1. Lecture 50 – Lecture 51 DNA: The Genetic Material Ozgur Unal
NIS - BIOLOGY
Lecture 50 – Lecture 51
DNA: The Genetic Material
Ozgur Unal

2. Discovery of the Genetic Material
Identify the differences in the phenotypes of the individuals below.

3. Discovery of the Genetic Material
After Mendel’s work was rediscovered in the 1900s, scientists began to search for the molecule involved in inheritance.
They knew that the genetic information was carried on the chromosomes in eukaryotic cells  two main components of chromosomes are DNA and protein.
Several experiments were performed to identify the source of genetic information:
Griffith
Avery
Hershey and Chase

4. Discovery of the Genetic Material
Griffith: Fredrick Griffith in 1928
The first major experiment that led to the discovery of DNA
Two strains of bacteria Streptococus pneumoniae
Griffith found that one strain could be transformed into the other form.
One strain, called S (smooth) strain had a sugar coat.
The other strain, called R (rough) strain did not have a sugar coat.
S strain kills the mouse – causes disease
R strain does not kill the mouse – does not
cause disease

5. Discovery of the Genetic Material
Griffith:
Check out Figure 12.2 for the experiment.
The experiment showed that R cells were transformed into S cells  What is the transforming substance?

6. Discovery of the Genetic Material
Avery: Oswald Avery in 1944
Avery and his colleagues isolated different macromolecules, such as DNA, protein and lipids from the killed S cells.
Then, the exposed the R cells to these macromolecules.
Only the R cells exposed to DNA were transformed into S strain  transforming substance is DNA
But their results were not widely accepted.

7. Discovery of the Genetic Material
Hershey and Chase: Alfred Hershey & Martha Chase in 1952
Definitive evidence that DNA is the transforming factor.
The experiment involved a bacteriophage  a type of virus that attacks bacteria
Bacteriophage in the experiment was made of DNA and protein.
Viruses cannot replicate themselves  they inject themselves into a living cell to reproduce
Hershey and Chase labeled both parts of virus (DNA and protein) by radioactive elements  they could
keep track of them

8. Discovery of the Genetic Material
Hershey and Chase:
Check out Figure 12.3 for the experiment.
Radioactive sulfur labels protein
Radioactive phosphorus labels
DNA

9. DNA Structure Nucleotides: Nucleic acids are complex macromolecules
that store and transmit genetic information  made of smaller subunits called nucleotides.
5-carbon sugar, a phosphate group and a nitrogenous base
There are 2 nucleic acids in cells: DNA
and RNA
DNA contains:
sugar deoxyribose
a phosphate
one of four nitrogenous bases: Adenine,
Guanine, Cytosine or Thymine

10. DNA Structure Nucleotides: RNA contains:
sugar ribose
a phosphate
One of four nitrogenous bases: Adenine, Guanine, Cytosine or Urasil
Guanine (G) and Adenine (A) are
double ringed bases  Purine base
Thymine (T), Cytosine (C) and
Uracil (U) are single ringed
bases  Pyrimidine base

11. DNA Structure Chargaff: Erwin Chargaff in 1950s
Erwin Chargaff analyzed the amount of A, G, T and C in the DNA of various species.
Check out Figure 12.5!!
He found that G = C and T = A

12. DNA Structure X-ray Diffraction:
Maurice Wilkins was working at King’s College on X-ray diffraction
Rosalind Franklin, also at King’s College, took “Photo 51” using X-ray diffraction.
This photo indicated that DNA was a double helix.
The specific structure of the DNA double helix was determined later by James Watson and Francis
Crick.

13. DNA Structure Watson and Crick:
Watson and Crick (at Cambridge Unv.) used Franklin’s picture and Chargaff’s data.
They built a model of the double helix:
1- Two outside strands consist
of alternating deoxyribose and
phosphate
2- C and G bases pair to each
other by 3 hydrogen bonds
3- T and A bases pair to each
other by 2 hydrogen bonds

14. DNA Structure DNA is often compared to a twisted ladder.
The pairs of bases (C-G and A-T) form the rungs of the ladder.
A purine always binds to a pyrimidine
base.
C + T = A + G

15. DNA Structure
Another unique feature of the DNA molecule is the direction of the two strands.
C atoms can be numbered in organic molecules.
Check out Figure 12.8!!
On the top rail, the orientation of the sugar has the 5’ C on the left and 3’ C on the right
The strand on the bottom runs
in opposite direction  3’ to 5’.
This orientation of the two
strands is called antiparallel.

16. Chromosome Structure http://www.johnkyrk.com/DNAanatomy.html
In eukaryotes DNA is organized into chromosomes.
In order to fit into the nucleus of a eukaryotic cell, the DNA tightly coils around a group of beadlike proteins called histones.
Histones and DNA form nucleosome.
The nucleosomes then group together into chromatin fibers, which supercoil to make
up the DNA structure recognized
as a chromosome.