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Chapter 11

Chapter 11 Study Guide

Biological Sciences
Course Code
Mark Fitzpatrick

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Chapter 11: DNA and Its Role in Heredity
11.1 What is the evidence that the Gene is DNA?
x By the 1920s, it was known that chromosomes consisted of DNA and proteins
x A new dye provided circumstantial evidence that DNA was the genetic material:
o It was in the right place
o It varied among species
o It was present in the right amount
x The convincing demonstration that DNA is the genetic material came from two sets of
The Transforming Principle
x Frederick Griffith was working with two strains of Streptococcus pneumonia and was trying
to develop a vaccine for it
x He was working with two strains of this bacteria: The S-strain (had a protective capsule)
which was virulent, and the R-]v~olZ}]ÀµovÁv[À]µov
x He found that after destroying the S-strain with heat, and then introducing R-strain to the
pool, the R-strain turned virulent
x Proved that there is a substance that causes a heritable change in R-strain
What was the substance?
x Oswald Avery
x His experiment consisted of treating different samples of heat-killed S-strain virus with
enzymes that destroy either RNA, Proteins, or DNA
x And only when the DNA was destroyed was the transforming principle lost
x Had little impact early on since bacterial genetics was a very new field, and people assumed
DNA lacked the complexity of proteins
Hershey-Chase Experiment
x Determined whether DNA or protein is the genetic material using bacteriophage T2 virus
x One group had a S-35 marked protein coat
x And another had the DNA marked with P-32
x Both groups were introduced into separate bacteria cultures
x After a while, the cultures were centrifuged, and the results showed that the protein coat
stayed outside the bacteria, while the DNA was inside the bacteria
x This proved that DNA was the hereditary material as viruses used it to reproduce
themselves via their hosts

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x The genetic transformation of eukaryotic cells by DNA can be demonstrated
x The key is to use a genetic marker, a gene whose presence in the recipient cell confers an
observable phenotype
11.2 What is the structure of DNA?
x One crucial piece of determining the structure of DNA came from X-ray crystallography
x A purified substance can be made to form crystals and the position of atoms in this crystal can
be inferred by the pattern of diffraction of X-rays passed through it
x Maurice Wilkins prepared a sample containing very uniformly oriented DNA fibres which were
used by Rosalind Franklin to produce cystallographs that suggested a spiral or helical molecule
x In 1950, Erwin Chargaff found in the DNA from many different species:
o Amount of A = Amount of T; C = G
o µvv}(µ]vAµvv}(Ç]u]]v~ZP(([Zµo
x Model Building the assembly of three-dimensional representations by possible molecular
structures using known dimensions and bond angles (originally used by Linus Pauling)
x Francis Crick and James Watson built the structure of DNA after compiling all the available data
x Four key features define:
o Double-stranded helix
Sugar-phosphate backbones of the polynucleotide chains coil around the
outside of the helix
Held together by complementary base pairing
The base pairs are flat and their stacking in the center of the molecule is
stabilized by hydrophobic interactions contributing to the overall stability of the
double helix
o Right-handed
o Antiparallel
Direction of strand is determined by the sugar-phosphate bonds
Phosphate groups connect to the 3 C of one sugar and the 5 C of the next sugar
At one end of the chain t (ñ[Z}ZP}µVZ}Zv(ï[
o Outer edges of nitrogenous bases are exposed in the major and minor groove
The C=O group in thymine, the N-group in adenine, and others offer hydrogen
bonding sites
Key to DNA-protein interactions in replication and gene expression
x Function of DNA:
o Storage of genetic material
o Susceptible to mutation t thus evolution
o Replication during cell division
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