Download Major Events in Genetics

Major Events in Genetics
A gene is a coding unit
A gene is a genetic sequence that codes for an RNA.
In protein coding genes, the RNA codes for a
protein.
DNA is the genetic material
of bacteria, viruses and
animal cells
How was this discovered?
1870: DNA (nuclein) was
first isolated from the nuclei
of lymphocytes by F.
Miescher. Alkali extraction.
1909: The T. Morgan
Drosophila experiments led
to the chromosomal theory
of heredity.
The word “gene” is first
introduced
GRIFFITH
• The bacterium Pneumoccocus kills mice by causing
pneumonia
• Pneumoccocus virulence is determined by its
capsular polysacharide
– component of cell surface allows the bacteria to escape
destruction by the host.
• Several types of pneumoccocus have different types
of polysacharides.
– Two groups S (smooth) and R (rough)
GRIFFITH
1928
The transforming Principle is
DNA
• Some properties from dead S-type
bacteria can transform the live R-type
bacteria and render it virulent (S-type)
Avery,
MacLeod e
McCarty
What is the chemical nature
of the transforming factor ?
Only DNase destroyed the transforming factor.
Bacteriophage
Bacteriophages attacking a
bacteria
Alfred Hershey and Martha Chase
– Performed experiments showing that DNA is the genetic
material of a phage known as T2
– Used radioactive isotopes for labeling
– Sulfur isotope, 39S, to label the protein
– Phosphorous isotope, 32P, to label the DNA
HERSHEY e CHASE
Protein labelling:
No radioactivity
in progeny
DNA labelling:
radioactivity in
progeny
1950 The Chargaff RULES
- The number of guanines and adenines equal the
number of cytosines and thymines.
- The relative amounts of the G/C and A/T vary
among different species.
X ray diffraction allows to obtain information on the
three dimensional structure of molecules.
Rosalind Franklin and Maurice Wilkins
• They were using a technique called X-ray crystallography to
study molecular structure
• Rosalind Franklin
– Produced a picture of the DNA molecule using this
technique
– Already determined that the sugar-phosphate ladder was
on the outside of the molecule
– Wilkins received Nobel Prize in 1962
– Franklin –and Chargaff- did not.
James Watson and Frances Crick
• Watson and Crick published an article in the same Nature
issue as Wilkins and Franklin in April 1953 that DNA was a
double helix through observations of the X-ray
crystallographic images of DNA
• Watson and Crick reasoned that there must be
additional specificity of pairing
– Dictated by the structure of the bases
• Each base pair forms a different number of
hydrogen bonds
– Adenine and thymine form two bonds,
cytosine and guanine form three bonds
Race to the structure
A
B
Z
A-DNA e B-DNA:
Doppie eliche destrorse che portano
rispettivamente 10,9 e 10 coppie di
basi per giro completo dell’elica.
Nella cellula il DNA si trova
tipicamente come forma B, la forma
A si riscontra solo in condizioni di
umidità relativamente bassa.
Z-DNA:
Elica sinistrorsa con 12 paia di basi per ogni
giro completo dell’elica.
Elica sottile ed allungata con solco minore
profondo e solco maggiore poco evidente.
Associato a particolari sequenze di basi, come
basi puriniche che si alternano a basi
pirimidiniche.
Properties of the genetic material:
-Potentially it contains a huge amount of information
-The information can be translated in a phenotype.
-The information can be copied in a faithfull manner
Specific structures of
RNA
Cromosomes of prokaryotes
-single chromosome of circular double stranded DNA
-in some cases, a major chromosome and several much smaller
chromosomes (plasmids)
Organized as NUCLEOID of supercoiled DNA
Eukaryotic chromosomes
-Organized in chromatin with histonic and non histonic
proteins
-DNA domain organized as loops bound to a structures linked to
the nuclear membrane = NUCLEAR MATRIX.
-DNA sequences associated to the nuclear matrix = MAR
(matrix attachment regions).
-MAR are generally associated to regions active in transcription
and early replication.
Chromatin:
-constitutive and facultative heterochromatin
-euchromatin