Chapter 16
The Molecular Basis of
Inheritance
Question?
• Traits are inherited on chromosomes, but
what in the chromosomes is the genetic
material?
• Two possibilities:
– Protein
– DNA
Qualifications
• Protein:
– very complex.
– high specificity of function.
• DNA:
– simple.
– not much known about it (early 1900’s).
Know For Testing:
• Name(s) of experimenters
• Outline of the experiment
• Result of the experiment and the importance
of the result
Griffith - 1928
• Pneumonia in mice.
• Two strains:
– S - pathogenic
– R - harmless
How does information resulting from Griffith's experiments with Streptococcus
pneumoniae support the idea that a heritable material (the identity of which was
unknown in 1928) transformed living, nonpathogenic "R" bacteria into pathogenic
"S" bacteria?
Result
• Something turned the R cells into S cells.
• Transformation - the assimilation of external
genetic material by a cell.
Problem
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Griffith used heat.
Heat denatures proteins.
So could proteins be the genetic material?
DNA - heat stable.
Griffith’s results contrary to accepted views.
Avery, McCarty and MacLeod - 1944
• Repeated Griffith’s experiments, but added
specific fractions of S cells.
• Result - only DNA transformed R cells into S
cells.
• Result - not believed.
Hershey- Chase 1952
• Genetic information of a virus or phage.
• Phage - virus that attacks bacteria and
reprograms host to produce more viruses.
Bacteria with Phages
Phage Components
• Two main chemicals:
– Protein
– DNA
• Which material is transferred to the host?
Used Tracers for Experiment
• Protein - CHONS, can trace with 35S.
• DNA - CHONP, can trace with 32P.
• Used phages labeled with one tracer or the
other and looked to see which tracer entered
the bacteria cells.
Compare the design of the Hershey-Chase experiment 1 to experiment 2.
Why use radioactive S in experiment 1 and radioactive P in experiment 2?
Discuss the results and conclusions of these experiments.
Based on the Hershey-Chase experiments, is it reasonable to assume that Griffith’s
“transforming factor” was DNA, not protein? Why or why not?
Result
• DNA enters the host cell, but the protein did
not.
• Therefore: DNA is the genetic material.
Picture Proof
Chargaff - 1947
• Studied the chemical composition of DNA.
• Found that the nucleotides were found in
certain ratios.
Chargaff’s Rule
• A=T
• G=C
• Example: in humans,
A = 30.9%
T = 29.4%
G = 19.9%
C = 19.8%
Why?
• Not known until Watson and Crick worked out
the structure of DNA.
Inquiry Questions
• How did James Watson
and Francis Crick know
what they knew?
• How was their
discovery based on a
great deal of work by
many scientists?
• What is the structure of
DNA, and how might its
structure reveal a
possible “copying
mechanism”?
Watson and Crick - 1953
• Used X-ray crystallography data (from Rosalind
Franklin)
• Used model building.
• Result - Double Helix Model of DNA structure.
(One page paper, 1953).
Rosalind Franklin
Book & Movies
• “The Double Helix” by James Watson- His
account of the discovery of the shape of DNA
• “Rosalind Franklin: The Dark Lady of DNA”
Biography by Brenda Maddox
• Movie – The Double Helix
DNA Composition
• Deoxyribose Sugar (5-C)
• Phosphate
• Nitrogen Bases:
– Purines
– Pyrimidines
DNA Backbone
• Polymer of sugar-phosphate.
• 2 backbones present.
Nitrogen Bases
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Bridge the backbones together.
Purine + Pyrimidine = 3 rings.
Constant distance between the 2 backbones.
Held together by H-bonds.
Chargaff’s Rule
• Explained by double helix model.
• A = T, 3 ring distance.
• G = C, 3 ring distance.
Watson and Crick
• Published a second paper (1954) that
speculated on the way DNA replicates.
• Proof of replication given by others.
Replication
• The process of making more DNA from DNA.
• Problem: when cells replicate, the genome
must be copied exactly.
• How is this done?
Models for DNA Replication
• Conservative - one old strand, one new strand.
• Semiconservative - each strand is 1/2 old, 1/2
new.
• Dispersive - strands are mixtures of old and
new.
Replication Models
Meselson – Stahl late 1950’s
• Grew bacteria on two isotopes of N.
• Started on 15N, switched to 14N.
• Looked at weight of DNA after one, then 2
rounds of replication.
Results
• Confirmed the Semiconservative Model of
DNA replication.
Replication - Preview
• DNA splits by breaking the H-bonds between
the backbones.
• Then DNA builds the missing backbone using
the bases on the old backbone as a template.
