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DNA: The Genetic Material
CHAPTER 12
SECTION 1
Discovery of the Genetic Material
 Genetic information is carried on the chromosomes
in eukaryotic cells
 Two main components of chromosomes are DNA
and protein
 Which macromolecules—nucleic acid (DNA) or
proteins—was the source of genetic information
Fredrick Griffith (1928)
 Studied two strains of bacteria Streptococcus
pneumoniae, which causes pneumonia.
 One strain that causes pneumonia is called the
smooth (S) strain. The other is called the rough (R)
strain.
 He found that one strain could be transformed, or
changed into the other form.
 He injected the strains into lab mice and discovered
that the R strain had changed into the S strain which
caused the death of the mice
 When the S bacteria was killed, DNA was released.
Some of the R bacteria incorporated this DNA into
their cells and this changed the bacteria in S bacteria
DNA Structure
 Nucleotides are the subunits of nucleic acids and
consists of a five-carbon sugar, a phosphate group,
and a nitrogenous base.
 The two nucleic acids found in living are DNA and
RNA.
 DNA nucleotides contain the sugar deoxyribose, a
phosphate and of four nitrogenous bases:
 Adenine (A), guanine (G), cytosine (C) and thymine
(T)
 RNA contains the sugar ribose, a phosphate, and of
four nitrogenous bases: adenine, guanine, cytosine
and uracil
 Adenine and Guanine are double-ringed bases called
purine bases
 Thymine, cytosine and uracil are single-ringed bases
called pyrimidine bases
The structure question
 Rosalind Franklin used x-ray diffraction data to help
determine the structure of DNA
 DNA is a double helix, or twisted ladder shape,
formed by two strands of nucleotides twisted around
each other
Watson and Crick
 Used Franklin’s data to determine the width of the
helix and the spacing of the bases.
 Together they built a model of the double helix and
discovered 3 important features:



Two outside strands consist of alternating deoxyribose and
phosphate
Cytosine and quanine bases pair to each other by three hydrogen
bonds
Thymine and adenine bases pair to each other by two hydrogen
bonds
Top, a GC base pair with three hydrogen bonds. Bottom, an AT base pair
with two hydrogen bonds. Non-covalent hydrogen bonds between the pairs
are shown as dashed lines.
Chromosome Structure
 Eukaryotic DNA is organized into individual
chromosomes.
 The length of a human chromosome ranges from 51
million to 245 million base pairs.
 If a DNA strand 140 million nucleotides long was
laid out in a straight line, it would be about 5 cm
long.
 In order to fit into the nucleus of a eukaryotic cell,
the DNA tightly coils around a group of beadlike
proteins called histones
 The phosphate groups in DNA create a negative
charge, which attracts the DNA to the positively
charged histone proteins and forms a nucleosome
 The nucleosomes than group together into
chromatin fibers, which super-coil to make up the
DNA structure recognized as a chromosome