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8/12/2019 Molecular Biology Lecture 1
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BIOL 321 - DNA Structure
Cristofre MartinDepartment of BiochemistrySt. George’s University
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What is the genetic material?
The material responsible for hereditary informationmust have the following characteristics:
1. It must contain, in stable form, the information for
an organism’s cell structure, function, developmentand reproduction.
2. It must replicate accurately so that progeny cellshave the same genetic information as the parentalcell.
3. It must be ca able of chan e.
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DNA was first identified in1868 by Friedrich Miescher, a
Swiss biologist, in the nuclei of pus cells obtained fromdiscarded surgical bandages.He called the substancenuclein , noted the presence of
phosphorous, and separated thesubstance into a basic part(which we now know is DNA)and an acidic part (a class ofacidic proteins that bind to
basic DNA).
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Frederick Griffith (1928):The infection of S. pneumoniae in mice
and the transformation of bacteria.
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Griffiths Experiments showed thatthere was a “transforming principle”from the killed virulent bacteria that
can alter the non-virulent bacteria to become virulent.
Griffith believed that this unknown“transforming principle” was protein.
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Peter J. Russell, iGenetics : Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Transformation is blocked when DNA isdestroyed; therefore DNA is the genetic material.
Oswald Avery (1944)
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Further experiments by Avery and his colleague supported
that the transforming factor was DNA.
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The results of theOswald Avery’s (a
Canadian scientist)experiment suggestedthat DNA was thegenetic material.
At the time we did notknow very much aboutDNA.
Oswald Avery received Nobel Prize for hisresearch.
8/12/2019 Molecular Biology Lecture 1
http://slidepdf.com/reader/full/molecular-biology-lecture-1 9/34Peter J. Russell, iGenetics : Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
In 1953, AlfredHershey and MarthaChase published
experiments using bacteriophage viruses(viruses that attack
bacteria) that furtherdemonstrated that DNAis the genetic material.
Evidence from Bacteriophage
8/12/2019 Molecular Biology Lecture 1
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Lytic life cycle of a virulent phage
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Hershey-Chase experiment demonstrating DNA isgenetic material
By radioactivelylabeling prot eins andnucleic acids (DNA)of bacteriophage itwas furtherdetermined that thehereditary material islocated on DNAmolecules.
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Relative Proportions (%) of Bases in DNAOrganism A T G CHuman 30.9 29.4 19.9 19.8Chicken 28.8 29.2 20.5 21.5Grasshopper 29.3 29.3 20.5 20.7Sea Urchin 32.8 32.1 17.7 17.3Wheat 27.3 27.1 22.7 22.8Yeast 31.3 32.9 18.7 17.1E. coli 24.7 23.6 26.0 25.7
DNA hydrolysis experiments by Erwin Chargaff showedthat all double stranded DNAs contained 50% purine bases(A & G) and 50% pyrimidine bases (C & T).
Chargaff’s Rule:
A = T
C = G
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An unknown piece of DNA contains 35%
adenine. What is the percentage of all othernucleotide in this same piece of DNA?
Answer:
Adenine = 35%Thymine = 35%
Cytosine = 15%Guanine = 15%
TOTAL = 100%
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Peter J. Russell, iGenetics : Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Structures of deoxyribose and ribose, and of thenitrogenous bases in DNA and RNA
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Peter J. Russell, iGenetics : Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
em ca structures o an
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Crystallographer, Dr.
Rosalind Franklin tookthe X-ray image of theDNA molecule thatWatson and Crick usedto deduce the structureof the DNA molecule.
Rosalind Franklin died of cancer in 1958 and thuswas unable to share in the award of the Nobel prizethat was awarded to Watson, Crick, and Wilkins in
1962
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Peter J. Russell, iGenetics : Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Molecular structure of DNAProperties of DNA:
1)The DNA moleculeconsists of two
polynucleotide chains
wound around each otherin a right-handed doublehelix.2) The two chains are
antiparallel. One strandis oriented in a 5’ to 3’way and the other strainis oriented 3’ to 5’.
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Peter J. Russell, iGenetics : Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Molecular structure of DNAProperties of DNA
(cont.):
4) The bases of theopposite strand are
bonded together byhydrogen bonds.
A-T base pairing forms twohydrogen bonds, while G-C
base pairing forms threehydrogen bonds
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Peter J. Russell, iGenetics : Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Structures of the complementary basepairs found in DNA
Properties of DNA(cont.):
5) Specific base pairing iscomplementary
such that A pairswith T, and G pairswith C.
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Properties of DNA (cont.):
5) Specific base pairing is complementary such that A pairswith T, and G pairs with C.
Example 1:
Strand 1: 5’-TATTCCGA -
3’
Strand 2: 3’ -ATAAGGCT - 5’
Example 2:
A DNA strand contains 15% G and 35% T.Therefore, the same strand will contain 15% Cand 35% A.
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Properties of DNA(cont.):
6) The base pairs are0.34 nm apart in the DNAhelix. A complete turntakes 3.4 nm; thereforethere are 10 bases per
turn. The externaldiameter of the helix is2nm.
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Funny DNA trivia:
6 X 10 9 base pairs (bp)/human cell bp are 0.34 X 10 -9 meters (m) apart
so the length of DNA/cell is6 X 10 9 X 0.34 X 10 -9 m = 2 meters
109 =billion10 -9 =one billionth
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Human karyotype
In order for the
extremely largeDNA molecules to
be packaged into a
tiny nucleus, theDNA is organizedinto physicalstructures calledchromosomes.
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Chromosomes are composed ofchromatin.
Chromatin is the complex of DNA andchromosomal proteins.
The most important chromosomal proteins are called histones.
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Peter J. Russell, iGenetics : Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Nucleosome Structure
Various histone proteins bind witheach other along withDNA to form astructure called thenucleosome. Thenucleosome core is
composed of twomolecules each ofH2A, H2B, H3 andH4 histone proteins.
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Nucleosomes can associate with each other toform a more compact structure that has been
termed, due to its size, the 30 nm fiber.
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Electron
micrographof ametaphase
chromosomedepleted ofhistones
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This figure shows theunraveled chromatin inthe upper inset with thedense protein core. Theindividual filaments areseen in the main
photograph and in thelower inset at higher
magnification.
The many different orders of chromatin packing that give
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The many different orders of chromatin packing that giverise to the highly condensed metaphase chromosome
Two types of chromatin have been defined based onchromosome staining
properties:
Euchromatin - decondensedand transcriptionally active
Heterochromatin - condensedand transcriptionally inactive
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