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
DNA, genetic material of
bacteria
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…
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
Bacteriophage
Bacteriophages attacking a
bacteria
Il fago T2 e il suo ciclo vitale
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
Animal cells
•  Cells that lack
Thymidine kinase
synthesize TK after
transfection and
survive in the
absence of TK
•  Not only DNA is the
genetic material of
the cells but DNA
can be transferred
among species and
remain functional
Rosalind Franklin
La diffrazione ai raggi X permette di ricavare
informazioni circa la struttura delle molecole.
Rosalind Franklin and Maurice Wilkins
•  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 w/ W & C in 1962
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
Race to the structure
•  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-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.
3 PROPRIETA’ DEL MATERIALE GENETICO:
- Consente di contenere grandi quantità di informazioni
- Consente una replicazione fedele con meccanismo di
copiatura
- Consente di tradurre le istruzioni in esso contenute in un
fenotipo.
Strutture particolari
dell’RNA e del DNA
Cromosomi dei procarioti
-singolo cromosoma di DNA doppio filamento circolare
-in alcuni casi un cromosoma principale ed uno o più
cromosomi più piccoli (se non è indsipensabile=plasmide)
Organizzato in NUCLEOIDE come DNA superavvolto
Cromosomi degli eucarioti
-tipicamente numero diploide di cromosomi in tutte le cellule
somatiche
-cromosomi organizzato in cromatina con proteine istoniche e
non-istoniche
- Domini di DNA ad ansa ancorati ad un’intelaiatura strutturale
filamentosa all’interno della membrana nucleare = MATRICE
NUCLEARE.
- Sequenze di DNA associate a proteine della matrice nucleare =
MAR (matrix attachment regions).
-MAR generalmente fiancheggiano geni trascrizionalemnte
attivi e regioni in attiva replicazione.
Cromatina:
-eterocromatina facoltativa e costitutiva
-eucromatina