fig. 16-1. dna – lots of it in a small space dna – a historical perspective 1865 – gregor...
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Fig. 16-1
DNA – lots of it in a small space
DNA – A Historical Perspective
• 1865 – Gregor Mendel – “Father of Heredity”
• 1869 – Johann Miescher (Swiss biochemist) – isolates DNA from WBC
• 1902 – Walter Sutton – American Geneticist – Columbia U. Theory of the Chromosome
• 1928 – Frederick Griffith – British Bacteriologist – discovers transformational factor
• 1944 – Oswald Avery et al. - Canadian-born American physician – shows that the transformational factor was not a protein but DNA
• 1952 – Alfred Hershey & Martha Chase – provide conclusive evidence that DNA is the transformational factor
• 1952 – Rosalind Franklin & Maurice Wilkins – use x-ray diffraction to analyze DNA
• 1953 – James Watson & Francis Crick construct double helix model of DNA
Johannes Friedrich Miescher 1844-1895
In 1869, first to isolate a substance he called nuclein from the nuclei of leucocytes or WBC
Collected these from pus he obtained from bandages at nearby hospitals.
He found that nuclein contained phosphorus and nitrogen, but not sulfur
Walter Sutton (1877-1916)
American geneticist & physician – Columbia University
Boveri-Sutton Chromosome Theory
Made connection – Mendel’s Laws of Heredity could be applied to chromosomes at the cellular level of living organisms
Thomas Hunt Morgan (1866-1945)
American geneticist and embryologist – Columbia U.
Studied the mutations in fruit flies, Drosophilia melanogaster
demonstrated that genes are carried on chromosomes and are the mechanical basis of heredity
Nobel Prize in Physiology or Medicine in 1933
Frederick Griffith 1871 - 1941
What is the transformational factor??? Is it DNA or Protein???
Griffith’s research, working with two strains of a bacterium, one pathogenic and one harmless, addresses this vital question
In 1941, Griffith was killed at work in his London laboratory as a result of an air raid in the London Blitz.
DNA – A Historical Perspective
Griffith and Transformation
1928 – British pathologist was researching
How certain types of bacteria produced pneumonia
He isolated 2 different strains: R which was harmless and S - virulent
Live S-strain kills mouse
Injection of Rough Colonies ( R)
Results in Live Mice
Heat-killed Smooth colonies (S)Result in Live Mice
Heat-Killed S + Live R =Dead Mice
Fig. 16-2
Living S cells (control)
Living R cells (control)
Heat-killed S cells (control)
Mixture of heat-killed S cells and living R cells
Mouse diesMouse dies Mouse healthy Mouse healthy
Living S cells
RESULTS
EXPERIMENT
Oswald Avery and DNA (1944)
Working along with Colin Macleod & Maclyn McCarty
Repeated Griffith’s work with modifications
Which molecule in the heat-killed was the transformational factor?
The components of the Ground up S were isolated, each mixed with R and injected into mice
In 1952, Alfred Hershey and Martha Chase performed experiments showing that DNA is the genetic material of a phage known as T2
To determine the source of genetic material in the phage, they designed an experiment showing that only one of the two components of T2 (DNA or protein) enters an E. coli cell during infection
They concluded that the injected DNA of the phage provides the genetic information
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Animation: Hershey-Chase ExperimentAnimation: Hershey-Chase Experiment
Alfred Hershey and Martha Chase. 1953
Fig. 16-3
Bacterial cell
Phage head
Tail sheath
Tail fiber
DNA
100
nm
Fig. 16-4-1
EXPERIMENT
Phage
DNA
Bacterial cell
Radioactive protein
Radioactive DNA
Batch 1: radioactive sulfur (35S)
Batch 2: radioactive phosphorus (32P)
Fig. 16-4-2
EXPERIMENT
Phage
DNA
Bacterial cell
Radioactive protein
Radioactive DNA
Batch 1: radioactive sulfur (35S)
Batch 2: radioactive phosphorus (32P)
Empty protein shell
Phage DNA
Fig. 16-4-3
EXPERIMENT
Phage
DNA
Bacterial cell
Radioactive protein
Radioactive DNA
Batch 1: radioactive sulfur (35S)
Batch 2: radioactive phosphorus (32P)
Empty protein shell
Phage DNA
Centrifuge
Centrifuge
Pellet
Pellet (bacterial cells and contents)
Radioactivity (phage protein) in liquid
Radioactivity (phage DNA) in pellet
Fig. 16-6
(a) Rosalind Franklin (b) Franklin’s X-ray diffraction photograph of DNA
Erwin Chargaff (1905-2002)and “Chargaff’s Rules”
The bases were not present in equal quantities
They varied from organism to organism.
