4.1 遗传物质的发现the discovery of genetic material在线视频

One of the fundamental properties of all living things

is the ability to reproduce

All organisms inherit the information

specifying their structure and function from their parents

Likewise, all cells arise from pre-existing cells

and the genetic material must be replicated

and passed from parent to progeny cells

The substance capable of carrying all genetic information

from one generation to another

is called the genetic material

To fulfil this function

the genetic material has to

have the following characteristics

The first: Replicability

The genetic material of all cells

must be replicated accurately

The second: Informative

It must contain all the biological information for an organism

The third: Mutability

The genetic material should be capable of

undergoing variations for evolution

It took the scientists long before they found

the substance that fulfilled the above requirements

Thanks to the pioneer research works of scientists

during mid to late 1,800s

and the early 1,900s

which led to the discovery of DNA

as the Genetic Material

In the first 40 years of the last century

chemical analysis revealed that

nucleus contained a unique molecule

the DNA

However, it was not understood that

DNA was the chemical constituent of genes

The breakthrough came from the Transformation Studies

form Fredrick Griffith

He was a medical officer

in the British Ministry of Health, in 1928

He performed experiments with several strains of

the bacterium Diplococcus pneumoniae

Some strains of this bacterium have polysaccharide coat

surrounding each cell that gives a smooth

shiny appearance to the colony of the bacteria

called S- type

These bacteria are not easily engulfed

and destroyed by the phagocytic cells

and thereby are able to multiply and cause pneumonia

Whereas some other lack polysaccharide coat

colonies of this strain are rough, called R-type

These non-capsulated bacteria are readily engulfed

and destroyed by the phagocytic cells

and so they are noninfectious

Injection of the R-bacteria did not affect

the mice and injection of S-bacteria caused death

Griffith further observed that

if the S bacteria were heat-killed before injection

the mice however, survived

Lastly, Griffith injected another batch of mice with

a mixture of living R-bacteria and heat killed S bacteria

In this case, the mice died

and living S bacteria were present in their blood

Griffith then concluded that

some R-bacteria had somehow

been transformed into smooth, infectious S-bacteria

by interaction with the dead S-bacteria cells

The transformed cells retained their infectious properties

in successive generations

indicating that the transformation was stable

In 1944, Oswald T. Avery, Colin M. Macleod

and Maclyn McCarty showed that

the transforming principle was DNA

by the improved version of

Griffith’s transforming experiment

They lysed S-bacteria cells and separated the lysate

into different cellular macromolecular components

like lipids, polysaccharides, proteins and nucleic acids

Each component then was checked

whether or not it could transform living R bacteria

and it was found that

only the nucleic acid component could

transform the R cells into S cells

Avery et al. further proved that

DNA was the actual genetic material

by digesting the nucleic acid components

with ribonuclease (RNase)

or de-oxy-ribonuclease (DNase)

and found that no transformation after DNase digestion

The second important evidence supporting DNA

as the genetic material

was provided by Alfred D. Hershey and Martha Chase

in 1953

with their bacteriophage experiment

It was known that each phage

consisted of only DNA and protein

The T2 phage was labelled with either sulphur 35S on proteins

or phosphorus 32P on DNA

They subsequently infected unlabeled E. coli

with the two types of radio labelled T2 phage, separately

In case of infection done by 32P labelled phages

most of the radioactivity could be found

within the bacteria soon after infection after centrifugation

However, when E. coli was infected with 35S-labelled phage

radioactivity neither appeared within the cell

nor in the progeny phages

Thus, Hershey and Chase proved that

DNA is the genetic material in phages

as it fulfilled the essential criteria like

replicability and informative

All the above mentioned evidences supporting the concept that

DNA is the genetic material

Soon after that

experiments of mutagenesis and DNA recombination

supported that DNA was also the genetic material in eukaryotes

Results in later experiments showed that

although DNA was the major form of

genetic material in most organisms

RNA could also perform this function in some virus

for example, poliovirus

How these RNA virus replicate

and transfer the genetic information will be addressed later

Alright, hope all of you today

enjoyed the fascinating research works

in the early years that

led to the discovery of the genetic material