Origins of Replication
• Specific sites on the DNA molecule that starts
replication.
• Recognized by a specific DNA base sequence.
Prokaryotic
• Circular DNA.
• 1 origin site.
• Replication runs in both directions from the
origin site.
Eukaryotic Cells
• Many origin sites.
• Replication bubbles fuse to form new DNA
strands.
DNA Elongation
• By DNA Polymerases such as DNA pol III
• Adds DNA triphosphate monomers to the
growing replication strand.
• Matches A to T and G to C.
Energy for Replication
• From the triphosphate monomers.
• Loses two phosphates as each monomer is
added.
Problem of Antiparallel DNA
• The two DNA strands run antiparallel to each
other.
• DNA can only elongate in the 5’--> 3’ direction.
Leading Strand
• Continuous replication toward the replication
fork in the 5’-->3’ direction.
Priming
• DNA polymerase III cannot initiate DNA
synthesis.
• Nucleotides can be added only to an existing
chain called a Primer.
Primer
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Made of RNA.
10 nucleotides long.
Added to DNA by an enzyme called Primase.
DNA is then added to the RNA primer.
Priming
• A primer is needed for each DNA elongation
site.
Lagging Strand
• Discontinuous synthesis away from the
replication fork.
• Replicated in short segments as more
template becomes opened up.
Okazaki Fragments
• Short segments (100-200 bases) that are
made on the lagging strand.
• All Okazaki fragments must be primed.
• RNA primer is removed after DNA is added.
Enzymes
• DNA polymerase I - replaces RNA primers with
DNA nucleotides.
• DNA Ligase - joins all DNA fragments together.
Other Proteins in Replication
• Topoisomerase – relieves strain ahead of
replication forks.
• Helicase - unwinds the DNA double helix.
• Single-Strand Binding Proteins - help hold the
DNA strands apart.
Video
• http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535
::535::/sites/dl/free/0072437316/120076/mic
ro04.swf::DNA%20Replication%20Fork
Video
• http://www.youtube.com/watch?v=teV62zrm
2P0
DNA Replication Error Rate
• 1 in 1 billion base pairs.
• About 3 mistakes in our DNA each time it’s
replicated.
Reasons for Accuracy
• DNA pol III self-checks and corrects
mismatches.
• DNA Repair Enzymes - a family of enzymes
that checks and corrects DNA.
DNA Repair
• Over 130 different DNA repair enzymes
known.
• Failure to repair may lead to Cancer or other
health problems.
Example:
• Xeroderma Pigmentosum -Genetic condition
where a DNA repair enzyme doesn’t work.
• UV light causes damage, which can lead to
cancer.
Xeroderma Pigmentosum
Cancer
Protected from UV
Thymine Dimers
• T-T binding from side to side causing a bubble
in DNA backbone.
• Often caused by UV light.
Excision Repair
• Cuts out the damaged DNA.
• DNA Polymerase fills in the excised area with
new bases.
• DNA Ligase seals the backbone.
Problem - ends of DNA
• DNA Polymerase can only add nucleuotides in
the 5’--->3’ direction.
• It can’t complete the ends of the DNA strand.
Result
• DNA gets shorter and shorter with each round
of replication.
Telomeres
• Repeating units of TTAGGG (100- 1000 X) at
the end of the DNA strand (chromosome)
• Protects DNA from unwinding and sticking
together.
• Telomeres shorten with each DNA replication.
Telomeres
Telomeres
• Serve as a “clock” to count how many times
DNA has replicated.
• When the telomeres are too short, the cell
dies by apoptosis.
Implication
• Telomeres are involved with the aging process.
• Limits how many times a cell line can divide.
Telomerase
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Enzyme that uses RNA to rebuild telomeres.
Can make cells “immortal”.
Found in cancer cells.
Found in germ cells.
Limited activity in active cells such as skin cells
Comment
• Control of Telomerase may stop cancer, or
extend the life span.
Chromatin Packing
1. Nucleosomes
2. 30-nm Chromatin Fibers
3. Looped Domains
4. Chromosomes
We will Focus on #1 & 4
Nucleosomes
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"Beads on a String”.
DNA wound on a protein core.
Packaging for DNA.
Controls gene reading
Protein Core
• Two molecules of four types of Histone
proteins.
• H1- 5th type of Histone protein attaches the
DNA to the outside of the core.
Chromosomes
• Large units of DNA.
• Similar to "Chapters" in the Book of Life.
Summary
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Know the Scientists and their experiments.
Why DNA is an excellent genetic material.
How DNA replicates.
Problems in replication.
Chromatin packing