Chapter 6 – The Genetic Material
The Basic Requirements for Genetic Material (i.e., the material that determines the inherited characteristics of a functional organism):
How do we know that DNA is the genetic material of most organisms?
Why is DNA suited for this job?
The Griffith Experiment with Streptococcus pneumoniae (1928); Figure 6-2:
What is S. pneumoniae and what disease does it cause in mice and man?
How does the bacterial capsule relate to the disease process?
Avery, MacLoed and McCarty experiment, fifteen years later, proved that DNA was the “transforming principle”.
Chargoff (1952) showed that DNA existed with enough structural complexity and diversity to allow it to be THE genetic material of all organisms.
The Blender Experiment – The Hershey – Chase Experiment:
First a few facts about phage/virus: See figures 6-5 and 6-6.
Note that the life cycle of the T-even phage was not known at the time of this experiment – all that was known was that viruses attached to the bacterial cells and then some time later the bacteria lysed and more virus was released.
They knew that phage were composed of only DNA and protein.
They knew that phage attached to the bacteria via a tail, and that the phage could be detached from the bacteria via violent agitation (i.e., a blender)
They did two experiments (See fig 6-7):
1. Label phage with 35S, infect E. coli (allow attachment), blenderize, and measure where the radioactivity went.
a. Most of the radioactivity detached from the E.coli
b. Any of the radioactivity that stayed with the E. coli was not found in the progeny virus
2. Label phage with 32P, infect E. coli (allow attachment), blenderize, and measure where the radioactivity went.
a. Most of the radioactivity stayed with the E. coli
b. Progeny phage that came out of the E.coli were also radioactive…genetic information was transmitted to the progeny!
Can you explain why, in neither of these experiments was the radioactivity partitioned completely in one place or another? That is: Why did not100% of the activity go with the phage in experiment 1 or 100% or the activity go with the bacteria in experiment 2?
Of course, DNA is not the only genetic material…there is also RNA.
Read about RNA viruses.
The Properties of DNA:
1. DNA stores and transmits genetic information … IT’S ALL IN THE SEQUENCE!
a. Look over the list on page 111. Note that the sequence info can be converted into new DNA, new RNA, and protein. Also note that specific sequences are involved in REGULATION.
b. DNA is copied with high-fidelity: only one mistake per every 109 – 1010 bases
2. DNA is stable
a. Very resistant to chemical attack
1. resistant to alkali treatment (RNA isn’t)
2. resistant to low pH, however it will begin to hydolyze below pH 2
b. Its double-strandedness insures that the information is redundant.
c. Double-strandedness also shelters the bases from chemical attack. Very hydrophobic internal environment.
d. One exception is that cytosine is somewhat unstable; can deaminate and turn into uracil (see page 113).
1. 100 cytosines/human genome/day do this!
Could this be bad? Why or why not?
2. Thus there is DNA excision/repair system that always removes uracil from DNA.
3. DNA does change, it evolves.
a. This process is called mutation.
b. It is caused by either a chemical alteration or a replication error