Telomerase
O Shorten after each replication
O Aging?
Lets see it!
O DNA Coiling and Replication
O DNA Replication (other links on left)
O Really good!
O Chargaff’s Ratio