Transcript Chapter 16 - HomeworkForYou

MOLECULAR BIOLOGY OF
INHERITANCE:
PART I: HISTORICAL
PERSPECTIVES
Chapter 16
Learning Objectives
1. Summarize the experiments and
conclusions conducted by Griffiths; Avery,
McCarty & MacLeod; Hershey & Chase.
2. Summarize the experimental evidence
of Franklin, Chargaff that were used by
Watson & Crick to propose the structural
model of DNA.
3. Describe the chemical and structural
features of DNA.
Concept 16.1: DNA is the genetic
material. Historical Context.
• DNA was first isolated from cells in 1863.
• Discovery of chromosomes:
• The movement of chromosomes during cell
division was discovered in 1883.
• the presence of chromosomes in reproductive
cells was discovered in 1902.
• chromosomes were the physical basis of
inheritance was discovered in 1903.
• The experimental evidence that genes are
located was published in 1903.
Hereditary Molecule: DNA or Protein?
• The monomers of DNA were
discovered in 1910 by Phoebus
Levine.*
• He thought that DNA was
structural component of
chromosomes and the genes
were thought likely to be made
of the protein component of
chromosomes.
(* Phoebus Levene. (2015, May 20). In Wikipedia,
The Free Encyclopedia. Retrieved 19:17, October
Phoebus Levene
15, 2015, from
https://en.wikipedia.org/w/index.php?title=Phoebu Image: Wikimedia Commons
s_Levene&oldid=663259196)
Experiments by Frederick Griffith
• The discovery of the genetic
role of DNA began with
research by Frederick
Griffith in 1928.
• Griffith was a bacteriologist
who provided the first
experimental evidence of
genetic transformation,
changing the form and
function of a bacterium.
Frederick Griffith. Image
Wikipedia Commons
Griffith’s Transformation Experiment
• He observed that mixing a
heat-killed pathogenic strain of
bacteria with a living
nonpathogenic strain can
convert some of the living cells
into the pathogenic form.
• He called this phenomenon
transformation, now defined as
a change in genotype and
phenotype.
• Griffith’s referred to a
“transforming principle” as
causing the change, but didn’t
know what it was.
Figure 16.2
Experiments by Avery, MacLeod &
McCarty
• In 1944, Oswald Avery, Maclyn McCarty,
and Colin MacLeod repeated Griffiths’
experiments and announced that the
transforming substance was DNA.
• Their conclusion was based on
experimental evidence that only DNA
worked in transforming harmless bacteria
into pathogenic bacteria.
• Many biologists remained skeptical, mainly
because little was known about DNA.
Experiments by Hershey & Chase
• More evidence for DNA
as the genetic material
came from studies of
viruses that infect
bacteria, called
bacteriophages (or
phages.
• In 1952, Alfred Hershey
and Martha Chase
research with T2 phages
proved that DNA was
Griffith’s transforming
principle.
Alfred Hershey
Image: Wikipedia
Commons
Martha Chase
Image: Wikipedia
Commons
T2 Phage: DNA & Protein
The T2 phage,
consisting almost
entirely of DNA and
protein, attacks E. coli
transforming the cells
into a T2-producing
factory that releases
phages when the cell
ruptures.
Figure 16.3
DNA is transforming principle.
• DNA contains phosphorus, whereas protein does
not.
• DNA was labeled with a radioactive phosphorus;
protein was labeled with a radioactive sulfur.
• The fraction containing the radioactive
phosphorus—DNA—was the only component that
could transform E. coli into phage producing
factories.
• Their results confirmed Avery, MacLeod&
McCarty’s results and confirmed that the DNA of
the phage was Griffith’s transforming principle.
Summary of Hershey & Chase Experiment
Figure 16.4
Erwin Chargaff’s Rules
• By 1947, Erwin Chargaff had
developed a series of rules
based on a survey of DNA
composition in organisms.
• Chargaff found a peculiar
regularity in the ratios of
nucleotide bases, known as
Chargaff’s rules, in all organisms
he studied.
• The number of A is
approximately equal to the
number of T and the number of
G is approximately equal to the
number of C; but A + T did not
equal C + G.
Erwin Chargaff
Image original source:
unknown
Determining the structure of DNA
• After DNA was accepted
as the genetic material,
the challenge was to
determine how its
structure accounts for its
role in heredity.
• Maurice Wilkins and
Rosalind Franklin were
using a technique called
X-ray crystallography to
study molecular
structure.
Maurice Wilkins.
Image: Cold Spring
Harbor Archives
Figure 16.6
Franklin’s X-ray Diffraction of DNA
• X-rays are diffracted as they
pass through crystalized DNA.
• The diffraction pattern can be
used to deduce the threedimensional shape of
molecules.
• An unpublished annual report
summarizing Franklin’s work,
she had concluded that the
sugar-phosphate backbones
were on the outside of the
double helix.
Figure 16.6
DNA Structural Features
• Watson & Crick built models
based on X-rays and chemistry of
DNA.
• Only a pyrimidine-purine pair
produces the 2-nm diameter
indicated by the X-ray data.
• Watson built a model in which the
backbones were antiparallel (their
subunits run in opposite
directions).
• The ladder forms a full turn of the
helix every ten base pairs, or
every 3.4 nm.
DNA Structural Features, cont.
• Watson & Crick determined
that chemical side groups of
the nitrogenous bases form
hydrogen bonds, connecting
the two strands.
• Based on details of their
structure, adenine forms two
hydrogen bonds only with
thymine, and guanine forms
three hydrogen bonds only
with cytosine.
• This finding explained
Chargaff’s rules.
Figure 16.1. James Watson and Francis Crick with their DNA model at the Cavendish
Laboratories in 1953.
Who won Nobel Prizes?
• Griffith—no.
• Avery—nominated, but never won; MacLeod--no,
& McCarty—no.
• Alfred Hershey—yes, but for previous work.
• Martha Chase—no.
• Erwin Chargaff-no, and he was not happy about
it.
• Watson—yes.
• Crick-yes.
• Wilkins—yes—but who remembers?
• Franklin—no (died before award given).