No matter where DNA came from — yeast, people, or salmon — the number of adenine bases always equaled the number of thymine bases and the number of guanine always equaled the number of cytosine bases.
He published a review of his experiments in 1950, calling the ratios — which came to be known as Chargaff’s Rules
Chargaff’s rules state that in any species there is an equal number of A and T bases, and an equal number of G and C bases
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Chargaff’s Rule
• American biochemist discovers that % of G and C bases are almost equal in any sample of DNA.
• The same thing is true for A and T
• [A]=[T] and [G]=[C]
Fig. 16-8
Cytosine (C)
Adenine (A) Thymine (T)
Guanine (G)
Fig. 16-UN2
Sugar-phosphate backbone
Nitrogenous bases
Hydrogen bond
G
C
A T
G
G
G
A
A
A
T
T
T
C
C
C
Fig. 16-7a
Hydrogen bond 3 end
5 end
3.4 nm
0.34 nm
3 end
5 end
(b) Partial chemical structure(a) Key features of DNA structure
1 nm
Fig. 16-5Sugar–phosphate
backbone
5 end
Nitrogenous
bases
Thymine (T)
Adenine (A)
Cytosine (C)
Guanine (G)
DNA nucleotide
Sugar (deoxyribose)
3 end
Phosphate
Purines•Adenine•Guanine
PurAsGold
Pyrimidines•Cytosine•Thymine•Uracil
PyCUT
Carbon 1 – bonds to nitrogen base
Carbon 3 – bonds to next nucleotide
Carbon 5 – bonds to phosphate group
Fig. 16-UN1
Purine + purine: too wide
Pyrimidine + pyrimidine: too narrow
Purine + pyrimidine: width consistent with X-ray data
Additional Evidence That DNA Is the Genetic Material
• It was known that DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group
• In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next
• This evidence of diversity made DNA a more credible candidate for the genetic material
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Animation: DNA and RNA StructureAnimation: DNA and RNA Structure
Building a Structural Model of DNA: Scientific Inquiry
• After most biologists became convinced that DNA was the genetic material, the challenge was to determine how its structure accounts for its role
• Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure
• Franklin produced a picture of the DNA molecule using this technique
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical
• The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases
• The width suggested that the DNA molecule was made up of two strands, forming a double helix
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Animation: DNA Double HelixAnimation: DNA Double Helix
• Watson and Crick built models of a double helix to conform to the X-rays and chemistry of DNA
• Franklin had concluded that there were two antiparallel sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• At first, Watson and Crick thought the bases paired like with like (A with A, and so on), but such pairings did not result in a uniform width
• Instead, pairing a purine with a pyrimidine resulted in a uniform width consistent with the X-ray
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Watson and Crick reasoned that the pairing was more specific, dictated by the base structures
• They determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C)
• The Watson-Crick model explains Chargaff’s rules: in any organism the amount of A = T, and the amount of G = C
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Concept 16.2: Many proteins work together in DNA replication and repair
• The relationship between structure and function is manifest in the double helix
• Watson and Crick noted that the specific base pairing suggested a possible copying mechanism for genetic material
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 16-9-1
A T
GC
T A
TA
G C
(a) Parent molecule
Fig. 16-9-2
A T
GC
T A
TA
G C
A T
GC
T A
TA
G C
(a) Parent molecule (b) Separation of strands
Fig. 16-9-3
A T
GC
T A
TA
G C
(a) Parent molecule
A T
GC
T A
TA
G C
(c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand
(b) Separation of strands
A T
GC
T A
TA
G C
A T
GC
T A
TA
G C
The Basic Principle: Base Pairing to a Template Strand
• Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication
• In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Animation: DNA Replication OverviewAnimation: DNA Replication Overview
Fig. 17-5Second mRNA base
Fir
st m
RN
A b
ase
(5
en
d o
f co
don
)
Th
ird
mR
NA
bas
e (3
e
nd
of
cod
on)
Enzymes involved in DNA Replication & Transcription
Enzyme FunctionHelicase “molecular zipper” – unwinds double helix;
breaks hydrogen bonds that holds base pairs together
Topoisomerase (gyrase) “molecular swivel”- relieves overwinding stress on DNA strands by working ahead of helicase and breaking, swiveling and rejoining small sections of the DNA molecule
DNA polymerase Using a parent DNA strand, adds free-floating nucleotides (A, T, G, & C’s) covalently to the new strand being constructed.
ligase “molecular glue” – joins fragments of the New DNA strand together
RNA polymerase (used in transcription) Uses one strand of DNA as a template to construct mRNA – adds free-floating nucleotide
Editase Fixes mistakes on DNA molecule
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