major events in genetics - unimi.it 3... · a gene is a genetic sequence that codes for an rna. in...

Major Events in Genetics

A gene is a genetic sequence that codes for an RNA. In protein coding genes, the RNA codes for a protein.

A gene is a coding unit

DNA is the genetic material of bacteria, viruses and

animal cells

DNA, genetic material of bacteria

•  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

GRIFFITH

1928…

•  Some properties from dead S-type bacteria can transform the live R-type bacteria and render it virulent (S-type)

Transforming Principle is DNA

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