40 Questions
What was the conclusion of Hershey and Chase's experiment?
The injected DNA of the phage provides the genetic information.
What was the purpose of using radioactive sulfur (35S) in Batch 1?
To track the phage protein.
What happened to the phage parts outside the E. coli cells after agitation?
They were freed from the cells.
What was the result of centrifuging the cells in Batch 2?
The radioactivity was found in the pellet.
What is the name of the phage studied by Hershey and Chase?
T2
Why was radioactive phosphorus (32P) used in Batch 2?
To label the phage DNA.
What was the significance of the experiment?
It proved that DNA is the genetic material of a phage.
What happened to the cells after centrifugation in both batches?
They formed a pellet.
What is a characteristic of the copying of DNA?
It is remarkable in its speed and accuracy
How many origins of replication are found in a eukaryotic chromosome?
Hundreds or even thousands
What is the direction of DNA replication from each origin?
In both directions
What is formed when the two DNA strands are separated at the origin of replication?
A replication bubble
What is the role of the enzymes and proteins in DNA replication?
To participate in the copying of DNA
What is the result of DNA replication?
Two daughter DNA molecules
What is the function of the origin of replication?
To initiate DNA replication
What is the shape of the replication fork?
Y-shaped
What is the result of the low error rate of DNA replication?
Permanent sequence changes
What is the role of mutations in the process of evolution?
They are the source of genetic variation
What type of chromosomes do most prokaryotes have?
Circular
What is the limitation of DNA polymerase?
It cannot complete the 5′ ends
What happens to the DNA molecules after repeated rounds of replication?
They become shorter with uneven ends
What is the significance of mutations in the evolution of species?
They are responsible for the formation of new species
What is the effect of proofreading and repair on the error rate of DNA replication?
It decreases the error rate but not to zero
Which of the following enzymes is involved in the replication process?
All of the above
What is the direction of the leading strand synthesis?
5′ → 3′
What is the function of the RNA primer in lagging strand synthesis?
To provide a free 3′ end for DNA synthesis
What happens to the RNA primers after the synthesis of the lagging strand?
They are removed and replaced with DNA
What is the outcome of the second round of replication?
The production of shorter and shorter daughter molecules
What is the direction of the lagging strand synthesis?
Discontinuous 5′ → 3′
What is the relationship between the new leading strand and the new lagging strand?
The new leading strand is complementary to the new lagging strand
What happens to the parental strands during DNA replication?
They are used as templates
What is the orientation of the 5′ and 3′ ends of the new DNA strands?
5′ end of the leading strand is opposite to the 3′ end of the lagging strand
What is the purpose of agitation in Experiment 1?
To free phage particles from outside cells
What is the outcome of Experiment 1?
Radioactivity is found only in the liquid
What is the difference between Batch 1 and Batch 2?
The type of radioactive material used
What is the purpose of radioactive phosphorus in Experiment 2?
To label phage DNA
What is the outcome of Experiment 2?
Radioactivity is found only in the pellet
What is the role of centrifugation in these experiments?
To separate phage components based on their density
What is the conclusion that can be drawn from these experiments?
Phage DNA is necessary for infection
What is the significance of the radioactivity in the pellet and the liquid?
It indicates the presence of phage DNA
Study Notes
Discovery of DNA as Genetic Material
- In 1952, Alfred Hershey and Martha Chase conducted an experiment that showed DNA is the genetic material of a phage (T2).
- They demonstrated that only one component of T2 (DNA or protein) enters an E. coli cell during infection.
- They concluded that the injected DNA of the phage provides the genetic information.
Experiment Design
- The experiment involved labeling phages with radioactive sulfur (35S) in protein and radioactive phosphorus (32P) in DNA.
- The labeled phages were allowed to infect E. coli cells, and then the cells were centrifuged to separate the phage parts from the cells.
- Radioactivity was measured in the pellet and the liquid to determine which component (DNA or protein) entered the cell.
DNA Replication
- DNA replication is a remarkable process that occurs with high speed and accuracy.
- More than a dozen enzymes and other proteins participate in DNA replication.
Initiation of Replication
- Replication begins at specific sites called origins of replication, where the two DNA strands are separated, opening up a replication "bubble".
- A eukaryotic chromosome may have hundreds or thousands of origins of replication.
- Replication proceeds in both directions from each origin until the entire molecule is copied.
Replication Process
- The replication process involves the separation of the double-stranded DNA molecule at the origin of replication.
- The two daughter strands are synthesized in opposite directions, forming a replication fork.
- Enzymes such as DNA polymerase and DNA ligase are involved in the replication process.
Errors and Mutations
- The error rate after proofreading and repair is low but not zero.
- Sequence changes can become permanent and be passed on to the next generation.
- These changes (mutations) are the source of genetic variation upon which natural selection operates.
Replicating the Ends of DNA Molecules
- The usual replication machinery provides no way to complete the 5' ends of linear DNA molecules in eukaryotic chromosomes.
- Repeated rounds of replication produce shorter DNA molecules with uneven ends.
- This is not a problem for prokaryotes, most of which have circular chromosomes.
This quiz is about the experiment conducted by Alfred Hershey and Martha Chase in 1952 to demonstrate that DNA is the genetic material of a phage. They showed that DNA, not protein, enters an E.coli cell during infection and provides genetic information.
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